Urban Water Cycle Processes and Interactions

Full text

INTERNATIONAL HYDROLOGICAL PROGRAMME
_____________________________________________________________
Urban water cycle
processes and interactions
By
J. Marsalek, B.E. Jiménez-Cisneros, P.-A. Malmquist,
M. Karamouz, J. Goldenfum and B. Chocat
IHP-VI
~
Technical Documents in Hydrology
~
No. 78
UNESCO, Paris, 2006

Published in 2006 by the International Hydrological Programme (IHP) of the
United Nations Educational, Scientific and Cultural Organization (UNESCO)
1 rue Miollis, 75732 Paris Cedex 15, France
IHP-VI Technical Document in Hydrology N°78
UNESCO Working Series SC-2006/WS/7
 UNESCO/IHP 2006
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Paris, France

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Table of Contents
Foreword................................................................................................................................................vii
Acknowledgements...............................................................................................................................viii
CHAPTER 1 Urban Water Cycle
1.1 Introduction ......................................................................................................................................1
1.2 Urban Water Cycle Concept.............................................................................................................2
1.3 Total Management of the Urban Water Cycle ..................................................................................5
CHAPTER 2 Urban Water Cycle Hydrologic Components
2.1 Water Sources...................................................................................................................................8
2.1.1 Municipal water supply .............................................................................................................8
2.1.2 Precipitation...............................................................................................................................8
2.1.2.1 Climatic aspects ..................................................................................................................8
2.1.2.2 Urban precipitation .............................................................................................................9
2.2 Hydrologic Abstractions.................................................................................................................10
2.2.1 Interception..............................................................................................................................10
2.2.2 Depression storage...................................................................................................................10
2.2.3 Evaporation and evapotranspiration.........................................................................................11
2.2.4 Infiltration................................................................................................................................11
2.2.5 Lumped hydrologic abstractions..............................................................................................12
2.3 Water Storage .................................................................................................................................12
2.3.1 Soil moisture............................................................................................................................12
2.3.2 Urban groundwater ..................................................................................................................12
2.4 Stormwater Runoff .........................................................................................................................12
2.5 Interflow and Groundwater Flow....................................................................................................14
2.6 Natural Drainage: Urban Streams, Rivers and Lakes .....................................................................14
2.7 Needs for Urban Water Infrastructure ............................................................................................14
CHAPTER 3 Urban Water Infrastructure
3.1 Demands on Water Services in Urban Areas..................................................................................17
3.2 Water Supply ..................................................................................................................................19
3.2.1 Historical development............................................................................................................20
3.2.2 Water demand..........................................................................................................................20
3.2.2.1 Water supply standards: quantity......................................................................................22
3.2.2.2 Water supply standards: quality........................................................................................23
3.2.3 Water supply sources...............................................................................................................23
3.2.3.1 Conjunctive use of sources and artificial recharge............................................................24
3.2.3.2. Supplementary sources of water ......................................................................................25
3.2.3.3 Water shortage..................................................................................................................26
3.2.4 Drinking water treatment .........................................................................................................27
3.2.4.1 Emerging technologies .....................................................................................................27
3.2.4.2 Desalination ......................................................................................................................28
3.2.4.3 Disinfection.......................................................................................................................29
3.2.5 Water distribution systems.......................................................................................................29
3.2.6 Drinking water supply in developing countries .......................................................................30
3.3 Urban Drainage...............................................................................................................................30
3.3.1 Flooding in urban areas............................................................................................................31
3.3.2 Stormwater...............................................................................................................................32
3.3.2.1 Stormwater characterisation..............................................................................................34
3.3.2.2 Stormwater management ..................................................................................................35
3.3.2.3 Special considerations for drainage in cold climate..........................................................35
3.3.3 Combined Sewer Overflows (CSOs).......................................................................................36
3.3.3.1 CSO characterisation ........................................................................................................36
3.3.3.2 CSO control and treatment ................................................................................................36
3.4 Wastewater and Sanitation..............................................................................................................37

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3.4.1 Problem definition ...................................................................................................................38
3.4.2 Technological development.....................................................................................................38
3.4.3 Ecological sanitation................................................................................................................39
3.4.4 Basic demands on wastewater management systems...............................................................40
3.4.5. Wastewater characterisation ...................................................................................................41
3.4.6 Wastewater systems without separation of wastewaters at the source.....................................41
3.4.6.1 Centralised systems...........................................................................................................41
3.4.6.2 Distributed (local) systems ...............................................................................................42
3.4.7 Systems with separation of wastewaters at the source.............................................................43
3.4.8 Water and wastewater reuse.....................................................................................................44
3.4.8.1 NEWater in Singapore......................................................................................................45
3.4 8.2 Shinjuku water recycling centre, Tokyo, Japan ................................................................45
3.4.8.3 Wetlands with fish production in Calcutta, India..............................................................45
3.4.8.4 Reuse of (untreated) sewage for agricultural irrigation in the Mezquital Valley (Mexico
City sewage disposal).........................................................................................................................45
3.4.8.5 Reuse of stormwater and greywater in Sydney, Australia ................................................46
CHAPTER 4 Impacts of Urbanisation on the Environment
4.1 Overview ........................................................................................................................................47
4.2 General Characterisation of Urbanisation Effects...........................................................................48
4.2.1 Increased ground imperviousness............................................................................................48
4.2.2 Changes in runoff conveyance networks .................................................................................49
4.2.2.1 Construction of runoff conveyance networks ...................................................................49
4.2.2.2 Canalisation of urban streams and rivers ..........................................................................49
4.2.2.3 Interfering transport infrastructures ..................................................................................50
4.2.3 Increased water consumption...................................................................................................50
4.2.4 Time scales of urbanisation effects..........................................................................................51
4.2.5 Spatial scales and types of receiving waters ...............................................................................51
4.3 Urbanisation Effects on the Atmosphere ........................................................................................52
4.3.1 Thermal effects (urban heat island phenomenon)....................................................................53
4.3.2 Urban air pollution...................................................................................................................53
4.3.3 Combined impacts ...................................................................................................................54
4.4 Urbanisation Effects on Surface Waters .........................................................................................54
4.4.1 Physical effects ........................................................................................................................54
4.4.1.1 Urbanisation effects on flows ...........................................................................................54
4.4.1.2 Urbanisation effects on sediment regime: erosion and siltation........................................55
4.4.1.3 Modification of the thermal regime of receiving waters...................................................55
4.4.1.4 Density stratification of receiving water bodies................................................................56
4.4.1.5 Combined physical effects................................................................................................56
4.4.2 Chemical effects ......................................................................................................................57
4.4.2.1 Dissolved oxygen (DO) reduction ....................................................................................57
4.4.2.2 Nutrient enrichment and eutrophication ...........................................................................57
4.4.2.3 Toxicity.............................................................................................................................58
4.4.3 Microbiological effects............................................................................................................59
4.4.3.1 Waterborne pathogens ......................................................................................................59
4.4.3.2 Indicators of microbiological pollution.............................................................................62
4.4.4 Combined effects on surface waters ........................................................................................62
4.4.5 Examples of urbanisation effects on specific types of receiving waters..................................63
4.4.5.1 Rivers................................................................................................................................63
4.4.5.2 Lakes and reservoirs .........................................................................................................65
4.5 Urbanisation Effects on Wetlands ..................................................................................................67
4.6 Urbanisation Effects on Soils .........................................................................................................69
4.6.1 Erosion.....................................................................................................................................69
4.6.2 Transport of pollutants in soils ........................................................................................
........70
4.6.3 Changes in water quality during percolation through soils......................................................71
4.6.4 Effects of sludge disposal on soils ...........................................................................................71
4.6.4.1 Sludge production.............................................................................................................72
4.6.4.2 Sludge quality ...................................................................................................................72

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4.6.4.3 Biosolids (sludge) application on land..............................................................................72
4.6.4.4 Sludge disposal .................................................................................................................73
4.6.4.5 New chemicals of concern in sludge.................................................................................73
4.7 Urban Impacts on Groundwater......................................................................................................73
4.7.1 Unintentional discharges into groundwater aquifers................................................................74
4.7.2 Intentional discharges into groundwater aquifers ....................................................................75
4.7.3 Impacts on aquifers..................................................................................................................76
4.8 Urban Impacts on Biota – Loss of Biodiversity.................................................................................76
4.8.1 General structure of water bodies and their biota ....................................................................76
4.8.2 Properties of the water bodies affecting flora and fauna..........................................................77
4.8.3 Effects of alterations of urban water bodies on biota...............................................................77
4.8.3.1 Rivers................................................................................................................................77
4.8.3.2 Lakes and reservoirs .........................................................................................................78
References.............................................................................................................................................81

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FOREWORD
Continuing urbanisation leads to ever increasing concentrations of population in urban
areas. General statistics indicate that of the current (2005) world population of about
6.5 billion people, more than 54% live in urban areas, and in some countries this
proportion reaches 90% or more. The process of urbanisation is particularly fast in
developing countries, which account for a disproportionately high number of megacities
with many millions of inhabitants. Consequently, the issue of urban environmental
sustainability is becoming critical, because urbanisation and its associated
environmental impacts are occurring at an unprecedented rate and scope.
These concerns have long been recognised by UNESCO in their International
Hydrological Program (IHP), which has addressed the role of water in urban areas,
effects of urbanisation on the hydrological cycle and water quality, and many aspects of
integrated water management in urban areas. The current phase of IHP (IHP 6) is
seeking solutions to water resource problems in urban areas by examining the means of
implementation of integrated water management in urban areas. Towards this end,
UNESCO held in 2001 a Symposium on Frontiers in Urban Water Management and the
resulting publication (Maksimovic and Tejada-Guibert, 2001) proposed the way forward
in this challenging field. At a subsequent meeting at UNESCO Headquarters, a
program comprising eight mutually related studies on urban water management was
initiated by UNESCO.
This report presents results of one of those studies; its main focus is on the
assessment of anthropogenic impacts on the urban hydrological cycle and the urban
environment, including processes and interactions in the urban water cycle. The need
for this study follows from the fact that effective management of urban waters should be
based on a scientific understanding of anthropogenic impacts on the urban hydrological
cycle and the environment. Such impacts vary broadly in time and space, and need to
be quantified with respect to the local climate, urban development, cultural,
environmental and religious practices, and other socio-economic factors. The final
product of this activity should be a guidance manual on anthropogenic alterations of the
urban water cycle and the environment, with reference to various climatic zones and
potential climate changes.
To address the broad range of conditions in urban water management, UNESCO
established a working group for this study with representatives of various professional
backgrounds and experience from various climatic regions. The Working Group for the
study of the urban water cycle processes and interactions comprised the following
members:
Mr. Gamal Abdo, Department of Civil Engineering, University of Khartoum,
Khartoum, Sudan
Mr. Bernard Chocat, INSA Lyon, Lyon, France
Mr. Joel Goldenfum, IPH/UFRGS, Porto Allegre, Brazil
Mr. K.V. Jayakumar, Water and Environment Division, Regional Engineering
College, Warangal, India
Ms. Blanca Jiménez-Cisneros, Environmental Engineering Department, Institute of
Engineering, Universidad Nacional Autónoma de Mexico, Mexico
Mr. Mohammad Karamouz, School of Civil Engineering, Amirkabir University
(Tehran Polytechnic), Tehran, Iran
Mr. Per-Arne Malmquist, Chalmers University of Technology, Goteborg, Sweden
Mr. Jiri Marsalek, National Water Research Institute, Burlington, Ontario, Canada.

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ACKNOWLEDGEMENTS
Many colleagues have contributed to the preparation of this report and their
contributions are gratefully acknowledged. In particular, the work of the following is
acknowledged:
UNESCO Secretariat: Mr. J.A. Tejada-Guibert, officer in charge of Urban Water
activities of the International Hydrological Programme VI (IHP-VI) and Deputy
Secretary of IHP, Mr. C. Maksimovic, adviser for the IHP Urban Water
component, Ms. B. Radojevic, consultant, and Mr. W. H. Gilbrich, consultant.
Mr. W.E. Watt, Emeritus Professor, Queen’s University, Kingston, Ontario, Canada,
who served as an external editor of the final report.
Mr. Q. Rochfort, Ms. J. Dziuba and Mr. P. McColl, National Water Research
Institute, Burlington, Ontario, Canada, for producing the print-ready version of
the report.
All the members of the Working Group for this project, and in particular, those who
provided written materials:
Prof. B. Chocat – contributed to Chapter 4
Prof. J. Goldenfum – contributed to Chapters 2 and 3
Prof. B.E. Jiménez-Cisneros – contributed to Chapters 3 and 4
Prof. M. Karamouz– contributed to Chapters 2 and 3
Dr. P.-A. Malmquist – contributed to Chapter 3
Dr. J. Marsalek – contributed to Chapters 1-4 and provided report integration and
editing.

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Chapter 1
Urban Water Cycle
1.1 INTRODUCTION
An urban population demands high quantities of energy and raw materials, and removal
of waste, some of which turns into environmental pollution. Indeed, all key activities of
modern cities: transportation, electricity supply, water supply, waste disposal, heating,
supply of services, manufacturing, etc., are characterised by the aforementioned
problems. Thus, concentration of people in urban areas dramatically alters material and
energy fluxes in the affected areas, with concomitant changes in landscape; altered
fluxes of water, sediment, chemicals, and microorganisms; and, increased release of
waste heat. These changes then impact on urban ecosystems, including urban waters
and their aquatic ecosystems, and result in their degradation. Such circumstances make
provision of water services to urban populations highly challenging, particularly in
megacities, which are defined as the cities with 10 million or more inhabitants. Yet, the
number of these megacities keeps growing, particularly in the developing countries, and
this further exacerbates both human health and environmental problems. The growth of
the number of megacities is illustrated in Table 1.1, listing megacities in 1975 and 2003,
and predictions for 2015.
Table 1.1 Megacities with more than 10 million people (after Marshall, 2005)
Megacities with more than 10 million people
1975 2003 2015
Tokyo, Japan (26.6) Tokyo, Japan (35.0) Tokyo, Japan (36.2)
New York, USA (15.9) Mexico City, Mexico (18.7) Mumbai, India (22.6)
Shanghai, China (11.4) New York, USA (18.3) Delhi, India (20.9)
Mexico City, Mexico (10.7) Sao Paulo, Brazil (17.9) Mexico City, Mexico (20.6)
Mumbai, India (17.4) Sao Paulo, Brazil (20.0)
Delhi, India (14.1) New York, USA (19.7)
Calcutta, India (13.8) Dhaka, Bangladesh (17.9)
Buenos Aires, Argentina (13.0) Jakarta, Indonesia (17.5)
Shanghai, China (12.8) Lagos, Nigeria (17.0)
Jakarta, Indonesia (12.3) Calcutta, India (16.8)
Los Angeles, USA (12.0) Karachi, Pakistan (16.2)
Dhaka, Bangladesh (11.6) Buenos Aires, Argentina (14.6)
Osaka-Kobe, Japan (11.2) Cairo, Egypt (13.1)
Rio de Janeiro, Brazil (11.2) Los Angeles, USA (12.9)
Karachi, Pakistan (11.1) Shanghai, China (12.7)
Beijing, China (10.8) Metro Manila, Philippines (12.6)
Cairo, Egypt (10.8) Rio de Janeiro, Brazil (12.4)
Moscow, Russian Federation (10.5) Osaka-Kobe, Japan (11.4)
Metro Manila, Philippines (10.5) Istanbul, Turkey (11.3)
Lagos, Nigeria (10.1) Beijing, China (11.1)
Moscow, Russian Federation (10.9)
Paris, France (10.0)

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Conflicting demands on resources necessitate integrated management of the
urbanisation process, which is a most challenging task. Within this complex setting,
this report focuses on the management urban waters, recognising that effective
management of urban waters should be based on a scientific understanding of
anthropogenic impacts on the urban hydrological cycle and the environment, and the
means of mitigation of such impacts, and full recognition of the socio-economic system.
Urbanisation impacts vary broadly in time and space, and need to be quantified with
respect to the local climate, urban development, engineering and environmental
practices, cultural religious practices, and other socio-economic factors.
Analysis of urban water management should be based on the urban water cycle,
which provides a unifying concept for addressing climatic, hydrologic, land use,
engineering, and ecological issues in urban areas. Furthermore, it was felt that the
analysis of the urban water cycle would be conducive to a later examination of modern
approaches to water management in urban areas, including total urban water cycle
management. In this approach based on water conservation, integrated management
measures are implemented, including integrated management and reuse of stormwater,
groundwater, and wastewater.
The report that follows represents the first step of a comprehensive project and aims
to develop a schematic representation of the urban water cycle (UWC), including the
environmental components, and identify the major fluxes of water, sediment, chemicals,
microorganisms, and heat, with reference to urban waters. Such a scheme may be
presented in many variations reflecting various climatic conditions, both the current and
the future ones (i.e., considering climate change). In the subsequent study phases, it is
expected that these fluxes will be quantified and described by water balance/quality
models approximating such processes. Connection between urban development and
these fluxes will be established, and principles for low impact developments and
restoration of the existing areas will be established. Some of the intermediate
steps/results in the overall study include: (a) identification of the components of the
urban water cycle and the effects of urbanisation on water resources, (b) quantification
of the imprint of human activities on the urban hydrological cycle and its interaction
with the environment under the present and future development scenarios, (c)
understanding of the processes at the urban water and soil interface, including the water
and soil interaction, with particular reference to soil erosion, soil pollution and land
subsidence, (d) hydrological, ecological, biological and chemical processes in the urban
water environment of sustainable cities of the future, (e) assessment of the impact of
urban development, land use and socio-economic changes on the availability of water
supplies, aquatic chemistry, (anthropogenic) pollution, soil erosion and sedimentation
and natural habitat integrity and diversity, and, (f) assessment of the preventive and
mitigation measures available for dealing with urban water problems.
The final product of this activity should be a guidance manual on anthropogenic
alterations of the UWC and the environment, with reference to various climatic zones
and potential climate changes. This manual should advance (a) the understanding of
processes that take place in the urban environment, and of the interactions of natural
suburban, rural and urban environments for the successful analysis, planning,
development and management of urban water systems, (b) development of innovative
analytical tools for addressing the problems of spatial and temporal variability, and (c)
assessment of the potential effects of climate variations and changes on urban water
systems.
1.2 URBAN WATER CYCLE CONCEPT
One of the most fundamental concepts in hydrology and indeed in the water resource
management is the hydrologic cycle (also referred to as the water cycle), which has
been speculated on since ancient times (Maidment, 1993). There is some diversity of
definitions of the hydrological cycle, but generally it is defined as a conceptual model
describing the storage and circulation of water between the biosphere, atmosphere,
lithosphere, and the hydrosphere. Water can be stored in the atmosphere, oceans, lakes,
rivers, streams, soils, glaciers, snowfields, and groundwater aquifers. Circulation of

3
water among these storage compartments is caused by such processes as
evapotranspiration, condensation, precipitation, infiltration, percolation, snowmelt and
runoff, which are also referred to as the water cycle components.
Combined effects of urbanisation, industrialisation, and population growth affect
natural landscapes and hydrological response of watersheds. Although many elements
of the natural environment are affected by anthropogenic factors with respect to
pathways and hydrologic abstractions (or sources of water), the principal structure of
the hydrological cycle remains intact in urban areas. However, the hydrologic cycle is
greatly modified by urbanisation impacts on the environment and the need to provide
water services to the urban population, including water supply, drainage, wastewater
collection and management, and beneficial uses of receiving waters. Thus, it was noted
that the hydrological cycle becomes more complex in urban areas, because of many
anthropogenic influences and interventions (McPherson, 1973; McPherson and
Schneider, 1974); the resulting “urban” hydrological cycle is then called urban water
cycle (UWC). The urban water cycle is shown pictorially in some detail in Fig. 1.1 and
schematically in Fig. 1.2, which displays just the major components and pathways.
The urban water cycle provides a good conceptual and unifying basis for studying the
water balance (also called the water budget) and conducting water inventories of urban
areas. In such studies, the above listed major components of the hydrological cycle are
assessed for certain time periods, with durations exceeding the time constants of the
system to filter out short-term variability. Water balances are generally conducted on
seasonal, annual, or multi-year bases (van de Ven, 1988), and in planning studies, such
balances are projected to future planning horizons. This approach is particularly
important for urban planning (i.e., providing water services to growing populations) and
for coping with extreme weather and climatic variations and potential climate change.
In fact, an understanding of water balances is essential for integrated management of
urban water, which strives to remediate anthropogenic pressures and impacts by
intervention (management) measures, which are applied in the so-called total
management of the urban water cycle (Lawrence et al., 1999).
Fig. 1.1 Urban water cycle

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Fig. 1.2 Urban water cycle – main components and pathways
Thus, water, sediment and chemical balance studies help establish and quantify the
urban water cycle, by addressing such issues as verification of pathways in the cycle;
quantifying flows and fluxes of sediment and chemicals along the pathways; assessing
component variations; and, assessing impacts of climatic, population and physiographic
changes on the urban water cycle (UWC). Examples of urban water balances were
offered by Hogland and Niemczynowicz (1980) and van de Ven (1988). A brief
description of principal components of the urban water cycle follows.
Two main sources of water are recognised in the UWC, municipal water supply and
precipitation. Municipal water is often imported from outside the urban area or even
from another catchment in widely varying quantities reflecting local water demands and
their management. Municipal water may bypass some pathways in the UWC; it is
brought into the urban area, distributed within the area, some fraction is lost to urban
groundwater, and the rest is used by the population, converted into municipal
wastewater, and eventually returned to surface waters. The second source, precipitation,
generally follows a longer route through the water cycle. It falls in various forms over
urban areas, is subject to hydrologic abstractions (including interception, depression
storage, and evapotranspiration), partly infiltrates into the ground (contributing to soil
moisture and recharge of groundwater) and is partly converted into surface runoff,
which may be conveyed to receiving waters by natural or constructed conveyance
systems. With various success and accuracy, flow components were quantified for
urban areas in studies of urban water balances (Hogland and Niemczynowicz, 1980).
Besides these clearly established (intentional) linkages among the various water
conveyance and storage elements, others (unintentional) may also develop (e.g., water
main leaks, sewer exfiltration) and have to be addressed in water management.
In addition to flow components of the urban water cycle, attention needs to be paid to
the fluxes of materials and energy conveyed by air, water or anthropogenic activities. In
general, these processes are less well known and quantified than those dealing with
water only, and their description in urban areas is complicated by numerous remote and
local sources and high variability in time and space. With respect to atmospheric
pollutants conveyed in wet form with precipitation and dry form as gases and
particulates, Novotny and Olem (1994) identified the major pollutants as acidity
(originating from nitrogen and sulfur oxides emitted from fossil-fuel combustion), trace
metals, mercury and agricultural chemicals (particularly pesticides and herbicides).
These chemicals may fall directly into receiving waters, or be deposited on catchment
surfaces and subject to scouring and transport into receiving waters during wet weather.
Other pollution sources include land use activities and poor housekeeping, including
transportation, construction activities, use of building materials, road maintenance,
attrition or elution or corrosion of hard surfaces, soil erosion, urban wildlife

5
(particularly birds) and pets, deficient solid waste collection, and others. Besides direct
deposition into urban waters (generally of secondary importance because of small water
surface areas), these materials may be washed off and transported by urban runoff as
dissolved or suspended pollutant loads, or as a bedload. During the transport,
depending on hydraulic conditions, settling and re-suspension takes place on the
catchment surface and in pipes, as well as biological and chemical reactions. These
processes are often considered to be more intense in the initial phase of the storm (first
flush effect); however, due to temporal and spatial variability of rainfall and runoff
flow, first flush effects are more pronounced in conveyance systems with pipes rather
than on overland flow surfaces.
While past studies of urbanisation and water management, particularly in developed
countries, focused on science and engineering, there is growing recognition of the
importance of the social conditions and links between the socio-economic system and
the water and the environment (Lundqvist et al., 2001). Furthermore it is recognised
that sustainable solutions to water related problems must reflect the cultural (emotional,
intellectual, and moral) dimensions of people’s interactions with water. Culture is a
powerful aspect of water resources management. Water is known as a valuable blessing
in most of the arid or semiarid countries and most religions. There are two cultural
aspects that cause direct impacts on water resources management in urban areas: urban
architecture and people’s life style.
Traditional architecture in urban areas often reflects the climate characteristics of the
area. However, the traditional architecture in many large cities is being replaced by
modern “western” architecture because of population increase and globalisation, with
concomitant changes in urban hydrology. The density of the population and buildings,
rainwater collection systems, material used in construction, and wastewater collection
systems are major factors among others that cause changes in the urban hydrologic
cycle.
Life style in urban areas affects the hydrologic cycle through the changes in domestic
water demands. Domestic water use per capita and water use in public areas such as
parks and green areas are the main characteristics that define the lifestyle in large cities.
Even though the economic factors are important for determining these characteristics,
the pattern of water use, tradition and culture have more significant effects on the life
style in urban areas.
1.3 TOTAL MANAGEMENT OF THE URBAN WATER CYCLE
The concept of the urban water cycle demonstrates the connectivity and inter-
dependence of urban water resources and human activities, and the need for integrated
management. Towards this end, the concept of total urban water cycle management
was introduced in Australia and further elaborated on by Lawrence et al. (1999). The
basic water management categories encompassed in this approach include:
x reuse of treated wastewater, as a basis for disposing potential pollutants, or a
substitute for other sources of water supply for sub-potable uses;
x integrated stormwater, groundwater, water supply and wastewater based
management, as the basis for: economic and reliable water supply; environmental
flow management (deferment of infrastructure expansion, return of water to
streams); urban water-scape/landscape provision; substitute sub-potable sources
of water (wastewater and stormwater reuse); and, protection of downstream
waters from pollution; and,
x water conservation (demand management) based approaches, including: more
efficient use of water (water saving devices, irrigation practices); substitute
landscape forms (reduced water demand); and, substitute industrial processes
(reduced demand, water recycling).
While many of these measures have been practiced in the past, what has been missing
was the understanding of the linkages among the various components, and the
implication of the practices for long-term quality of groundwater, soils, and
environmental flows.

6
Finally, it should be emphasised that the concept of the urban water cycle and of its
total management applies to all climatic, physiographic, environmental, and socio-
cultural conditions, and the levels of development, with appropriate modifications.
Naturally, depending on local circumstances, different measures may attain different
priorities, but the general principle of identifying the main sources of water, sediments,
chemicals and biota, the applicable pathways or changes, and intervention measures,
serving the integrated management of natural resources, remain the same and will be
explored in the following chapters.
Referring to Fig. 1.2, the discussion of UWC is organized accordingly: after a general
introduction of the UWC concept in Chapter 1, hydrological components of UWC are
addressed in Chapter 2, urban infrastructure and water services in Chapter 3, and
urbanisation effects on the environment in Chapter 4.

7
Chapter 2
Urban Water Cycle Hydrologic Components
Urbanisation contributes to changes in the radiation flux and the amount of
precipitation, evaporation and evapotranspiration, infiltration into soils, and
consequently causes changes in the hydrological cycle. The effects of large urban areas
on local microclimate have long been recognised and occur as a result of changes in the
energy regime, air pollution, and air circulation patterns caused by building and/or
transformation of land cover. The changes in the rainfall-runoff components of the
hydrologic cycle can be summarised as follows:
x transformation of undeveloped land into urban land (including transportation
corridors),
x increased energy release (i.e., greenhouse gases, waste heat, heated surface
runoff), and
x increased demand on water supply (municipal and industrial).
Fig. 2.1 shows rainfall-runoff components of the hydrologic cycle. Each of these
components and the related processes are briefly explained in the following section;
detailed descriptions can be found elsewhere (Viessman et al., 1989).
Fig.2.1 Rainfall–runoff components of hydrologic cycle

8
Further discussion of urbanisation impacts on water resources is presented in
Chapters 3 and 4
2.1 WATER SOURCES
2.1.1 Municipal water supply
Two main sources of water in urban areas are recognised, municipal water supply and
precipitation. Municipal water is often imported from outside of the urban area (or even
outside the watershed in which the urban area is located), in quantities ranging from 50
to 700 L/capita/day, as demanded by municipal water users. Municipal water use is
usually categorised as residential, commercial, industrial and “other” water demands,
where “other” includes water lost through leakage, unaccounted for water uses (e.g., fire
fighting and distribution system flushing), and water not assigned to the above three
categories. Thus, the quantity of imported water depends largely on the population
served by municipal water supply systems, and on institutional, commercial and
industrial activities. Import of water supply (particularly from other catchments)
represents a major influence on the urban water cycle; with typical urban water uses
being non-consumptive, most of the imported water is discharged into groundwater
(contributing to rising groundwater tables) or local receiving bodies as wastewater
effluents with major impacts on such water resources. Under some circumstances, some
of the sub-potable water demands can be supplied by reused or recycled water, thus
conserving potable water sources (see Section 3.4.8). Furthermore, the water provided
through municipal water supply systems may be locally consumed, or exported as
virtual water in various products, or used in ground irrigation, or converted into
wastewater, which may or may not be treated prior to discharge into receiving waters.
A more detailed discussion of water supply is presented in Chapter 3.
2.1.2 Precipitation
The second important source of water is precipitation, which occurs in greatly varying
quantities depending on local climate. The effects of large urban areas on local
microclimate have long been recognised (Geiger et al., 1987); these occur as a result of
changes in the energy regime, air pollution, and air circulation patterns, which are
caused by buildings, land transformations, and by release of greenhouse gases. These
factors contribute to changes in the radiation balance and the amounts of precipitation
and evaporation, and consequently to changes in the hydrologic cycle. Such effects,
with respect to changes in annual precipitation, air temperatures, and evaporation rates
are also described in Section 4.3.
2.1.2.1 Climatic aspects
Climate is defined as the long-term behaviour of the weather in a region. The
hydrologic processes in different climates are affected by the hydrometeorological
variables, which attain different ranges of magnitude in different climates. There are
four world climate categories corresponding to subtropical, continental, rain-shadow,
and cool coastal arid lands.
Subtropical areas (e.g., Sahara, Arabia, Australia, and Kalahari) are characterised by
clear skies with high temperatures. The summers are hot and the winters are mild, so the
seasonal contrasts are evident with low winter temperatures due to freezing. Convective
rainfalls develop only when moist air invades the region.
Continental interior areas (e.g., arid areas of Asia and western USA) have seasonal
temperatures ranging from very cold winters to very hot summers. Snow can occur.
Rainfall in the summer is unreliable in this climate.
Rain-shadow areas (mountain ranges such as the Sierra Nevada, the Great Dividing
Range in Australia and the Andes in South America) are characterised by conditions
similar to those in continental areas with diverse behaviour, but their climatic conditions
are not as extreme as in the continental interior areas.

9
Cool coastal areas (e.g., the Namib Desert on the south-western coast of Africa and
the Pacific coast of Mexico) have reasonably constant climatic conditions with a cool
humid environment. When temperate inversions are weakened by upward moving moist
air, thunderstorms can develop.
Arid climate is one of the more important climate types of the world. Rozanov (1994)
suggested that the major cause of aridity was explained through the global atmospheric
circulation patterns, with certain local effects imposed by topography; Thompson
(1975) listed four main processes explaining aridity: (a) high pressure, (b) wind
direction, (c) topography, and (d) cold ocean currents.
The majority of semi-arid and arid regions are located between latitudes 25 and 35
degrees, where high pressure causes warm air to descend, resulting in dry stable air
masses. Aridity caused by orographic causes is common in North and South America,
where high mountain ranges are perpendicular to the prevailing air mass movements.
These air masses are cooled as they are forced up the mountains, reducing their water
holding capacity. Most of the moisture is precipitated at high elevations of the
windward slopes. The relatively dry air masses warm up as they descend on the leeward
side of the mountain ranges, increasing their water-holding capacity and reducing the
chance of any precipitation. This orographic aridity is referred to as the rain shadow
effect (Dick-Peddie, 1991). The positioning over a continent where distance from
oceans lessens the chance of encountering moisture-laden air masses is the cause of the
semiarid and arid conditions in central Asia. Cold ocean currents cause the coastal arid
regions of Chile and Peru and the interior part of northern Argentina, where cold ocean
currents in close proximity to the coast supply dry air that comes on shore, but as the
mass is forced up the mountain sides there is no moisture to be lost as the air mass
cools. The desert climate is another important arid climate of the world.
2.1.2.2 Urban precipitation
Precipitation represents one of two primary water inputs to the urban water cycle and is
derived from atmospheric water. Recognising that large amounts of water may
accumulate in clouds without precipitation, the processes of condensation and
precipitation are sometimes considered individually. Among the various causes of
condensation, dynamic or adiabatic cooling is the most important cause, which produces
nearly all precipitation. The condensation of water vapour into droplets occurs on
condensation nuclei, whose occurrence is related to air pollution. Large urban areas
affect the local microclimate as a result of changes in the energy regime, air pollution
(providing condensation nuclei), air circulation patterns, and releases of greenhouse
gases (Marsalek et al., 2001). Earlier studies have shown that total annual precipitation
in, or downwind of, large industrialised cities is generally 5-10% higher than in the
surrounding areas, and for individual storms, this increase in precipitation can be as
high as 30%, particularly on the downwind side of large metropolitan areas (Geiger et
al., 1987). Further changes are expected as a result of climate change, with global
circulation models predicting either increasing or decreasing precipitation, depending on
the specific location, and greater climatic variability with more pronounced extremes
(Van Blarcum et al., 1995).
Among the various forms of precipitation, convective storms with high rainfall
intensities are particularly important in the design of urban minor drainage elements or
infrastructure (the sizing of conveyance elements), cyclonic precipitation may be more
important in design of major drainage and storage facilities.
While climates were traditionally considered as either non-varying or changing very
slowly, recent research on greenhouse effects indicates some imminent climate changes.
Solar radiation reaching the earth is partly absorbed by the earth, but a substantial part is
reflected back into space. The heat absorbed is radiated by the earth as infrared
radiation. The greenhouse gases such as water vapour, carbon dioxide, methane, nitrous
oxides, etc. absorb the infrared radiation and in turn re-radiate it in the form of heat.
The amount of greenhouse gases in the atmosphere is increasing due to anthropogenic
activities (Houghton et al., 1996). Thus, the emission of greenhouse gases contributes
to climate change. It is forecasted that increased greenhouse gas concentration will lead
to increasing average temperature, by 3-5qC in some regions by the year 2050. As

10
concentrations of greenhouse gases increase even more, further climate changes can be
expected. In urban areas, which are affected by global climate changes, some
researchers suggest that the magnitude of the effects of changing climate on water
supplies may be much less important than changes in population, technologies,
economics or environmental regulations (Lins and Stakhiv, 1998).
Air temperature is also of interest in studies of precipitation, because it determines the
form of precipitation (e.g., rain or snow). The urban heat island effect increases air
temperatures over urban areas by as much as 4-6qC, compared to surrounding localities
(see Section 4.3.1). These thermal phenomena then explain higher evaporation rates (by
5-20%) in urban areas (Geiger et al., 1987) and other related effects.
2.2 HYDROLOGIC ABSTRACTIONS
The most important component of the UWC with respect to drainage and flood
protection is stormwater runoff. To determine runoff, one needs to consider water input
(i.e. rainfall or snowmelt), hydrologic abstractions (sometimes called losses), and the
routing of net water input in the catchment. Such routing is strongly affected by
storage, which modifies the inflow hydrograph. A brief overview of such processes
follows.
A significant fraction of precipitation is returned to the atmosphere by evaporation or
evapotranspiration, depending on local landscape and water resources. The remaining
water may infiltrate into the ground (recharging groundwater), or be converted into
runoff and streamflow. Generally, rainwater infiltration in urban areas is reduced by
high imperviousness of urban areas and this contributes to increased runoff and higher
risk of flooding and erosion in receiving streams (Marsalek, 2003a). Reduced
hydrologic abstractions and increased surface runoff are recognised as typical impacts
of urbanisation on the hydrologic cycle (Leopold, 1968). Furthermore, urban runoff
becomes polluted during overland flow and transport in storm or combined sewers, and
consequently exerts water quality impacts on the receiving waters (Marsalek, 2003a).
Therefore, during the last 30 years, stormwater management has been introduced, with
the main goal of reducing anthropogenic impacts on the hydrologic cycle and
mobilisation and transport of sediments and pollutants. Typical stormwater
management measures are discussed later in Chapter 3.
2.2.1 Interception
Interception is defined as that part of water input that wets and adheres to above ground
objects until it evaporates and returns to the atmosphere (Viessman et al., 1989). Water
abstractions by interception are particularly important in vegetated (forested)
catchments, where the amount intercepted depends on species, age and density of the
vegetation, storm event characteristics, and the season of the year (Geiger et al., 1987).
Interception abstractions occur early during rain storms and quickly diminish. In urban
areas with low tree cover, interception is insignificant and often neglected. Although
there are formulae for calculating interception as a function of rainfall and vegetation
characteristics (Chow, 1964), the estimated interception if often included in the initial
abstraction and deducted from the storm rainfall (Geiger et al., 1987). Traditional urban
development with high imperviousness and low vegetation or tree cover reduces
interception and its importance in urban runoff analysis.
2.2.2 Depression storage
Depression storage (also called surface storage) accounts for water that is trapped in
small depressions on the catchment surface and retained until it infiltrates or evaporates.
In some hydrology handbooks, wetting abstractions (i.e., water used for the initial
wetting of catchment surface) are combined with depression storage and called the
initial abstraction (Geiger et al., 1987). Depression storage depends on catchment
surface characteristics, including the type of surface and its slope. On impervious urban
surfaces, depression storage ranges from 0.2 mm (smooth asphalt pavement) to 2.8 mm

11
(an average value for small urban areas); on pervious surfaces, depression storage
ranges from 0.5 mm (bare clay) to 15 mm (wooded areas and open fields). The relative
significance of depression storage depends on the storm rainfall (or snowmelt); the
larger the rainfall, the less significant is depression storage in stormwater runoff
calculations. A detailed listing of depression storage values for more than 25 types of
surfaces can be found in Geiger et al. (1987).
2.2.3 Evaporation and evapotranspiration
Evaporation is the process occurring along the water-air or soil-air interface by which
water in liquid or solid state transforms into water vapour escaping into the atmosphere.
Higher rates of energy consumption and higher air temperatures in cities contribute to
higher rates of evaporation in urban areas (by 5-20%) (Geiger et al., 1987).
Transpiration is the process of vaporisation of water at the surface of plant leaves after
the soil water has been transported through the plant (Overton and Meadows, 1976).
For simplification, the process of transpiration is sometimes combined with evaporation
from water and soil surfaces into evapotranspiration, which can be estimated by the
Penman equation (Viessman et al., 1989). Furthermore, it is commonly assumed that
water supply to these processes is not limited which permits the treatment of
evapotranspiration at its potential rate (Geiger at el., 1987). Land use changes in urban
areas lead to a reduced extent of green areas in cities and thereby contribute to reduced
total transpiration from trees and vegetation. While evaporation and evapotranspiration
are important in water budget calculations (note that in the Sahelian zone, the daily
evaporation can be as high as 15 mm/day), during urban stormwater runoff, both
abstractions are rather small and justifiably neglected.
2.2.4 Infiltration
Infiltration is the process of water movement into the soil under gravity and capillary
forces. Through this process, shallow aquifers are recharged and, by discharging to
surface waters, contribute to streamflow during dry periods. Two basic approaches to
describing infiltration include a soil physics approach relating infiltration rates to
detailed soil properties (e.g., hydraulic conductivity, capillary tension and moisture
content) and a hydrological approach, which is parametric and utilises lumped soil
characteristics to estimate infiltration rates. The latter approach is commonly used in
urban runoff calculations. For more information on infiltration calculations methods
used commonly in urban runoff modelling (Horton, Green-Ampt, Philip, and Holtan
approaches) see Viessman et al. (1989).
Compared to natural areas, infiltration rates decrease in urban areas because of the
following factors:
x increased imperviousness of urban catchments (pavements, rooftops, parking
lots, etc.),
x compaction of soils in urban areas, and
x presence of a man-made drainage system providing for quick removal of ponded
water, without allowing water enough time to infiltrate into the ground.
In the first in-depth analysis of the urban hydrological cycle provided by Leopold
(1968), it was noted that increased imperviousness of urban catchments contributed to
lower infiltration and thereby to reduced groundwater recharge, reduced
interflow/baseflow, and higher rates of surface runoff. Higher rates of runoff then
contribute to higher incidence of flooding. However, recent studies indicate that
urbanisation may result in a net gain in overall groundwater recharge, mostly because of
losses from water supply mains, leaking sewer systems, and stormwater infiltration
(Lerner, 2004).

12
2.2.5 Lumped hydrologic abstractions
In some empirical procedures for runoff calculation, hydrological abstractions are
lumped into empirical coefficients. Examples of such approaches include the runoff
coefficient, which applies to the runoff peak flow, the )-index applied to pervious areas
and accounting for interception, evaporation, wetting, depression and infiltration
abstractions (Chow, 1964), and the runoff curve number method of the Soil
Conservation Service (SCS) (U.S. Department of Agriculture, SCS, 1975).
2.3 WATER STORAGE
Water infiltrating into the ground contributes to soil moisture and to groundwater
recharge.
2.3.1 Soil moisture
Large parts of urban areas are covered by impervious surfaces, reaching the
imperviousness of 100% in downtown areas, and resulting in reduced infiltration and
evapotranspiration, because of reduced vegetated areas. Consequently, the vadose zone
in urban areas differs significantly from that in natural areas. Another factor
contributing to changes in soil moisture in urban areas is a partial removal of topsoil
during the urban development and construction, and changes in soil structure resulting
from the use of heavy machinery. After development, less topsoil may be returned and
lower soil layers may have been compacted, with reduced soil moisture storage and
greater need for irrigation.
2.3.2 Urban groundwater
Urbanisation affects not only surface waters, but also groundwater, with respect to both
its quantity and quality. The state of groundwater then impacts on the water balance of
an urban area and on the operation of the urban infrastructure, including storm, sanitary
and combined sewers, stormwater management facilities, and sewage treatment plants.
A detailed analysis of urban groundwater and its pollution can be found in Lerner
(2004); a brief introduction of the groundwater issues is included here for the sake of
completeness.
Groundwater can be characterised according to its vertical distribution into two
zones, the zone of aeration and the zone of saturation (below the water table). The zone
of aeration is divided, from top to bottom, into the soil-water, vadose and capillary
zones (Todd, 1980). In urban areas, groundwater interacts with surface waters and
urban infrastructure, and is further affected by land use activities. An Internet-based
Urban Groundwater Database (www.utsc.utoronto.ca/~gwater/IAHCGUA/UGD/
)
indicates that urban groundwater issues and problems vary depending on the climate,
urban area, land use activities, environmental practices, and other local conditions. In
drier climates and developing countries, the typical trend is toward overexploitation of
groundwater for municipal and industrial water supply, with the resulting lowering of
groundwater table, land subsidence, salt water intrusion in coastal areas, and
groundwater pollution. In developed countries, urban aquifers are generally not used
for water supply; the water tables may still decline due to insufficient recharge caused
by high imperviousness of urban areas. Less commonly, urban water tables may be
rising due to low withdrawal of groundwater and leakage from water mains (e.g.,
Nottingham, UK). In all urban areas, the occurrence of pollution from various sources
has been reported (Lerner, 2004).
2.4 STORMWATER RUNOFF
Changes of runoff regime represent one of the most significant impacts of urbanisation.
Urbanisation affects surface runoff in three ways: (a) by increasing runoff volumes due
to reduced rainwater infiltration and evapotranspiration, (b) by increasing the speed of

13
runoff, due to hydraulic improvements of conveyance channels, and (c) by reducing the
catchment response time and thereby increasing the maximum rainfall intensity causing
the peak discharge. Thus urbanisation changes the catchment hydrologic regimen.
These changes were quantified in the literature, with the mean annual flood increasing
from 1.8 to 8 times, and the 100-year flood increasing from 1.8 to 3.8 times, due to
urbanisation (Riordan et al., 1978). Stormwater direct runoff volume increased for
various return periods up to 6 times. In general, the magnitude of such increases
depends on the frequency of storms, local climate and catchment physiographic
conditions (soils, degree of imperviousness, etc.), as partly illustrated in Fig. 2.2. Fig.
2.3 then shows two runoff hydrographs from the same catchment – before and after
urban development. The figure demonstrates changes in the runoff hydrograph caused
by urbanisation. As discussed later in Section 3.3.2.2, the post-development runoff
peak can be controlled by storage. Note that storage reduces the peak, but not the
volume of runoff, which contributes to increased runoff flows over extended time
periods, with concomitant effects on channel erosion in downstream areas (see Section
4.4.1.2).
Fig. 2.2 Effects of watershed development on flood peaks (Marsalek, 1980)
Fig. 2.3 Runoff hydrograph before and after urbanisation

14
2.5 INTERFLOW AND GROUNDWATER FLOW
Reduced infiltration due to high imperviousness of urban catchments should contribute
to smaller interflows. However, the situation is more complicated in urban areas
because of another source of water to shallow aquifers - leakage from water distribution
and wastewater collection networks. Leakage from water mains is particularly
important, because such pipes are pressurised. Even low water losses, expressed as
15% of volume input, provide water volumes equivalent to groundwater recharge by
several hundred millimetres of rainfall (Lerner, 2004). Exfiltration from leaky sewer
pipes depends on the relative positions of sewers and the groundwater table, which
determines the direction of water transport and may vary in time. Leaky sewers in dry
soils will function as a source of groundwater, but leaky sewers below the groundwater
table will drain aquifers and convey this flow to sewage treatment plants. Finally, other
sources of inputs to groundwater are stormwater infiltration facilities, which are applied
in modern stormwater management. Such facilities include porous or permeable
pavements, permeable manholes, drainage swales, and infiltration wells, trenches and
basins.
There may be also an influx of groundwater into the urban area, or into municipal
sewers (Hogland and Niemczynowicz, 1980), and such waters may contribute to
increased volumes of municipal sewage treated at the local plant and the effluent
discharged into receiving waters.
2.6 NATURAL DRAINAGE: URBAN STREAMS, RIVERS AND LAKES
The final components of the urban water cycle are sinks, in the form of receiving waters
representing elements of the natural drainage system. Two types of receiving waters are
commonly recognised, receiving surface waters and groundwater. In both cases, there
are conflicts arising from multiple water uses. Receiving waters generally provide
beneficial water uses, including source water for water supply, fishing, recreation and
ecological functions (e.g., aquatic habitat), but they also serve to transport/store/purify
urban effluents conveying pollution. Similar conflicts were reported for groundwater,
which may serve for water supply as well as (often unintentionally) for disposal of some
pollutants. Thus, to protect downstream water uses, it is necessary to manage urban
effluents with respect to their quantity and quality, in order to lessen their impact on
water resources.
Wastewater from various municipal sources, including residential, commercial,
industrial and institutional areas is collected by sewers or open drains and conveyed to
treatment facilities, or discharged directly into receiving waters. Depending on local
climate and population density, on an annual basis, the municipal effluent volume may
exceed the volume of stormwater runoff from the urban area.
In urban drainage design, urban streams are considered as elements of the major
drainage system, and are often modified to accommodate increased flows resulting from
urbanisation. The situation concerning urban lakes is similar, with urbanisation
changing their hydrological regime. In most cases, however, the main water
management challenge is dealing with the water quality impairment (particularly
siltation of streams and eutrophication of lakes), as discussed further in Chapter 4.
2.7 NEEDS FOR URBAN WATER INFRASTRUCTURE
Extensive changes of the hydrological regime in urban areas have been historically
managed by building an urban infrastructure, starting with water supply aqueducts,
followed by stormwater and sewage collection, and eventually sewage treatment plants.
Such systems providing water services, including water supply, drainage and sewage
management, then in turn interactively affect the hydrological cycle in urban areas. For
example, import of drinking water into urban areas changes the urban water budget,
particularly through leakage from water mains into urban aquifers. Increased
stormwater runoff has to be managed in urban areas, sometimes by source controls, but
more often by enhancing the conveyance capacity of natural channels and by building

15
new ones and underground sewers. These measures were implemented to manage local
flooding in urban areas, but they also contributed to faster hydrological response of
urban catchments and further increases in peak stormwater flows. Finally, most of
water imported into and used in urban areas is transformed into wastewater, which is
discharged into receiving waters and further increases demands on their capacity, with
respect to conveyance, storage, and self-purification. In developed countries with long
tradition in providing urban water services, infrastructure systems have been built over
centuries and do provide good services to urban dwellers, as long as their operation and
maintenance is adequately funded. As an alternative to these “central” systems, new
distributed systems are currently promoted and may represent an attractive alternative in
developing countries without the central systems, or funds to build and maintain them.
The issues of urban infrastructure and provision of water services are addressed in
Chapter 3.

17
Chapter 3
Urban Water Infrastructure
3.1 DEMANDS ON WATER SERVICES IN URBAN AREAS
Urban areas are highly dynamic and complex entities. They require various resources
including water, food, energy and raw materials, and produce wastes, which need to be
safely disposed of. With respect to urban water management, such a continuous
movement, use, and disposal of energy and materials can be visualized schematically as
flows of water, food and wastes into and out of urban areas, or as ecocycles of water
and nutrients supporting urban areas (Fig. 3.1).
Fig. 3.1 Ecocycles of water and nutrients supporting urban areas
Sustainable operation of urban areas includes the provision of sustainable water
services. Historically, the main components of urban water systems and the provision of
related water services, including water supply, drainage, sewage collection and
treatment, and receiving water uses, were addressed separately. Their interactions were
often disregarded or underestimated; such an approach is obviously untenable.
Consequently, an integrated approach to water management, sometimes referred to as
an ecosystem approach, has evolved. The interdependency and interactions among the

18
principal system components are fully recognised and used in the development of
solutions to water problems. The uses of the receiving waters, including natural
functions, in-stream uses and withdrawals (e.g., for water supply) are often the driving
force dictating the level of control of urban drainage and wastewater effluents.
Recently, the depletion and degradation of urban water resources has led to the
advocacy of a sustainable urban water system, characterised by lower water
consumption, preservation of natural drainage, reduced generation of wastewater
through water reuse and recycling, advanced water pollution control, and preservation
and/or enhancement of the receiving water ecosystem. Specifically, sustainable urban
water systems should fulfil the following basic goals:
x supply of safe and good-tasting drinking water to the inhabitants at all times,
x collection and treatment of wastewater in order to protect the inhabitants from
diseases and the environment from harmful impacts,
x control, collection, transport and quality enhancement of stormwater in order to
protect the environment and urban areas from flooding and pollution, and
x reclamation, reuse and recycling of water and nutrients for use in agriculture or
households in case of water scarcity.
Most of the goals of sustainability have been reached or are within reach in North
America and Europe, but are far from being achieved in developing parts of the world.
The Millennium Development goals put strong emphasis on poverty reduction and
reduced child mortality. The two specific goals related to water are:
x to halve, by the year 2015, the proportion of people who are unable to access or
afford safe drinking water, and
x to stop the unsustainable exploitation of water resources by developing water
management strategies at local, regional, and national levels, which promote both
equitable access and adequate supplies.
In addition to these goals, the proportion of people lacking access to adequate
sanitation should be halved by 2015. Sanitation systems must be designed to safeguard
human health as well as the health of the environment.
The urban populations of the world, especially in Africa, Asia, and Latin America,
are expected to increase dramatically. The African urban population is expected to more
than double over the next 25 years, while that of Asia will almost double. The urban
population of Latin America and the Caribbean is expected to increase by almost 50%
over the same period (WHO & UNICEF, 2000). Consequently, urban services will face
great challenges over the coming decades to meet the fast-growing needs. Water
management considerations in provision of urban water services, including the basic
requirements on urban water infrastructure, are presented in this chapter. Three urban
infrastructure sub-systems are considered in this discussion: water supply, drainage, and
wastewater management and sanitation.
Finally, it is the provision of urban water services and construction of related
infrastructure which changes components of the hydrological cycle in urban areas and
leads to its replacement by the urban water cycle. Specifically, water supply generally
involves import of large quantities of water into urban areas, sometimes from remote
catchments. Some of this water finds its way into urban aquifers, via losses from the
water distribution networks. Most of the remaining imported water is used within the
urban area and turned into wastewater. Increased catchment imperviousness and
hydraulically efficient urban drainage contribute to higher volumes and flow rates of
runoff, and reduced recharge of groundwater. The collection of sewage also captures
some groundwater through sewer infiltration and thereby reduces groundwater tables in
urban areas. For most cities in developed countries, collected wastewater is treated at
sewage treatment plants, which discharge their effluents into receiving waters, thus
contributing to higher export of water from the urban area and potentially causing
pollution of receiving waters. Thus, elements of the urban water infrastructure and their
interactions with the hydrological cycle are of particular interest when dealing with
urban water cycle.

19
3.2 WATER SUPPLY
In the “Global Water Supply and Sanitation Assessment 2000 Report” (WHO and
UNICEF, 2000), the state of urban water services worldwide has been assessed. A main
finding was that the percentage of people served with some form of improved water
supply rose from 79% in 1990 to 82% in 2000. At the beginning of 2000, about one-
sixth of the world’s population (1.1 billion people) was without access to improved
water supply services. The majority of these people lived in Asia and Africa. Even if the
situation in rural areas is generally worse than in urban areas, the fast-growing cities of
the world create special challenges for improving the urban water services. The urban
drinking water supply coverage ranges from 85% in Africa to 100% in Europe and
North America. These numbers should not be regarded as reliable since the definition of
urban areas with their large fringe areas is imprecise. The term “improved water supply”
is also fairly generous, comprising not only household connections but also public
standpipes, boreholes, protected dug wells, and protected springs and rainwater
collection.
In most cities in North America and Europe and in a large number of cities in other
parts of the world, however, the citizens benefit from a water supply service that fulfils
the major requirements on water quantity and quality. In these cities, water is normally
treated in a water treatment plant and distributed by a pipe system to households. The
challenge is to provide similar services to all urban inhabitants in the world, using
similar technologies or appropriate alternatives.
The report “Water and sanitation in the world’s cities – local action for global goals”
(UN-HABITAT, 2003) describes the situation worldwide and gives many illustrative
examples from the cities in different parts of the world. A sample of water supply data
for the 10 most populous cities in the world (in 2000) is presented in Table 3.1. Points
of interest include (a) a large variation in water use per capita (130-570 L/capita/day),
(b) large withdrawal rates (25-88 m
3
/s), (c) significant losses (15-56%), and (d) seven
out of the 10 cities listed were in developing countries.
Table 3.1 Water supply in the world’s 10 most populous cities (2000 data) (UN-HABITAT, 2003)
City and Country Inhabitants in 2000
(million)
Water supply
(m
3
/s)
Water supply
(L/captia/day)
Tokyo, Japan 27.9 81 250
Mexico City, Mexico 19.7 69 331
Sao Paulo, Brazil 17.8 63 306
Shanghai, China 17.2 81 407
New York, USA 16.4 57 300
Mumbai, India 16.4 25 130
Beijing, China 14.2 39 239
Lagos, Nigeria 13.4 (missing data) (missing data)
Los Angeles, USA 13.1 88 570
Calcutta, India 12.7 25 171
Water losses were reported just for 5 cities, ranging from 15 to 56%.
Explanations of basic terminology used in water supply, provided by the U.S.
Geological Survey, can be found in Table 3.2 (Mays, 1996).

20
Table 3.2 Definitions of water use terms (Mays, 1996)
Term Definition
Consumptive use
The part of withdrawn water that is evaporated, transpired, incorporated into
products or crops, consumed by humans or livestock, or otherwise removed from
the immediate water environment.
Conveyance loss
The quantity of water that is lost in transit from a pipe, canal, conduit, or ditch by
leakage or evaporation.
In-stream use
Water that is used, but not withdrawn from a ground or surface-water source for
such purposes as hydroelectric-power generation, navigation, water quality
improvement, fish propagation, and recreation.
Off-stream use
Water withdrawn or derived from a ground or surface-water source for public water
supply, industry, irrigation, livestock, thermoelectric-power generation, and other
uses.
Return flow
The water that reaches a ground or surface water source, after release from the point
of use, and thus becomes available for further use.
Withdrawal
Water removed from the ground or delivered from a surface–water source for off-
stream use.
3.2.1 Historical development
Some milestones in the development of modern, centralised drinking water systems are
worthwhile to review (WaterWorld and Water & Wastewater International, 2000).
Already some 3000 years BC the drinking water was distributed in lead and bronze
pipes in Greece. In 800 BC the Romans built aqueduct systems that provided water for
drinking, street washing and public baths and latrines. In the beginning of the 19
th
century, the first public water supply systems were constructed in North America and in
Europe. The early cities were Philadelphia, USA, and Paisley, Scotland. In the middle
of the 19
th
century, filter systems were introduced in some cities where the water quality
started to be a problem. The spread of cholera necessitated the use of disinfection; and
the first chlorination plants were installed around 1900 in Belgium and New Jersey,
USA. During the 20
th
century, all large cities in North America and Europe introduced
successively more and more advanced treatment of centrally supplied drinking water,
including physical, biological and chemical treatment. This was primarily done for
surface waters; groundwater supplies in many countries even at present either require
minimal treatment, or no treatment at all (e.g., in Slovenia and Denmark). Towards the
end of the 20
th
century, microfiltration of raw drinking water was introduced. It is an
innovative treatment process being employed on an increasing scale. Water treatment
and distribution is effectively governed by various laws and regulations, such as the
U.S. Safe Drinking Water Act in USA (introduced in 1974, amended in 1986 and 1996)
and the 1998 European Union Drinking Water Directive 98/83/EC (CEC, 1998). A
broader, internationally developed guidance on drinking water quality can be obtained
from the World Health Organization (WHO, 2004).
3.2.2 Water demand
The provision of adequate water supply and sanitation to the rapidly growing urban
population is a problem for government authorities throughout the world. In many
developed or developing parts of the world, locating new sources or expanding existing
sources is becoming more difficult and costly, and is often physically and economically
infeasible. The actual cost of water per cubic metre in second and third generation water
supply projects in some cities has doubled, compared to the first and second generation
projects (Bhatia and Falkenmark, 1993).
The ability to manage existing water resources and plan for developing new water
resources is tied directly to the ability to assess both the current and future water use.
The main objective of water demand management is to improve the efficiency and

21
equity in water use and sanitation services. For this purpose, different instruments have
been developed and these can be generally classified into the following categories:
x water conservation measures,
x economic measures,
x information and educational measures, and
x legal measures.
The efficiency of each of these instruments depends highly on local conditions. In
different sections of this chapter, various aspects of managing water demands for
domestic, industrial and agricultural purposes are discussed.
Water demand is the scheduling of quantities that consumers use per unit of time (for
particular prices of water). Water use can be classified into two basic categories:
consumptive and non-consumptive uses; the former remove water from the immediate
water source, in the latter, water is either not diverted from the water sources, or it is
diverted and returned immediately to the source at the point of diversion in the same
quantity as diverted and meets water quality standards for the source.
Municipal water uses include residential/domestic use (apartments and houses),
commercial (stores and small businesses), institutional (hospitals and schools),
industrial, and other water uses (fire-fighting, swimming pools, park watering). These
uses require withdrawal of water from surface or groundwater sources, and some parts
of the withdrawn water quantity may be returned to the source, often in a different
location and time, and with different quality. Further explanation of individual water
uses follow.
Domestic water use includes water used for washing and cooking, toilet flushing,
bath and shower, laundry, house cleaning, yard irrigation, private swimming pools, car
washing and other personal uses (e.g., hobbies). Public services water use includes
water used in public swimming pools, institutional uses by government agencies and
private firm offices, educational institutions (such as schools, universities and their
dormitories), fire fighting, irrigation of parks and golf courses, health services
(hospitals), public hygienic facilities (public baths and toilets), cultural establishments
(e.g., libraries and museums), street cleaning and sewer flushing, entertainment and
sport complexes, food and beverage services (restaurants), accommodation services
(hotels), and barber shops and beauty parlours. Small industries include laundries,
workshops, and similar establishments. Transportation water use includes water used
for operation of taxis, buses, and other transportation means (stations and garages),
ports and airports, and railways (stations and workshops).
Good estimation of municipal water demands can be obtained by disaggregating the
total delivery of water to urban areas into a number of classes of water use and
determining separate average rates of water use for each class. This method is called
disaggregate estimation of water use. The disaggregate water uses within some
homogenous sectors are less variable than the aggregate water use. Therefore, a better
accuracy of estimation of water use can be obtained.
In order to estimate the total municipal water use in a city, the study area should first
be divided into homogenous sub-areas, on the basis of water pressure districts or land-
use units, and then the water use rates can be assumed to be constant for different users
within each sub-area. Temporal (annual, seasonal, monthly, etc.) variation should also
be considered in disaggregating the water uses.
The most commonly used method for water use forecasting is regression analysis.
The independent variables of the regression model should be selected on the basis of the
available data for different factors affecting the water use and their relative importance
in increasing or decreasing water uses. For example, the most important factor in
estimating water use in an urban area is the population of each sub-area. A multiple
regression method can also be used to incorporate more variables correlated with water
use in municipal areas for estimating and forecasting the water demand in the future.
Population, price, income, air temperatures, and precipitation are some of the variables
that have been used by different investigators (Baumann et al., 1998).
Time series analysis has also been used to forecast future water demands. For this
purpose, time series of municipal water use and related variables are used to model the

22
historical pattern of variations in water demand. Long memory components, seasonal
and non-seasonal variations, jumps, and outlier data should be carefully identified and
used in modelling water demand time series.
In recent years, more attention has been given to conserving water rather than
developing new water sources. In many countries, this has been accepted for both
economic and environmental reasons as the best solution for meeting the future water
demands. Therefore, water demand management is a proper strategy to improve
efficiency and sustainable use of water resources taking into account economic, social,
and environmental considerations (Wegelin-Schuringa, 1999; Butler and Memon,
2005). In water demand management, increasing attention is paid to water losses and
unaccounted for water, which generally include: (i) leakage from pipes, valves, meters,
etc.; (ii) leakage/losses from reservoirs (including evaporation and overflows); and, (iii)
water used in the treatment process (back-wash, cooling, pumping, etc.) or for flushing
pipes and reservoirs. Typically, the losses may vary from 10-60% (See Table 3.1), with
the higher values reported for developing countries. Other measures focus on water
saving technologies, such as dual-flush toilets, flow restrictors on showers and
automatic flush controllers for public urinals, automatic timers on fixed garden
sprinklers, moisture sensors in public gardens, and improved leakage control in
domestic and municipal distribution systems. All these measures are practical, but
regulations and incentives are needed for their implementation.
Costs of water supply services and technological developments designed to lower
these costs have a major influence on the level of water demand in the developing
countries. In rural areas, the distance from households to the standpipes, or the number
of persons served by a single tap or well, are the major factors controlling the demand.
3.2.2.1 Water supply standards: quantity
Adequate quantities of water for meeting basic human needs are a prerequisite for
human existence, health, and development. If development is to be sustained, an
adequate quantity of water must be available. In fact, as development increases, in most
instances, the demand for water on a per capita basis will also increase for personal,
commercial, industrial and agricultural purposes.
Domestic consumption of water per capita is the amount of water consumed per
person for the purposes of ingestion, hygiene, cooking, washing of utensils and other
household purposes including garden uses. Per capita water consumption can be
measured (or estimated) through metered supply, local surveys, sample surveys or total
amount supplied to a community divided by the number of inhabitants. As sustained
development can be achieved without or with a limited increase in per capita water
consumption, the per capita water use is usually limited by locally specific regulations
or standards. The actual domestic water use rates broadly vary, from a minimum of 50
L/capita/day (Gleick, 1998) to 500 L/capita/day (or more), depending on water
availability, pricing, traditional water use and other factors. Urban areas with high
water use can create large reserves by introducing and practicing demand side
management (Baumann et al., 1998).
The supply source plus storage facilities in urban areas are planned to yield enough
water to meet both the current daily demands and the forecasted consumptions in the
near future. Water supply systems in urban areas should satisfy some quantitative
guidelines and standards. As a general rule, when using surface supplies, the tributary
watershed should yield the estimated maximum daily demand for ten years into the
future, and the storage capacity of a supply reservoir should be equal to at least 30 days
maximum daily demand five years into the future. Ideally, for well supplies there should
be no mining of water; that is, neither the static groundwater level nor the specific
capacity of the wells (litres per minute per metre of drawdown) should decrease
appreciably as demand increases. These values should be constant over a period of five
years except for minor variations that correct themselves within one week.

23
3.2.2.2 Water supply standards: quality
The quality of water is assessed in terms of its physical, chemical, and biological
characteristics and its intended uses. Water to be used for public water supplies must be
potable (drinkable), that is without polluting contaminants that would degrade the water
quality and constitute a hazard or impair the usefulness of the water. To ensure drinking
water safety, the so-called multiple barrier approach is advocated; it consists of an
integrated system of measures that safeguard water quality from the source to the tap.
For safety, a redundancy of protection measures is built into these systems. The quality
of drinking water is prescribed by the appropriate standards. In this context, the term
standard represents a definite rule, principle or measure established by an authority.
Where health is of concern and scientific data are limited, precautionary standards may
be justified.
Quality criteria for drinking water have been presented in many documents.
Particularly well known are the regulations mandated by the U.S. Environmental
Protection Agency, title 40, parts 141 and 143 and of the Safe Drinking Water Act
(1974, amended in 1986 and 1996). These are the current regulations for evaluating the
suitability of surface or groundwater resources for public water supply in the USA (for
the latest version, visit www.epa.gov/safewater/mcl.html). These guidelines provide
primary and secondary standards; the primary standard is for human health protection
and the secondary standard implies a regulation that specifies the maximum
contamination levels that are permissible in order to protect the public welfare, but may
adversely affect appearance or odour of water.
On the international forum, the most authoritative document on drinking water
quality is the World Health Organization (WHO) guidelines for drinking water quality
(WHO, 2004). These guidelines address a framework for safe drinking-water, health-
based targets, water safety plans, surveillance, applications of guidelines in specific
circumstances, microbial aspects, chemical aspects, radiological aspects and
acceptability aspects. Specific circumstances include emergencies and disasters, large
buildings, packaged/bottled water, travellers, desalination systems, food production and
processing, and water safety on ships and in aviation. The guidelines are suitable for use
by both developed and developing countries. Most frequent concerns about the
drinking water quality are those posed by pathogens and arsenic.
Requirements on industrial water supply quality depend on the type of industry, and
may even differ in various segments of a particular industrial sector. For a detailed
description of water quality requirements in various types of industries, the reader is
referred to Corbitt (1990).
There are many water uses that can be met by sub-potable water. Typical examples
are irrigation of urban landscape, agricultural irrigation, aquaculture, some domestic
uses (e.g., toilet flushing), industrial reuse (e.g., cooling waters or process waters),
recreational waters and groundwater recharge. Sub-potable water requirements can be
met by reclaimed urban wastewater, with the main benefits consisting in saving potable
water and reducing pollution discharges into receiving waters. In water reuse, the most
important issue is to specify the quality of water to be reused for a particular purpose.
Such specifications then determine the required level of treatment. Some guidance for
water reuse can be obtained from WHO guidelines, which are under review (WHO,
1989). Further discussion of wastewater reclamation and reuse is presented in Section
3.4.8.
3.2.3 Water supply sources
The gap between society's needs for water and the capability to meet such needs is
continually widening. Water supply, an important element of the overall urban water
cycle, attempts to bridge this gap. Water needed in urban areas may come from
groundwater or surface water sources such as lakes, reservoirs, and rivers. It is called
untreated or raw water, which is usually transported to a water treatment plant. The
degree of treatment depends on the raw water quality and the purpose that this water
will be used for. Different water quality standards for municipal purposes have been

24
developed and used during the past several decades. After treatment, the water is
usually distributed via a water distribution network.
Four major characteristics of water supply are quantity, quality, time variation, and
price. If the quantity and time distribution of raw water conformed to the water use
patterns in an urban area, then there would be no need to store water or regulate its
distribution by man-made structures or devices. But in almost all urban areas, the time
variation of available water resources does not follow demand variations. Therefore,
certain facilities should be implemented to store the excess water during wet (high flow)
seasons to be consumed in low flow periods.
Costs of initial investment in, and operation and maintenance costs of, the water
supply should be incorporated in the economic studies for development planning. In the
same way, if water quality does not satisfy the standards for different water uses,
treatment plants should be implemented and their costs should be incorporated in the
urban water resources development studies. In general, water supply methods can be
classified into the following categories:
x large-scale and conventional methods of surface and groundwater resources
development,
x non-conventional methods.
Conventional methods of water supply include large-scale facilities such as dam
reservoirs, water transfer structures, and well fields. Dam reservoirs are the most
important source of water in many large cities around the world. Quite often the dam
reservoirs serving for water supply of urban areas are located tens or hundreds of
kilometres away from the areas served, sometimes in another river basin.
Water supply storage facilities range from large reservoirs created by building dams
to small scale storage tanks. The term “reservoir” has a specific meaning with regard to
water supply systems modelling and operation. It is an “infinite” source that can supply
or accept water with such a large capacity that the hydraulic grade elevation of the
reservoir is unaffected and remains constant.
In many large cities around the world, water demand has exceeded the total water
resources in the basin in which the city is located. In such cases, one of the approaches
to water supply management is to develop new inter-basin water transfer schemes in
order to keep ahead of the ever-increasing requirements due to the growing population
and improved standards of living. A different approach is referred to as demand side
management, in which water conservation is practiced to the maximum practical extent
to reduce demands on the water supply and manage the needs for infrastructure
expansion (Butler and Memon, 2005).
3.2.3.1 Conjunctive use of sources and artificial recharge
Conjunctive use of surface water and groundwater can sometimes offer an attractive and
economical means of solving water supply problems where the exploitation of either
resource is approaching or exceeding its optimum yield. The volumes of groundwater
naturally replaced during each year are relatively small because of the slow rates of
groundwater movement and the limited rate of infiltration. Artificial recharge can be
used to reduce the adverse groundwater conditions such as progressive lowering of
water levels or saline water intrusion. As shown in Table 3.3, the percentage of
population in selected countries relying on groundwater supply ranges from 15 to 98%;
the rest is served by surface water sources.

25
Table 3.3 Percentage of the population supplied by groundwater in selected countries
Country Population supplied by groundwater (%)
Denmark 98
Portugal 94
Italy 89
Mexico 75
Switzerland 75
Belgium 67
Netherlands 67
Luxemburg 66
Sweden 49
USA 40
United Kingdom 35
Canada 25
Spain 20
Norway 15
3.2.3.2. Supplementary sources of water
Three supplementary sources of water are of particular importance in urban areas:
rainwater harvesting, bottled water and wastewater reclamation and reuse. The first
source is addressed in this section, the second one in Section 3.2.6, and the third one in
Section 3.4.8.
Rainwater harvesting is a supplementary or even primary water source at the
household or small community level, especially in places with relatively high rainfall
and limited surface waters (e.g., small islands). Also, the use of roof runoff for irrigation
or other uses is becoming of interest in highly developed urban areas, as one of the
measures supporting environmental sustainability, by reducing water supply demands
for irrigation and reducing urban runoff and its impacts. Roofs of buildings are the most
common collecting surfaces. Natural and artificial ground collectors are also used in
different places around the world. In designing rainfall harvesting systems the following
issues should be considered.
x Quantity issues: Rainwater collection systems often suffer from insufficient
storage tank volumes or collector areas. Leakage from tanks due to poor design,
selection of materials, construction, or a combination of these factors is a major
problem of the rainwater collection systems.
x Quality issues: The rainwater quality in many parts of the world is good. But,
water quality problems may arise within the collection systems. Physical,
chemical, and biological pollution of rainwater collection systems occurs where
improper construction materials have been used or where maintenance of roofs
and other catchment surfaces, gutters, pipes, and tanks is lacking (Falkland,
1991).
Rainwater cisterns or other rainwater collection devices have been used in many parts
of the world for centuries. At present, this approach is used widely in Australia and
India. In rural areas of Australia, many farms are not connected to water supply systems
nor have adequate well water; hence, the rainwater tank has become a symbol of the
Australian outback culture. Local councils and urban water authorities are increasingly
encouraging these systems in urban areas, and in some cases offering rebates to
customers who use rainwater for sub-potable uses. The most feasible reuse of rainwater
in urban areas is for garden irrigation, which accounts for 35 to 50% of domestic water
use in many large cities of the world. Reuse of rainwater in the garden requires a
relatively simple system, with very low environmental risks, and it is therefore
encouraged by many water authorities. An example of a rooftop rainwater harvesting
system is shown in Fig. 3.2 (Karamouz et al., 2003).
Further savings of potable water can be achieved when rainwater is used for toilet
flushing (about 20% of domestic water use), as well as in the laundry, kitchen and
bathroom. It can also be used in pools, and for washing cars. In some situations (e.g., in
some rural areas), it may be possible to use rainwater for most domestic uses, without
relying on the public water supply. In all these cases, strict regulations for reclaimed
water quality must be followed and safety systems employed, particularly in connection

26
with drinking water, which should be protected by the so-called “multiple barrier
system”. In this approach, multiple barriers are used to control microbiological
pathogens and contaminants that may enter the water supply system, and thereby ensure
clean, safe and reliable drinking water.
Fig. 3.2 An example of a rainwater collection system (after Karamouz et al., 2003)
3.2.3.3 Water shortage
Urban demands on water supplies are continually increasing as a result of growing
urban populations and higher standards of living. When demands exceed the available
water, water shortages result, with significant social, political, and economic
implications. In general, there are two main reasons for water shortage in urban areas:
(a) climatic or hydrologic drought, and (b) inability of the supplier to provide the
required water.

27
Social impacts of water shortage refer to the effects of water deficit on the public
health and life styles; such impacts are exacerbated when the equity in water
distribution is disturbed. As a part of the long-term planning, urban water suppliers
strive to ensure the appropriate level of reliability of water supply meeting the needs of
various categories of customers during both normal and dry water years.
The water supply system failure to meet the customer demands is measured by
different performance parameters, such as reliability (reflecting the probability that the
system will meet demands), resiliency (the speed of returning the system to the fully
operational state), and vulnerability (reflecting the consequences of the failure)
(Hashimoto et al., 1982).
In comprehensive water supply planning, the following items are considered to
mitigate water shortages: (a) Planning horizon, (b) planning criteria (including
reliability, cost, and water quality), (c) demand projections, (d) water availability under
the current conditions (the minimum water supply is estimated for the driest water year
in the planning horizon, all supply opportunities such as recycled water and water
transfer are considered, and plans for replacing water resources at risk are assessed), (e)
long-term water supply strategy, (f) water quality considerations, (g) treatment and
production facilities, and (h) contingency plans (may include water use restrictions and
rationing, prioritising competing uses, and utilising alternative sources). Water
shortages may lead to conflicts among the stakeholders, which need to be resolved.
3.2.4 Drinking water treatment
Water treatment is generally required to make raw water drinkable. As high quality
sources of water are depleted, water utilities are increasingly using lower quality source
water requiring more treatment and consuming more water during the treatment.
Detailed discussion of water treatment is beyond the scope of this report and can be
found elsewhere (Pontius, 1990). Instead, the material presented here focuses just on
emerging technologies, desalination, and disinfection.
3.2.4.1 Emerging technologies
Rapid development of microfiltration (membrane filtration) has created new treatment
options that were not feasible earlier. One alternative system delivers raw source water,
or just primarily treated water, directly to the user, who further treats this water in small,
local treatment units near the point of consumption. The main advantages of this system
are that the deterioration of the water quality during transport is avoided and each
consumer can treat the water to the level specifically needed for their requirements.
Needless to say, the water quality does not need to be the same for such purposes as
human consumption (drinking water), process water for industry, cooling water,
irrigation water, or water for flushing toilets. Small, locally used microfiltration units
are rapidly becoming competitive in price. Both conventional central and on-site water
treatment systems are illustrated in Figs. 3.3 and 3.4.
Fig. 3.3 A conventional layout of water treatment and distribution systems

28
Fig. 3.4 An alternative layout of drinking water distribution and treatment system using small, local
membrane filter units
3.2.4.2 Desalination
Among unconventional water supply methods, the use of seawater treated by
desalination is the most widely used method. Seawater has a salinity of about
35,000 mg/L that is attributed mostly to sodium chloride. Desalination was first
adopted in the early 1960s, using multi-stage flash evaporation (MSF). Several plants
using various desalination technologies are currently operating in the USA, Caribbean
and Middle East. Desalination technologies have increasingly become cheaper and
more reliable, and their current costs may be competitive with those of other high-tech
treatment technologies, especially if water sources are sparse or very remote.
The following methods are used for removing dissolved solids:
x Distillation: In this method, the water is heated to its boiling point to convert it
into steam; the steam is then condensed yielding salt-free water.
x Reverse Osmosis (RO): In this method, the water is forced through a semi-
permeable membrane under pressure; the dissolved solids are held back.
x Electrodialysis: In this method, ions are separated from the water by attraction
through selective ion-permeable membranes using an electrical potential.
Among the above methods, the first two are more common for desalination of
seawater; electrodialysis is usually preferred for treating brackish groundwater. During
the last decade the development of membrane technologies has been rapid and has
resulted in the development of new, cheaper and high-performance membranes. Plants
using membrane technologies (reverse osmosis, RO, and electro-dialysis) have been
built and the cost of finished water is steadily decreasing. The main advantage of
membrane technologies over distillation technologies is that they use much less energy.
Often, pre-filtration is used in order to remove hydrocarbons to reduce the fouling of the
RO membranes. The cost of reverse osmosis desalination has decreased substantially
over the past decade, dropping down to between US$0.25 and US$1.00 per cubic metre.
Even then, seawater desalination remains more expensive than most other sources of
water supply (where available), but in arid locations close to the sea and far away from
suitable surface or groundwater sources, seawater desalination may be the most
economical option for an urban water supply. On a household level, small-scale
membrane units have been developed and sold in all parts of the world to offices and
single households. The cost of such units (typically around US$2,000/unit) is decreasing
but still too high for common use. As the technology further improves (particularly for
reverse osmosis), these costs may further decrease. In addition to high rates of energy
consumption, a major problem at inland desalination plants is the disposal of rejected
brine. Evaporation ponds, injection in deep wells, and transfer to the ocean are the
common methods depending on the volume of reject brine, site location and
geographical and climatic conditions.

29
3.2.4.3 Disinfection
The introduction of chlorine in disinfection of drinking water in the early 20th century
drastically reduced waterborne diseases in western cities. Even today, the struggle to
remove pathogens from potable water and deliver a healthy municipal drinking water is
of primary concern all over the world. In remote or poor regions of the world, boiling of
drinking water will continue to be used for a long time to safeguard against diseases.
Alternative disinfection methods have been developed and used, most often by
chlorination using hypochlorite, chlorine dioxide and chloramines.
One major drawback of adding chlorine is the formation of carcinogenic by-products
during the disinfection process. Among such products, most common are
trihalomethanes (THM), haloacetic acids, bromate, and chlorite. Numerous studies
have demonstrated that these byproducts may increase the cancer risk (e.g., Batterman
et al., 2002; Gibbons and Laha, 1999; Goldman and Murr, 2002; Korn et al., 2002).
Consequently, U.S. EPA limits the presence of THM in drinking water to 80 μg/L
(Gibbons and Laha, 1999). When considering the risks associated with these
substances, one should keep in mind that such risks are much smaller than
microbiological risks incurred if water is not disinfected. To avoid the problems
associated with chlorination byproducts, other disinfection methods have been
introduced, including UV irradiation, ozonation and solar disinfection. In developing
countries, the first two methods may not be affordable, but the third has a great
potential, because of its simplicity and low costs.
In solar disinfection, solar radiation is used to destroy pathogenic microorganisms
which cause waterborne diseases. This process is best suited for treating small
quantities of water, usually placed in transparent plastic bottles and exposed to full
sunlight for some extended time periods. The actual treatment occurs through solar UV
irradiation and increased water temperature, ideally above 50°C, which accelerates
bacteria die-off.
UV irradiation and ozonation may require pre-treatment and before discharging
disinfected water into the distribution system, some chlorine may have to be added to
the treated water to maintain chlorine residuals during transport in the distribution
network and prevent growth of bacteria originating from biofilms found in the pipe
system. In many countries the residual chlorine concentration at the tap is taken as a
measure of safety.
3.2.5 Water distribution systems
Water distribution systems include the entire infrastructure from the treatment plant
outlet to the tap. The actual layout of supply mains, arteries, and secondary distribution
feeders should be designed to deliver the required fire flows in all built-up parts of the
municipality, above and beyond the maximum daily rate usage. Similar considerations
must be given to the effects that a break, joint separation, or other main failure could
have on the water distribution system operation. In urban areas, most of the water
quantity standards relate to the water distribution system, as further discussed below, on
the basis of general practices in this field.
In evaluating a water distribution system, pumps should be considered at their
effective capacities when discharging at normal operating pressures. The pumping
capacity, in conjunction with storage, should be sufficient to maintain the maximum
daily use rate plus the maximum required fire-fighting flow with the single most
important pump out of service.
Storage is frequently used to equalise pumping rates in the distribution system as well
as provide water for fire fighting. In determining the fire flow from storage, it is
necessary to calculate the rate of delivery during the specified period. Even though the
volume stored may be large, the flow to a hydrant cannot exceed the carrying capacity
of the mains, and the residual pressure at the point of use should not be less than 140
kpa (20 psi).
Depending on specific practices, the recommended water pressure in a distribution
system is typically 450 to 520 kPa (65 to 75 psi), which is considered adequate for
buildings up to ten stories high as well as for automatic sprinkler systems for fire

30
protection in buildings of four to five stories. For a residential service connection, the
minimum pressure in the water distribution main should be 280 kPa (40 psi); pressure in
excess of 700 kPa (100 psi) is not desirable, and the maximum allowable pressure is
1030 kPa (150 psi). Fire hydrants are installed at spacing of 90-240 m and in locations
required for fire fighting.
Although a gravity system delivering water without the use of pumps is desirable
from a fire protection standpoint, the reliability of well-designed and properly
safeguarded pumping systems can be developed to such a high degree that no
distinction is made between the reliability of gravity-fed and pump-fed systems. Electric
power should be provided to all pumping stations and treatment facilities by two
separate lines from different sources. For more detailed information about the
quantitative standards in water distribution systems, the reader is referred to the water
distribution text books such as Walski et al. (2003). The use of standby emergency
power is also recommended.
3.2.6 Drinking water supply in developing countries
It is estimated that over one-third of the urban water supply systems in Africa, Latin
America and Asia operate intermittently. An intermittent water supply is a significant
constraint on the availability of water for hygiene and encourages the low-income urban
population to turn to alternatives such as water vendors.
Many of the intermittently operating systems do not deliver water more than half the
time, and there are large variations in water quality. One way of overcoming the lack of
water supply is to construct local water reservoirs, from which water is delivered to
outdoor taps or into the households. Problems arise from the facts that these reservoirs
are poorly protected against tampering and seldom cleaned. Further, the variations of
pressure in the pipe systems may cause intrusion of contaminated water. Considerable
risks for spreading of diseases exist in such systems (WHO and UNICEF, 2000).
Among the less developed but commonly used technologies are unprotected wells
and springs, vendor-provided water, bottled water and tanker truck provision.
Vendor-provided water is rapidly expanding in many countries and raises many
questions concerning water quality and price. It is often argued that for the price of the
vendor-provided drinking water, more efficient communal water supply systems could
be constructed. However, water vendors today play an important role in many regions
of the world.
The bottled water industry is growing quickly all over the world and has become an
important market. In developed countries, the price of bottled water is about 1,000 times
higher than that of the public water delivered at the tap. Nevertheless the market is
growing, influenced by trendy consumers and by people who do not trust the quality of
the tap water. In developing countries, bottled water, where affordably priced, may
solve acute drinking water problems or problems with low quality water.
Tanker truck provision is common in many regions where the public distribution
system does not exist. Typical examples may be small villages or peri-urban areas.
Generally (e.g., in India), this water is supplied from public water supplies by
contractors.
3.3 URBAN DRAINAGE
Urban drainage serves to reduce the risk of flooding and inconvenience due to surface
water ponding, alleviate health hazards, and improve aesthetics of urban areas.
Traditionally, drainage development was based on a steady expansion of the drainage
infrastructure without consideration of the impacts of drainage discharges on receiving
waters and their beneficial uses. Two interconnected urban drainage systems are
recognised; the major system serving to alleviate major flooding and the minor system
providing convenience by reducing water ponding in urban areas. Drainage systems
have evolved throughout the history and existing systems represent a compromise
among providing flood protection, improving quality of life, technical abilities,
ecological needs, and availability of resources.

31
Two types of urban sewerage systems exist - combined and separate. The combined
system conveys both surface runoff and municipal wastewaters in a single pipe. In dry
weather, the entire flow is transported to the sewage treatment plant and treated. In wet
weather, as the runoff inflow into the combined sewers increases, the capacity of the
collection system is exceeded and the excess flows are allowed to escape from the
collection system into the receiving waters in the form of the so-called combined sewer
overflows (CSOs), which pollute receiving waters.
In the separate system, surface runoff is transported by storm sewers and discharged,
with or without passive treatment, into the receiving waters, and the municipal
wastewater is transported by sanitary sewers to the wastewater treatment plant (WWTP)
and usually treated prior to discharge into the receiving waters. Both drainage systems
exist in many variations.
Urban drainage interacts with other components of the urban water cycle, and
particularly with receiving waters. Fast runoff from impervious surfaces, together with
hydraulic improvements of urban drainage in the form of street gutters, storm sewers
and drains, results in an increased incidence and magnitude of stormwater runoff. The
resulting high flows affect the flow regime, sediment regime, habitat conditions and
biota in receiving waters. Urban drainage also affects low flows. Reduced infiltration
leads to reduced groundwater recharge, lowered groundwater tables and reduced base
flows in rivers. Low flows reduce the self-purification capacity of rivers, limit the
dilution of polluted influents and consequently are characterised by poor water quality.
Where groundwater is withdrawn for urban water supply, aquifers are often
overexploited and land subsidence may occur.
Drainage also interacts with other water infrastructures and water resources. For
example, cross-connections between storm and sanitary sewers either allow the influx of
municipal sewage into separate storm sewers with concomitant pollution of stormwater,
or the influx of stormwater into sanitary sewers increases the flow rates, which may
exceed the WWTP capacity, result in sewage bypasses, and the pollution of receiving
waters. All types of sewers may interact with groundwater, particularly if not
watertight. Leaky sewers below the water table suffer from groundwater infiltration and
essentially drain groundwater aquifers. Infiltration of groundwater into sanitary sewers
increases sewage flows reaching the WWTP and thereby increases the cost of treatment
and the risk of sanitary sewer overflows. On the other hand, exfiltration from sanitary
sewers pollutes groundwater in urban areas and may impact on sources of drinking
water. Sudden increases in wet weather flows in combined sewers produce hydraulic
and pollution shocks on the treatment plants and may reduce the treatment efficiency,
particularly of biological treatment by shortening the reaction time and reducing the
return sludge flow. In addition, the biomass is diminished as sludge is flushed into the
final clarifier. All these factors can lead to reduced treatment efficiencies and increased
discharge of pollutants into the receiving waters.
Concerning drainage flows, urban water managers are interested in both water
quantity and water quality issues, as discussed in the following sections.
3.3.1 Flooding in urban areas
Floods are naturally occurring hydrological events characterised by high discharges
and/or water levels leading to inundation of land adjacent to streams, rivers, lakes, or
coastal areas. Where such areas are occupied by human settlements, disasters may
occur and result in loss of human life and material damages. Two types of floods are
distinguished in urban areas – those locally generated by high intensity rainfall, and
those generated in larger river catchments and passing through urban areas, where they
may inundate flood plains which have been encroached upon. Other floods may occur
in coastal areas, in the form of storm surges or tsunamis with catastrophic impacts.
Only the first flood type, locally generated, will be addressed here; river floods or
flooding of coastal areas are beyond the scope of this report.
Locally generated floods usually result from catchment urbanisation as discussed
later in Chapter 4. High catchment imperviousness, hydraulically efficient flow
conveyance, and reduced concentration times causing runoff generation by high-

32
intensity rainfall all contribute to high rates of surface runoff and risk of local flooding.
In developed countries, these issues have been addressed with various degree of success
by applying both non-structural and structural measures incorporated in a master
drainage plan. The methodology for urban drainage planning is well developed and
described in Geiger et al. (1987). These plans are prepared at two planning levels,
short-term (5-10 years) and long-term (25-50 years). A master drainage plan represents
a technical layout of the sewerage systems (drainage and sanitation) for the entire urban
area as it may further develop within the planning horizon. With respect to drainage,
the master plan should be part of a catchment plan and incorporate the whole drainage
system including the connections and interactions between the minor and major system
components.
The minor system comprises swales, gutters, stormwater sewers, open drains and
surface and subsurface storage facilities, and conveys runoff from frequent events with
return periods up to 10 years. The minor system reduces the frequency of
inconvenience (water ponding) and its failure has minimal implications. The major
system consists of natural streams and valleys, as well as large constructed drainage
elements, such as large swales, streets, channels and ponds. The major drainage system
greatly reduces the risk of loss of life and property damage in urban areas, and
consequently, its failure has serious consequences. It is typically designed for a 50-year
event, or even a 100-year event (Geiger et al., 1987).
Typical master drainage plans include such points as purpose and background of the
study, identification of drainage related problems, definition of study objectives,
database for planning, methods for planning and design, identification and investigation
of drainage alternatives, impact of the future drainage system, final design of individual
structures, and implementation. Much success has been achieved with master drainage
planning in developed countries; however, the situation is different in developing
countries, where the principles of master drainage planning are rarely followed.
Urbanisation in developing countries occurs too fast and unpredictably, and often
progresses from downstream to upstream areas, which increases flood problems
(Dunne, 1986). Urbanisation of peri-urban areas is largely unregulated, often without
the provision of any infrastructure, many public lands are occupied and developed
illegally, and flood-risk areas (flood plains) are occupied by low-income population
without any protection. WHO (1988) reported spontaneous housing developments in
flood-prone areas of many cities in humid tropics, including Bangkok, Mumbai,
Guayaquil, Lagos, Monrovia, Port Moresby and Recife.
Other problems include lack of funding for drainage and other services, lack of solid
waste collection (which may end up in and block drainage ditches), no prevention of
occupation of flood-prone areas, lack of knowledge on coping with floods, and lack of
institutions in charge of flood protection and drainage (Dunne, 1986; Ruiter, 1990).
Tucci and Villanueva (2004) suggested solutions including introduction of better
drainage policies (which would control flow volumes and peaks), and planned
development, in which space is retained for flow management measures. Also in flood
plains, non-structural measures should be applied by emphasising green areas, paying
for relocation from flood-prone areas, and public education about floods.
3.3.2 Stormwater
Stormwater is mostly rainwater running off impermeable surfaces in urban areas,
including roofs, sidewalks, streets and parking lots. It is drained from urban areas by
sewers or open channels to avoid local inundation. During this process, stormwater
becomes polluted and its discharges into receiving waters cause environmental
concerns. A schematics of runoff generation and pollution is shown in Fig. 3.5.

33
Fig. 3.5 Flows of water and pollutants in stormwater systems
Stormwater may be transported either by combined sewers, together with domestic
and industrial wastewaters, or by separate sewers discharging to the nearest stream or
lake. In combined sewers, high stormwater inflows exceed the pipe capacity and excess
flows have to be diverted by flow regulators as combined sewer overflows (CSOs) to
the nearest receiving waters. CSOs contain not only the stormwater, but also untreated
wastewater and sewer sludge; their direct discharges into receiving waters cause serious
pollution problems.
The stormwater contribution to the wet weather flow reaching the wastewater
treatment plant also increases the concentrations of heavy metals and other
contaminants in the WWTP effluent and in the sludge (biosolids). This is one of the
main reasons why the use of sludge from European WWTPs as a fertiliser in agriculture

34
has been questioned. So, which sewer system is better – the separate system in which
polluted stormwater may be discharged directly into the receiving water or the
combined system, in which the stormwater is conveyed to the WWTP for treatment, but
CSOs occur? There are no general answers to this question, as the definite comparisons
of separate and combined systems performance depend on local conditions. The
separate sewer system could be improved by implementing separate stormwater
treatment plants, but that may be costly and inefficient in view of highly variable
infrequent inflows with low concentrations of pollutants. Similarly, the combined
systems can be improved by incorporation of CSO pollution abatement.
3.3.2.1 Stormwater characterisation
The literature on urban stormwater quality is very extensive. So far, more than 600
chemicals have been identified in stormwater and this list is growing. Makepeace et al.
(1995) identified about 140 important contaminants, which can be found in stormwater
and would affect human health (i.e., mostly through contamination of drinking water
supply) and aquatic life. This list contains solids, trace metals, chloride, nutrients (N
and P), dissolved oxygen, pesticides, polycyclic aromatic hydrocarbons, and indicator
bacteria. Typical concentrations of many such constituents were reported in the
literature, mostly for developed countries. Summaries of data from two large databases
appear in Table 3.4.
Stormwater quality data were also reported for less developed countries, but usually
in small data sets. Examples of such data include those from Sao Paulo, Brazil (cited in
Tucci, 2001), Johor, Malaysia (Yusop et al., 2004), Bandung, Indonesia (Notodarmojo
et al., 2004) and Beijing, China (Che et al., 2004). In general, such data indicate
significantly higher concentrations than those in Table 3.4, which may be caused by
problems with infrastructure, such as cross-connections between storm and sanitary
sewers. In any case, such data suggest that pollution loads conveyed by storm sewers in
developing countries are larger than indicated by the literature data published for
developed countries.
Table 3.4 Quality of urban runoff and combined sewer overflows: stormwater worldwide data (Duncan,
1999), U.S. NURP stormwater data (U.S. EPA, 1983) and European CSO data (Marsalek et al., 1993).
Urban stormwater Chemical constituent Units
Mean of
Duncan’s
dataset (1999)
U.S. NURP
Median site
(U.S. EPA,
1983)
European CSO
data
(Marsalek et al.,
1993)
Total suspended solids (TSS) mg/L 150 100 50-430
Total phosphorus mg/L 0.35 0.33 2.2-10
Total nitrogen mg/L 2.6 - 8-12
Chemical Oxygen Demand, COD mg/L 80 65 150-400
Biochemical Oxygen Demand, BOD mg/L 14 9 45-90
Oil and grease mg/L 8.7 - -
Total lead (Pb) mg/L 0.140 0.144 0.01-0.10
Total zinc (Zn) mg/L 0.240 0.160 0.06-0.40
Total copper (Cu) mg/L 0.050 0.034 -
Faecal coliforms FCU/100 mL 8,000 - 10
4
-10
7
Elevated pollutant concentrations (compared to the data in Table 3.4) are observed
during periods of snowmelt, when the pollutants accumulated in snowpacks are rapidly
released and conveyed by storm sewers to the receiving waters (Viklander et al., 2003).
In older separate sewer systems, urban surface runoff is conveyed by storm sewers to
the nearest receiving waters without any control or treatment. Only during the last 30
years, has stormwater management been introduced and practised by reducing runoff
generation by allowing more rainwater to infiltrate into the ground, balancing runoff
flows by storage and providing some form of runoff quality enhancement. Among the
main pollutants of concern in stormwater, one could name suspended solids, nutrients
(particularly P), heavy metals, hydrocarbons, and faecal bacteria.

35
3.3.2.2 Stormwater management
As a result of high discharges of stormwater, and pollutant concentrations and loads
conveyed by stormwater, and their potential impacts on the environment, alternative
techniques have been developed for stormwater management during the last several
decades (Azzout et al., 1994; Baptista et al., 2005; Parkinson and Mark, 2005; Schueler,
1987; Urbonas, 1994), including the following:
x infiltration facilities,
x ponds and wetlands,
x swales and ditches,
x oil and sediment separators, and
x real-time control operation systems.
Infiltration may be applied in the so-called percolation basins which are specially
designed as underground gravel units. This technology has been used for a very long
time particularly on a small scale in rural settlements. Only during the last few decades
it has been further developed and used in urban areas on a larger scale. Stormwater
infiltration helps keep the groundwater table at a natural level, which promotes good
conditions for vegetation and a good microclimate. The construction costs of drainage
systems with infiltration facilities are also cheaper than those of conventional systems.
Infiltration is also implemented on grass or other permeable surfaces, and in drainage
swales and ditches. The use of this measure is steadily growing in many countries.
Ponds and wetlands have become in many countries a common and accepted means
of attenuating drainage flows and treating stormwater by removal of suspended solids,
heavy metals and, to some extent, nitrogen and phosphorus. The cost of construction
and operation of such facilities is often low compared to the environmental benefits.
The sediments from ponds may contain high concentrations of heavy metals. However,
ponds and wetlands should be considered as stormwater treatment facilities and not as
natural water bodies, even if they often provide aesthetic values to the urban area.
Swales and ditches are applied commonly in the upstream reaches of drainage to
control runoff flows and provide runoff quality enhancement. Flow control is obtained
by stormwater infiltration into the ground, quality enhancement by filtration through the
turf, solids deposition in low flow areas, and possible filtration through a soil layer.
Oil and sediment (grit) separators are used to treat heavily polluted stormwater from
highways or truck service areas, or where polluted stormwater is discharged into
sensitive receiving waters. The efficiency of these units in trapping oil, sediments and
chemicals attached to the sediments is often poor, because of under-sized units or lack
of flow-limiting devices preventing the washout of trapped materials.
Real-time control operation of sewer systems has been developed during the last two
decades and implemented in some Canadian, European, Japanese, and U.S. cities. The
applications are often in combined sewer systems and the purpose is mainly to reduce
combined sewer overflows and/or overloading of wastewater treatment plants, by the
maximum utilisation of the dynamic capacities of the system (Colas et al., 2004).
3.3.2.3 Special considerations for drainage in cold climate
In countries with a cold climate (i.e., occurrence of freezing temperatures over periods
of several months) the precipitation falls as snow during a significant part of the year.
When the snow is cleared from streets in the cities, it is either brought to local snow
dump sites, or to a central deposit site outside the city, or it is dumped into
watercourses. When the snow melts, the meltwater runs off in the same way as
stormwater. However, the impacts may be more severe due to the following facts:
x Snowmelt may generate high flows, causing surcharging of the sewer systems and
possibly flooding in the receiving waters.
x The meltwater often has higher concentrations of heavy metals, sand and salt than
stormwater (sand and salts being used for de-icing of urban roads and streets).
x The impacts of urban snowmelt on streams, lakes and ponds may be exacerbated
by ice covers of such water bodies and densimetric stratification. High salt

36
concentrations and oxygen depletion were noted at a number of locations
(Marsalek et al., 2003).
Various technologies have been adopted for treating urban meltwater. Examples are
ponds, oil and grit separators, and infiltration facilities. They all have to be designed and
operated with considerations of the special effects of temperature, ice and snow
conditions, and the elevated pollutant concentrations (Viklander et al., 2003).
3.3.3 Combined Sewer Overflows (CSOs)
Even though combined sewer overflows are highly polluted, they are often discharged
into nearby receiving waters without much treatment and cause serious pollution. The
magnitude of annual CSO discharges depends on the extent of combined sewers
(percent of the total), climate, and design policies and practice. The extent of combined
sewers varies from country to country, in the range from 20 to 90%. Generally
combined sewers are more common in climates with lower annual rainfall; for high
rainfalls, the system would be too overloaded and collect a low percentage of total
flows. Finally, the overflow setting, typically in multiples of dry weather flow, greatly
influences the spilled CSO volume. Typical settings vary from 2 to 6 times dry weather
flow (Marsalek et al., 1993).
3.3.3.1 CSO characterisation
The pollution characteristics of CSOs, while somewhat similar to those of stormwater, are
strongly affected by domestic sewage and sewer sludge washout from combined sewers.
Consequently, CSOs are particularly significant sources of solids, biodegradable organic
matter, nutrients, faecal bacteria, and possibly some other chemicals originating from
local municipal/industrial sources. During the early phase of runoff, referred to as the
first flush, the CSOs characteristics approximate or even exceed pollutant
concentrations in raw sanitary sewage. After the first flush, pollutant concentrations in
CSOs subside. Their impacts on receiving waters are similar to those described in the
preceding section, but much stronger in terms of oxygen depletion, eutrophication and
increased productivity, and faecal pollution. It is desirable, therefore, to control CSOs
prior to their discharge into the receiving waters.
CSOs are not routinely monitored, except for special studies of local significance.
This follows from the diffuse and intermittent nature of these sources, for which large
scale monitoring programs would be prohibitively expensive. Nevertheless, over the
years, a fair number of studies attempting to assess these sources have been undertaken
in a number of municipalities, or regions. A summary of such data from European
sources was presented earlier in Table 3.4.
In comparison to stormwater, the pollution strength of CSOs is similar to that of
stormwater for TSS, but greater for BOD, indicator bacteria, TN and TP, and generally
smaller for unconventional pollutants, including heavy metals, PAHs and
organochlorine pesticides.
3.3.3.2 CSO control and treatment
CSOs are caused by excessive inflows of stormwater into the sewer system, so any
measure discussed in the preceding section for reducing stormwater runoff and its
inflow into combined sewers would also help abate CSOs. Such helpful measures
include all lot-level measures, infiltration measures (pits, trenches, basins, porous
structures) and porous pavements (Urbonas, 1994). The mitigation of actual overflows
is accomplished by various forms of flow storage and treatment; flow storage serves to
balance CSO discharges, which may be returned to the treatment plant after the storm,
when flows have subsided below the plant capacity (Marsalek et al., 1993).
CSO storage can be created in a number of ways: by maximising the utilisation of
storage available in the existing system (e.g., through centrally controlled operation of
dynamic flow regulators in real time - Schilling, 1989), as newly constructed storage
on-line or off-line (on-line storage includes oversized pipes or tanks; off-line storage
includes underground storage tanks or storage and conveyance tunnels), or even in the

37
receiving waters (the so-called flow balancing systems created by suspending plastic
curtains from floating pontoons, in a protected embayment in the receiving waters)
(WPCF, 1989). Some storage facilities are designed for treatment, more or less by
sedimentation, which can be further enhanced by installing inclined plates. Stored
flows are returned to the wastewater treatment plant, which must be redesigned/
upgraded for these increased volumes. Without such an upgrade, the plant may become
overloaded, its treatment effectiveness impaired and the benefits of CSO storage would
be defeated.
CSO storage tanks have become a common design feature in many European
sewerage systems, and are increasingly being used in North America. In this approach,
an additional "storage" volume for flow and pollutant load retention is included in the
form of an oversized sewer pipe or storage chamber, which is incorporated into the
sewerage at the points of overflow. Tanks are normally either on-line (continually in
operation) or off-line (to which flow is diverted during high-flow periods via a diversion
structure).
CSO treatment takes place either at the central plant, together with municipal sewage,
or can be done in satellite plants dedicated to this purpose. Various processes have been
proposed or implemented for the treatment of CSOs, including settling (plain, inclined
plate and chemically aided), hydrodynamic separation, screening, filtration, dissolved
air flotation, and the Actiflo process with coagulation and ballasted settling (Zukovs
and Marsalek, 2004). Furthermore, the treated effluents may be disinfected, either by
conventional chlorination (sometimes followed by dechlorination), or by UV irradiation
(WPCF, 1989).
Treatment technologies are available to achieve almost any level of CSO treatment,
but proper cost/benefit considerations are crucial for achieving the optimal level of CSO
pollution abatement, within the given fiscal constraints. Reductions in treatment
capacities are obtained by flow balancing of inflows by storage (Marsalek et al., 1993).
From the maintenance point of view, the operators (municipalities) prefer relatively
simple treatment systems, with more or less automatic operation, and minimum
maintenance requirements.
The most cost-effective CSO abatement schemes deal with the entire urban area (and
all system components) and represent combinations of various source controls, storage
and treatment measures, allowing various degrees of control and treatment, depending
on the event frequency of occurrence (Marsalek et al., 1993). More frequent events
should be fully contained and treated; less frequent events may be still fully or partly
contained and treated to a lower degree, and finally, infrequent events would still cause
overflows, but of reduced volumes and could receive some pre-treatment prior to their
discharge into the receiving waters.
The complexities of combined sewer systems, and the dynamics of flow, storage,
loads and treatment processes, make it particularly desirable to control the
sewerage/treatment/ receiving water systems in real time. Real time control (RTC) was
found particularly useful in systems with operation problems varying in type, space and
time, and with some idle capacity (Schilling, 1989). The best developed types of RTC
are those for wastewater quantity and the associated modelling. The remaining
challenges include RTC of quality of wastewater and receiving waters, and reliable
hardware Colas et al., 2004). It was suggested that in typical wastewater systems with
no control (i.e., control by gravity only), approximately 50% of the system capacity
remained unused during wet weather. By applying RTC, about half of this potential
could be realized (Schilling, 1989).
3.4 WASTEWATER AND SANITATION
Urban populations require access to adequate sanitation and disposal of generated solid
and liquid wastes. Such objectives are achieved by wastewater management and
sanitation.

38
3.4.1 Problem definition
According to the “Global Water Supply and Sanitation Assessment 2000 Report”
(WHO and UNICEF, 2000) the proportion of the world’s population with access to
excreta disposal facilities increased from 55% in 1990 to 60% in 2000. At the
beginning of 2000, 2.4 billion people lacked access to improved sanitation. The
majority of these people lived in Asia and Africa; for example, less than one-half of the
Asian population had access to improved sanitation. In developing countries, the
percentage of the population served by wastewater treatment hardly reaches 15% (U.S.
EPA, 1992) and such treatment almost always consists of a primary or inefficient
secondary level treatment. Effluents are frequently destined for agricultural irrigation or
discharged into soils, rivers or lakes, ultimately reaching the sea. Agricultural reuse of
treated wastewater effluents is the most common option in arid and semi-arid areas
because of the lack of water, but also because farmers know that sewage increases
productivity because of its nitrogen, phosphorus and organic matter content.
Nevertheless, this practice also considerably increases health risks.
The global sanitation coverage with “improved sanitation” is estimated to be around
60%. The coverage in urban areas is significantly higher than in rural areas, ranging
from 78% in Asia to 100% in North America. These numbers must be considered
unreliable, due to the vague definition of urban areas and to the fact that the term
“improved sanitation” includes not only connections to public sewers but also
connections to septic systems, pour-flush latrines, simple pit latrines, and ventilated,
improved pit latrines.
Water quality in receiving waters is affected mainly by wastewater discharges, solid
wastes and wet-weather flow pollution. Impacts of such discharges depend on the type
of pollutant, the relative magnitude of the discharge vs. the receiving waters, and the
self-purification capacity of receiving waters.
In developed countries, even though the wastewater is treated at a secondary level,
treated effluents still pollute.
3.4.2 Technological development
Some milestones in the development of modern, centralised wastewater systems are
summarised below (Wolfe, 2000).
Around 3,500 BC, brick stone stormwater drain systems were constructed in streets
of Mesopotamia. In Babylonia manholes and clay piping were used to connect in-house
bathrooms to street sewers around 3,000 BC. In Rome the famous cloacas were
constructed starting 600 BC (Cloaca Maxima). The construction of sewers in Paris
started in the 14
th
century. An underground sewer for draining cellars and carrying away
wastewater was constructed in Boston, around 1700. The City of Hamburg is credited
with having built the first city sewerage system around 1840. Sewage pumps were
introduced around 1880, followed by the development of screens and grit chambers to
remove solid matter. The modern WC (water closet) was developed by Thomas
Twyford in 1885.
Wastewater treatment technologies were successively developed from the end of the
19
th
century. Among the important developments were contact beds (1890), trickling
filter (1901), sludge digestion (1912), activated sludge process (1913), and surface
aeration (1920). Biological as well as chemical treatment systems have been steadily
improved and installed, removing solids, organic matter, and nutrients from the
wastewater. A schematic of such a conventional system is shown in Fig. 3.6. At the
beginning of the 21
st
century much interest is being paid to combinations of biological
treatment and different kinds of microfiltration (membrane technology). The protection
of the environment from harmful discharges of wastewater is governed by laws and
directives, such as, for example, the European Union Waste Water Treatment Directive
91/271/EEC (CEC, 1991), and many national regulations.

39
Fig. 3.6 A conventional system for managing wastewater and organic waste
3.4.3 Ecological sanitation
There is a fundamental connection between the present state in water supply, sanitation,
solid (organic) waste management and agricultural development worldwide. While
sustainable provision of water and sanitation for a growing population is in itself a
formidable challenge, the new target is to develop technologies and management
strategies that can make organic residuals from human settlements useful in rural and
urban agriculture for production of food. Traditional methods used in water resources
development and in provision of sanitation were, and still are, unable to satisfy the fast
growing needs of developing countries.
In other words, a strong coordination between urban and rural components for
nutrient and biomass recycling is required, particularly in a sustainable society. After
consumption, the question is how to deal with urine and faeces and utilise them in the
recycling of nutrients and biomass. One solution is a separation of urine from other
household wastewater. This process is called ecological sanitation – a dry toilet with
separation of urine and faeces at the source (Winblad and Simpson-Hebert, 1998). The
separated urine can then be transferred to the nutrient processing plants, where nitrogen
and phosphorus are recovered and transformed into chemical fertilisers.
If the urine and faeces are collected and used in agriculture, the remaining wastewater
is called greywater, which is easier to treat by conventional wastewater treatment
processes. Pipe systems are not necessary for collecting greywater and open channels
are still acceptable for minimum conditions. The collected greywater can be treated
locally and discharged to surface waters in the vicinity of the collection points
(Maksimovic and Tejada-Guibert, 2001). The nutrient content of greywater is
comparable to waters that by different standards are regarded as “clean” (Gunther,
2000). However, concerns about pharmaceuticals and personal care products and
therapeutics in greywater exist and need to be considered in greywater reclamation and
reuse. Also sludge revalorisation causes some concerns due to associated health risks,
particularly in developing countries. Spreading diarrheic diseases can be a health
concern unless the pathogen content in the ecosanitation sludge is greatly reduced
(Jiménez et al., 2006). That is why EcoSanRes (2005) and Schönning and Stenström
(2005) recommend that, even if treated, ecosanitation black matter (faeces) should be
handled safely and not used to fertilise vegetables, fruit or root crops that will be
consumed raw.
A greywater reuse system should be able to receive the effluent from one or more
households during all seasons of the year. Where garden soils of low permeability

40
become saturated by winter rainfall, there should be opportunities to divert excess water
to sewers or to arrange an alternative disposal. The reuse system needs to protect public
health, protect the environment, meet community aspirations and be cost-effective
(Anda et al., 1996).
Esrey et al. (2001) summarised the basic characteristics of ecological sanitation
systems as follows:
x Prevent disease - must be capable of destroying or isolating faecal pathogens;
x Protect the environment - must prevent pollution and conserve valuable water
resources;
x Return nutrients - must return plant nutrients to the soil;
x Culturally acceptable - must be aesthetically inoffensive and consistent with
cultural and social values;
x Reliable - must be easy to construct and robust enough to be easily maintained in
a local context;
x Convenient - must meet the needs of all household members considering gender,
age and social status; and,
x Affordable - must be financially accessible to all households in the community.
3.4.4 Basic demands on wastewater management systems
Basic demands to be met by an urban wastewater management system are pollution
control, public health protection, avoidance of flooding, and recycling of nutrients.
Pollution control aims to protect the receiving waters, including creeks, streams,
rivers, lakes and the sea, against discharges of wastewater that may cause
eutrophication, oxygen depletion, toxicity and other negative impacts that decrease the
biological diversity or impair beneficial uses of the receiving waters for such purposes
as e.g., drinking water supply for downstream settlements. Pollution control is one of
the major reasons for the construction of wastewater treatment plants (WWTPs).
Public health protection has historically been the major driving force behind the
construction of WWTPs. In western countries as well as in developing countries the
application of wastewater treatment has resulted in drastic improvements in the health
standards of people living in affected areas. In particular, the introduction of
disinfection has been successful, as further elaborated below.
Flooding in urban areas, which is caused either by locally generated floods or by
floods generated in the upstream catchment was discussed earlier in Section 3.3.1.
Local (pluvial) flooding can be exacerbated by poorly designed and functioning sewer
systems. Flood damages caused by local flooding are often less severe in drainage
systems using open channels, ditches or swales for flow conveyance, rather than
underground sewers.
Recycling of nutrients has been included in the basic demands since the sustainability
requirements were formulated at United Nations sponsored meetings in Oslo, Rio, and
Johannesburg. The nutrient contents of domestic wastewater may be valuable as
fertiliser in agriculture (or aquaculture). Associated problems are the contamination of
the sludge with heavy metals and certain organic substances.
Additional demands on wastewater systems are that they should be affordable,
accepted by the public and convenient.
Affordable wastewater systems provide such services, which the users are able to pay
for. The distribution of service costs is essential, the rates should be equitable and fair to
all connected. The consumers must be willing to pay the costs. As a general guideline,
the World Bank recommends that the cost of water and sanitation should not exceed 5%
of total family income.
The services provided must also be accepted by the consumers, regarding the service
delivery, water quality and prices. The water and wastewater systems must be socially
and culturally acceptable to the consumers. Different traditions exist in each country or
region. For example, in some cultures, dry sanitation is not acceptable.
The systems must also be convenient to use. The carrying of drinking water or
wastewater products has strong gender implications and should be avoided in
sustainable systems.

41
3.4.5. Wastewater characterisation
Municipal sewage is a mixture of domestic, commercial and industrial wastewaters. In
developed countries, industrial wastewaters are pre-treated prior to discharge into
municipal sewers; in developing countries, industrial wastes are often not treated at all.
In spite of this, the impact of industrial discharges in developing countries is smaller
than in developed ones, because of a lower level of industrialisation. Typical
composition of municipal sewage is described in Table 3.5.
Table 3.5 Typical composition of sewage (after Metcalf and Eddy, 2003)
Concentration
Parameter
Minimum Average Maximum
Total solids (mg/L) 350 720 1,200
Dissolved solids, total (mg/L) 250 500 850
Fixed solids (mg/L) 145 300 525
Volatile solids (mg/L) 105 200 325
Total suspended solids (mg/L) 100 220 350
Fixed solids (mg/L) 20 55 75
Volatile solids (mg/L) 80 165 275
Settleable solids (mL/L) 5 10 20
Biochemical Oxygen Demand, BOD
5
(mg/L)
110 220 400
Chemical Oxygen Demand, COD (mg/L) 250 500 1,000
Total Organic Carbon (mg C/L) 80 160 290
Total nitrogen (mg N/L) 20 40 85
Organic nitrogen (mg N/L) 8 15 35
Free ammonia (mg N/L) 12 25 50
Total phosphorus (mg/L) 4 8 15
Grease and oil (mg/L) 20 100 150
Alkalinity (mg CaCO
3
/L) 510 100 200
As shown in Table 3.6, pathogen concentrations in developing countries wastewater
are much higher than those in wastewater in developed countries.
Table 3.6 Pathogen concentration in wastewater from developing and developed countries (Chávez et al.,
2002)
Microorganism Developed countries Developing countries
Salmonella (MPN/100 mL) 10
3
-10
4
10
6
-10
9
Enteric viruses (PFU/100 mL) 10
2
-10
4
10
4
-10
6
Helminth ova (HO/L) 1-9 6-800
Protozoa cysts (organisms/L) 28 10
3
Characteristics of industrial wastewaters vary greatly, depending on the type of
industry. To protect receiving waters from industrial pollutants, it is important to
develop efficient pre-treatment programs in order to have influents to wastewater
treatment plant with "relatively controlled" characteristics. Without such pre-treatment,
industrial pollutants might pass through conventional wastewater treatment plants
without much abatement and cause damage in receiving waters.
3.4.6 Wastewater systems without separation of wastewaters at the source
Systems without separation of wastewaters at the source manage the total mixture of
wastewaters, including blackwater and greywater. In terms of system architecture, they
can be designed as conventional centralised systems, or less common distributed
systems.
3.4.6.1 Centralised systems
Currently, most cities in all parts of the world have a centralised sewerage system with
some kind of treatment. Exceptions are the poorest cities and unofficial “squatter”
settlements in peri-urban areas in Africa, Asia and Latin America. They all have the
same basic features: collection of the wastewater in or near the houses, transport by

42
gravity sewers or pressure sewers to a treatment plant, and discharges to the receiving
waters by longer or shorter outfalls. In many of the old sewer systems, domestic and
industrial wastewater is mixed with stormwater in combined sewers. In principle,
wastewater collection and treatment systems are similar in all large cities of the world.
The degree of use of modern technologies may vary, depending on financial resources
and political will.
However, it is reported from many developing countries that although they have
access to central sewer systems and sewage treatment, the facilities do not operate
properly and in some places not at all due to the lack of management, maintenance,
funding and training.
Currently, the interest in research and technology advancement involves:
x further development and refinement of biological methods for nutrient removal
from wastewater and the recovery of nutrients,
x the use of membrane technology for wastewater treatment,
x development of anaerobic methods for sludge digestion and treatment,
x incineration of sewage sludge (biosolids), and
x control of new chemicals of concern, including endocrine disruptors,
pharmaceuticals (including antibiotics), and personal care and therapeutic
products.
3.4.6.2 Distributed (local) systems
Conventional (centralised) sewerage systems are common in the central parts of most
large cities, but the approaches to wastewater management are quite different in small
towns or in the peri-urban areas on the outskirts of cities. Open canals are commonly
used to transport human wastes and discharge them to receiving waters without
treatment. Such systems are a severe threat to public health since the water in receiving
streams, rivers or canals may be used further downstream for cleaning and washing, or
even as a drinking water source. In densely populated areas, polluted rivers cause the
same kind of a threat.
In some parts of the world, simplified and cheaper solutions are sought for the
management of wastewater on a smaller scale. Examples of such technologies are listed
below:
x Wastewater infiltration. After sedimentation, the wastewater is infiltrated into
the soil in a constructed filter plant, seeping down to the groundwater. The
reduction of organic matter, nutrients and bacteria may not reach the standards of
a high-tech WWTP, but may be a significant improvement compared to the
existing conditions. This technology should not be used for industrial effluents or
for wastewaters having high contents of dissolved substances (heavy metals,
organic toxic compounds, pharmaceutical residues, etc.). Also, wastewater
infiltration should not be used where the affected groundwater is used as a source
of drinking water.
x Constructed wetlands. The term “wetlands” is commonly used for many
technologies: simple open ponds, several ponds in a series with or without
vegetation, reed-beds with horizontal or vertical flow, and some others. In the
developed countries wetlands are most commonly used as a polishing step after a
WWTP, mainly for nutrient removal. The microorganisms in the wetland nitrify
and denitrify the nitrogen to a certain degree, depending on the size of the
wetland, the type of vegetation and the ambient temperature. In the Nordic
countries, the uses of wetlands are increasing as a low-cost, natural technology,
but the efficiency of the treatment is low during the winter. The major drawback
of this technology is that it requires a large space.
x Biological ponds. These have been used for a very long time and their
performance has been improved considerably. Further improvements of their
performance, especially in cold climates, have been achieved and demonstrated
by addition of precipitation chemicals.
x The Living Machine. In few places in North America, domestic wastewater is
transported to hydroponics plant beds in a greenhouse and subject to treatment,
while at the same time, the nutrients in the wastewater are used for cultivating

43
various green plants. Experimental facilities based on a similar concept also
operate in Europe (Todd and Todd, 1993).
3.4.7 Systems with separation of wastewaters at the source
Originating in Sweden in the 1990s, the idea of separating domestic and other
wastewaters at the source has been developed and tested in a small scale. Since then,
this concept has spread and facilities exist in many other European countries, including
Denmark, Germany, Switzerland and The Netherlands, although in all cases at a small
scale. Several ongoing research projects deal with the refinement of these systems. The
separation systems have also been applied in many places in developing countries,
especially in rural areas where they may substitute for pit latrines or no sanitation at all.
The advantages of ecological sanitation in rural areas in developing countries are
striking: the hygienic conditions are improved compared to simpler solutions, water is
only used for cleaning purposes, and the wastewater products, urine and/or excreta, can
be used as fertilisers after some minimum period of storage. Characteristics of various
domestic wastewater sources are listed in Table 3.7. Although the chemical
characteristics of various household wastes in developed and developing countries are
similar, the biological characteristics greatly differ. For example, with reference to
developing countries, the helminth ova content in untreated faeces may be as high as
3000 ova/g total solids (Strauss et al., 2003) and in treated ones from 0 to almost
600 ova/g total solids (Jiménez et al., 2005). A similar point was also emphasised in
section 3.4.5.
The use of separation systems in densely populated areas has been discussed but not
proven in reality. Opponents argue that the cost for redesigning the sewerage systems in
houses and in the streets will be huge, and that the transport of collected urine and/or
excreta will cause additional costs, nuisance and air pollution in the cities. Advocates
argue on the other hand that the cost/benefits of the system are favourable when
compared to the conventional system, particularly after accounting for the recycling of
nutrients and the use of natural resources. It would seem that the separation system may
be feasible in peri-urban areas and “informal” settlements where sewerage systems still
do not exist and the extension of the central system to the outlying parts of the city
would be too costly or almost impossible due to the lack of space and financial
resources.
Table 3.7 Composition of urine, faeces, greywater, household wastewater and compostable household
waste in Sweden (in kg/person equivalent/year) (Jönsson et al., 2005)
Parameter Urine Faeces, incl.
toilet paper
Greywater
total
Household
wastewater
Compostable
household
waste
TSS 71926 53 25
VSS 31715 35 21
COD
tot
32323 49 34
BOD
7
21212 27 12
N
tot
4.0 0.5 0.6 5.1 0.6
P
tot
0.33 0.18 0.25 0.76 0.10
S
tot
0.26 0.06 0.17 0.48 0.05
K
tot
0.88 0.33 0.29 1.49 0.23
A schematic of blackwater separation is shown in Fig. 3.7.

44
Fig. 3.7 Blackwater separation
3.4.8 Water and wastewater reuse
Water scarcity in many parts of the world causes stress on water supplies, which occurs
when the demand for water exceeds the available amount during certain periods or
when poor quality restricts the use of water. Water stress causes deterioration of fresh
water resources in terms of quantity (aquifer over-exploitation, dry rivers, etc.) and
quality (eutrophication, organic matter pollution, saline intrusion, etc.) (EEA, 1999). In
urban areas this has emphasised the need for developing other kinds of water resources,
such as desalination of seawater, collection of rainwater and reclamation of used water.
The reuse of treated wastewater has been practiced in many countries over a long
time, mostly for recharge of over-exploited aquifers by infiltration. Moreover, the reuse
of untreated wastewater is also practiced in some regions of the world due to the lack of
water and economic resources to treat wastewater before reuse. It is estimated that at
least 21 million ha are irrigated with treated, diluted, partly-treated or untreated
wastewater (Jiménez and Asano, 2004). In some urban areas, wastewater has been used
for agricultural irrigation. The “urban agriculture” is practiced in urban and peri-urban
areas of arid or humid tropic countries due to wastewater availability, local demand for
fresh produce and the need to support people living on the verge of poverty.
Wastewater flowing in open channels is used to irrigate very small plots of land where
trees, fodder or any other crops that can be brought to the market in small quantities
(flowers and vegetables) or be used as part of the family diet are grown (Cockram and
Feldman, 1996; Ensink et al., 2004a). One-tenth or more of the world’s population
consumes crops irrigated with wastewater (Smit and Nasr, 1992). The use of
wastewater can vary considerably from one region to another; for example, in Hanoi,
Vietnam, up to 80% of vegetables locally produced are irrigated with wastewater
(Ensink et al., 2004b).
More recently, direct use of treated stormwater and wastewater has been introduced
in some countries in most regions of the world. On a small scale, reuse of stormwater
and greywater has been applied in some European countries (The Netherlands, UK, and
Denmark) for garden irrigation and for flushing of toilets. In some cases, certain
drawbacks have been observed; most are due to cross-connections with the drinking
water supply system, causing risk of spreading waterborne diseases. In 2003 the use of
treated greywater was prohibited in the Netherlands for this particular reason.
In other parts of the world, the need for developing complementary water resources
has forced water authorities to go one step further by applying high-tech treatment and
delivering reclaimed water to the customers for various uses. Some examples are given
below.
Various types of waters, including reclaimed wastewater, are used in aquaculture to
produce fish and grow aquatic crops. Some guidance can be obtained from WHO
guidelines (WHO, 1989), which are currently under review.

45
3.4.8.1 NEWater in Singapore
In February 2003, Singapore started to replenish about 1 percent of its total daily water
consumption with reclaimed wastewater, which was named NEWater. NEWater is
mixed and blended with raw water in the reservoirs before undergoing conventional
treatment at the waterworks for supply to the public for potable use. The amount will be
increased progressively to about 2.5% of total daily water consumption by 2011.
Singapore suffers from a water shortage and buys more than half of its water demand
from neighbouring Malaysia under decades-old treaties, which will start expiring in
2011. The water trade has caused many arguments between the two nations over pricing
and other issues. The wastewater used is the product from a multiple barrier water
reclamation process. Treatment steps are conventional wastewater treatment,
microfiltration, reverse osmosis and finally ultraviolet disinfection. The quality of the
reclaimed water fulfils all requirements and is in most aspects better than the raw source
water currently used (more information can be obtained from Public Utilities Board and
the Ministry of the Environment and Water Resources, Singapore,
www.pub.gov.sg/NEWater/).
3.4 8.2 Shinjuku water recycling centre, Tokyo, Japan
The water recycling centre in the Shinjuku district of Tokyo, distributes advanced
treated wastewater for toilet flushing in high rise office buildings in the districts of
Shinjuku and Nakano-Sakaue. The reclaimed water is supplied from the Ochiai
wastewater treatment plant (WWTP), located 2 km from the water-recycling centre. The
treatment process applied in reclamation includes conventional secondary wastewater
treatment (primary sedimentation, activated sludge process and secondary
sedimentation) supplemented by rapid sand filtration. At present, the water recycling
system operates in 26 high-rise buildings. Differentiated fees for water supply were
implemented to support the choice of recycled water, rather than municipal drinking
water, for toilet flushing (Asano et al., 1996).
3.4.8.3 Wetlands with fish production in Calcutta, India
The City of Calcutta, India has ancient wetland traditions, and the existing techniques
are used for direct recycling of nutrients in the city, which is one of the largest in the
world. The sewage system in central Calcutta was constructed in the 1870s, and is
completely inadequate today. Some parts of the city constructed in later stages of the
city development have no sewers at all. There is also frequent flooding during the
monsoon period. Currently, there is one wastewater treatment plant operating in
Calcutta, and two more plants are under construction.
Most of the wastewater in Calcutta remains untreated today, and will remain
untreated when the ongoing project is completed. However, some of the untreated
wastewater is diverted into Calcutta’s wetlands, which are part of the largest
aquaculture area in the world. Wastewater is pumped directly to huge wetlands where
fish are cultivated and sold on the market. Use of wetlands has a centuries old tradition
in India. In the 1980s, systematic follow-up studies of the wetlands were initiated, under
the auspices of the newly established Institute for Wetland Management and Ecological
Design (IWMED). The area is enormous, corresponding to 12,000 ha, and produces one
sixth of all the fish consumed in Calcutta (Ghosh, 1999).
3.4.8.4 Reuse of (untreated) sewage for agricultural irrigation in the Mezquital Valley
(Mexico City sewage disposal)
Mexico is a country with apparent water sufficiency at the national level. However, two
thirds of the territory suffers from a lack of water. Frequently, municipal wastewater is
used for irrigation. In 1995, a total of 102 m
3
/s of wastewater was used to irrigate about
257,000 ha in the country. An example of this practice is Mexico City sewage, which
has been used to irrigate the Mezquital Valley, north of the city, since 1896; 52 m
3
/s of
wastewater without any treatment has been used to irrigate several crops and has
permitted the economic development of the region. This is the largest and oldest scheme
of agricultural irrigation using urban wastewater in the world (Mara and Cairncross,

46
1989). As a result of this practice, the water table of the aquifer underlying the irrigation
zone has been rising. The unplanned artificial recharge is about 25 m
3
/s and springs
with capacities of 100 to 600 L/s started appearing 35 years ago (Jiménez and Chávez,
2004). This “reclaimed water”, treated only with chlorine, is being used to supply
300,000 inhabitants of the region for human consumption. Several studies have shown
that the water meets potable norms and another 288 parameters from WHO human
consumption guidelines, including toxicological tests (Jiménez et al., 2001). Also, this
wastewater is enriched in nutrients, as confirmed by increased yields of agricultural
crops (see Table 3.8).
Table 3.8 Yield increase as a result of irrigation with wastewater in the Mezquital Valley, Mexico
(Jiménez et al., 2005)
Yield (tonnes / ha)Crop
Wastewater Fresh Water
Increase
(%)
Maize corn 5.0 2.0 150
Barley 4.0 2.0 100
Tomato 35.0 18.0 94
Oats for forage 22.0 12.0 83
Alfalfa 120.0 70.0 71
Chili 12.0 7.0 70
Wheat 3.0 1.8 67
3.4.8.5 Reuse of stormwater and greywater in Sydney, Australia
Serious water shortages in Sydney, Australia during summer prompted Sydney Water to
encourage the reuse of stormwater and greywater, besides the ongoing intensive water
saving and conservation campaigns. The use of reclaimed water for garden irrigation is
encouraged, either by using treated greywater or stormwater collected in separate tanks.
Advice to house owners can be found on the website www.sydneywater.com.
Sydney Water also introduced the distribution of reclaimed water. One example is the
Rouse Hill Development area. These homes have two water supply systems, reclaimed
water and drinking water. To ensure that the drinking water is not confused with the
recycled water it is delivered by a separate distribution system. The reclaimed water
taps, pipe-work and plumbing fittings are coloured lilac for easy identification.
Reclaimed water is passed through a complex treatment train including ozonation,
microfiltration and chlorination, in addition to the customary high level of treatment.
The reclaimed water is subject to strict guidelines that limit its use to toilet flushing and
outdoor purposes, such as car washing and garden irrigation (NSW Health, 2000).

47
Chapter 4
Impacts of Urbanisation on the Environment
4.1 OVERVIEW
Urbanisation affects many resources and components of the environment in urban areas
and beyond. Even though the discussion in this report focuses on water, brief
discussions of some connected issues, or other environmental compartments interacting
with urban water, cannot be avoided. Thus, for the sake of completeness, the discussion
covers, in various degrees of detail, impacts of urban areas on the atmosphere, surface
waters, wetlands, soils, groundwater and biota.
Urban areas produce air pollution and release heat into the atmosphere. In turn, air
pollution of local or remote origin may be a significant source of pollutants found in wet
and dry urban precipitation and in urban waters. Heat releases in urban areas lead to the
so-called heat island phenomenon with elevated air temperatures, which then affect
local climate and snowmelt.
The soil-water interface is important with respect to soil erosion, leaching of
chemicals from soils into water, and on-land disposal of residue from stormwater or
wastewater treatment. These interactions may affect both the water and the soil.
Most of the published literature on the effects of urbanisation addressed impacts on
surface waters, which may include streams, rivers, impoundments, reservoirs, lakes,
estuaries and near-shore zones of seas and oceans. The transition from aquatic to
terrestrial ecosystems is provided by fresh or salt water wetlands, which are discussed
separately from surface waters.
During the past 10-20 years, a great deal of attention has been paid to urban
groundwater, which is affected by urbanisation with respect to both quantity and
quality. Depending on local circumstances, urbanisation affects aquifers and
groundwater tables through either reduced recharge (increased runoff) or increased
recharge (leaking water mains, leaking sewers, stormwater infiltration), and pollution by
infiltrating effluents or accidental spills.
Finally, changes in the urban environmental compartments greatly affect the urban
biota, particularly the fish and wildlife, with respect to health, abundance and
biodiversity.
Each of the urban environmental compartments can be subject to various types of
impacts, which occur in the urban environment in various combinations and magnitude.
For convenience, it is useful to discuss these impacts under such headings as physical,
chemical, and microbiological, but the overall effect is generally caused by the
combined impacts of the earlier named categories. Considering the great diversity of
topics discussed in this chapter, some guidance to following the material in this chapter
is offered in Table 4.1, which provides classification of urbanisation impacts on
environmental compartments and illustrates this classification by the examples listed in
the table.

48
Table 4.1 Classifications and examples of impacts
Environmental Compartment Type of
Impact
Atmosphere Surface Waters Wetlands Soil Ground-water Biota
Physical
Heat island,
increased
precipitation
downwind,
dry deposition
Increased
surface runoff
and flooding,
higher water
temperature
Changes in
water balance
Increased
erosion,
changes in
physical
structure
Lower or
higher water
table
Loss of
habitat,
benthic
organism
burial
Chemical
Acid or toxic
rain
Pollution of
streams and
lakes
Pollution Soil pollution
DNAPL
contamination
Toxic effects,
loss of
biodiversity
Micro-
biological
Small risk of
exposure
during sludge
handling
Faecal pollution
of beaches or
drinking water
sources
Changes in
bacterial
ecology
Changes in
bacterial
ecology due
to sludge
application
Polluted
drinking
water
Risk of biotic
impacts
(diseases)
Combined Smog
Loss of
biodiversity,
impairment of
beneficial uses
Loss of
biodiversity,
impacts on
biota
Landfills
Degraded
aquifers
Loss of
abundance,
loss of
biodiversity
4.2 GENERAL CHARACTERISATION OF URBANISATION EFFECTS
The process of urbanisation changes the landscape as well as material and energy fluxes
in the urban areas, thereby affecting the urban environment. Changes in landscape and
runoff conveyance are particularly important with respect to surface runoff and its
characteristics. Other changes are caused by construction of urban infrastructures,
increased water consumption in urban areas, and releases of solids, chemicals,
microorganisms, and waste heat. Water leaves urban areas in the form of urban
wastewater effluents, (UWWE), which include stormwater, CSOs and municipal
wastewaters. Such types of effluents differ in their physical, chemical and
microbiological characteristics; consequently, their effects also differ and will be
discussed separately within a common framework of impacts. Because of the dynamic
nature of UWWE discharges and the associated pollutant levels, loads and effects, the
temporal and spatial scales of individual effects are also important (Lijklema et al.,
1989). Some effects manifest themselves instantaneously; others may become apparent
only after periods of many years. With respect to spatial scales, the magnitude of
discharges and the number of outfalls vis-à-vis the type and size of receiving waters are
also of great importance. Further discussion of these factors follows.
4.2.1 Increased ground imperviousness
Perhaps the most visible consequence of urbanisation is the increase in the extent of the
impervious ground cover that strongly limits the possibility of water infiltration. High
imperviousness is particularly noticeable in downtown areas, where it reaches almost
100%. In many countries, the rapid increase in catchment imperviousness is a relatively
recent phenomenon; in France, for example, the area of the impervious surface has
increased tenfold between 1955 and 1965.
Increased imperviousness affects runoff in several ways. Firstly, it increases runoff
volumes. This effect is often cited when explaining urban floods. However, if the runoff
volume increase plays an important role for frequent storm events, or even for the
events corresponding to the return periods considered in the design of minor drainage
systems (generally about 10 years), it is not the most important factor for extreme
events.

49
Indeed, the infiltration capacity of the majority of pervious soils, in the absence of a
dense forest cover, or except for sandy grounds, is much lower than the rainfall
intensities than can be observed during exceptional rainstorms. Thus, in this type of a
situation, permeable soils often yield specific runoff volumes (volume of runoff per unit
area), which approach those of the impermeable soils. For example, during an extreme
flood on the Yzeron River in the Lyon area in April 1989, the runoff coefficient of the
rural part of the catchment was estimated at 50% and the corresponding value for the
urban part was 60%.
Another significant consequence of increasing ground imperviousness is the lack of
recharge of groundwater aquifers (Leopold, 1968). This phenomenon can be
accentuated when water is withdrawn from the same aquifer for urban water supply.
More importantly, besides the direct effect of depletion of the water resource, the
lowering of the water table is likely to cause land subsidence, which in some cases can
reach several metres, as reported for example in the Mexico City (Figueroa Vega,
1984). Such extreme subsidence then affects the stability of buildings. For example,
during the drought spell in France in the early 1990s, the allowances paid by the
insurance companies for damages to buildings (cracks, fissures, etc.) were ten times
higher than the monetary losses in agriculture. However, in certain cases, exfiltration
from urban water infrastructures (drinking water mains, sewers and stormwater
management measures) can partially compensate the deficit in rainwater infiltration. For
example, in an urban agglomeration of 50 km
2
, with imperviousness of 50% and water
consumption of 100,000 m
3
per day, the water supply distribution network losses
(leakage) of 20% are equivalent to a groundwater recharge by infiltration of 300 mm of
rainfall per year. Similar values were reported by Lerner (2004).
4.2.2 Changes in runoff conveyance networks
As the urbanising area develops, there are profound changes in runoff conveyance, by
replacing natural channels and streambeds with man-made channels and sewers. In
general, these changes increase the hydraulic efficiency of runoff conveyance by
increasing the speed of runoff. This process starts with overland flow in headwaters of
the catchment and progresses to the receiving streams and rivers, which are canalised to
increase their hydraulic capacity and protect their beds against erosion. Finally, the
general drainage pattern of the catchment is also affected by transportation corridors
required in urban areas.
4.2.2.1 Construction of runoff conveyance networks
In urban areas, a natural drainage network, which may be temporary and comprising
sinuous waterways partly blocked by vegetation, is replaced with an artificial
conveyance network, which is often oversized in the upstream parts and characterised
by a straight layout to limit its length, and laid on significant slopes to decrease drain
sizes (and thus reduce costs) and improve its self-cleansing. The same process also
takes place in peri-urban areas, with respect to the drainage of soils and the canalisation
of the brooks, creeks and ditches. This canalisation, which is generally presented as an
effective means of preventing flooding, often has its origin in the occupation of the
flood zone (i.e., a major stream bed) by buildings or roads. However such major stream
beds constitute a natural part of the flood plain, and thus play a role in regulating the
flows transported to the downstream reaches. The increase in runoff speed and the
resulting shortening of the catchment response time contribute to higher runoff peaks
through two mechanisms: (a) faster transport processes, and (b) greater intensities of the
critical rainfall, which apply to the shortened response times.
4.2.2.2 Canalisation of urban streams and rivers
For various reasons, urbanisation usually leads to modification of river courses, by
damming, widening and training. Little brooks are gradually canalised, covered and
buried. Most important watercourses are enclosed between high embankments, which
completely isolate them from the city. In many cities, after centuries of progressing

50
urbanisation, urban rivers are now regarded only as "virtual sewers". The results of this
state of affairs are twofold.
x Urban rivers are gradually forgotten by the citizens who only perceive their
harmful effects.
x Urban rivers are enclosed in a too narrow “corset” and thereby have lost any
"natural" possibility of spilling onto natural flood plains in the case of floods.
Consequences can be catastrophic; the city, which is appropriately protected as long
as the water levels remain below the top of the embankments or dams, is suddenly
inundated when the flow increases, or these protective structures fail. No longer
accustomed to the presence of water, the city then reveals and manifests its increased
vulnerability by incurring damages of sensitive equipment located underground
(telephone switchboards, electric transformers, pumping stations, etc.), damages to
subways and in underground car garages, loss of important supplies of vulnerable goods
on ground floors, sweeping away of cars by floodwaters (because of their buoyancy),
inexperience of urban dwellers in coping with floods, etc. All the above factors help
transform the crisis into a catastrophe.
From an ecological point of view, the anthropogenic changes and river training also
have important consequences. A river is indeed a “living” entity, which must be
considered in all its temporal and spatial dimensions. From the spatial point of view the
river equilibrium depends on many conditions:
x upstream-downstream continuity (longitudinal dimension),
x habitat diversity (nature of the banks, width of the bed, speed of flow, depth of
the river, etc.),
x connections between the major stream bed and hydraulically connected water
bodies (lateral dimension), and
x flow exchange between rivers and aquifers (vertical dimension).
Construction of embankments and dams, bed dredging, canalisation, and construction
of new underground structures and foundations, all impoverish the river habitat and
decrease its capacity to be regenerated.
Temporal dynamics of rivers must also be considered. The succession of high and
low water stages, either episodic or cyclic (temporal dimension) is necessary for river
equilibrium. For that reason, the training and regulation of watercourses, building dams
to reduce floods and/or to maintain low flows, can be extremely harmful.
4.2.2.3 Interfering transport infrastructures
The third important consequence of urbanisation is the construction or expansion of
transportation corridors (motorways, railways, etc.). These projects often involve large
earthworks; the resulting infrastructures can be either very high, compared to the
original ground elevations, or very low in the form of deep cuts. As a consequence,
these earthworks superimpose a relief on the natural one, which, particularly in flat
terrain, can considerably modify the surface runoff and drainage patterns in two ways,
both of which contribute to increased flood risk:
(a) When the linear infrastructure is laid perpendicularly to the slope and the natural
direction of flow of water, transportation corridors constitute physical barriers
(dams) that force the runoff towards the provided flow openings (culverts), which
are generally superimposed on natural and obvious waterways (brooks, main