Understanding the Poor Performance of Urban Sewerage Systems: A Case of Coldstream High Density Suburbs, Chinhoyi, Zimbabwe

Abstract

The population of Coldstream High Density Suburbs in Chinhoyi, Zimbabwe is increasing and the sewerage system shows signs of poor performance. The frequency of reported sewerage system blockages in the suburb increased from 43 per month to 65 per month between April 2012 and March 2013. The suburb has also been experiencing water shortages. An increasing population and low water supply versus overflowing sewage is a potential health risk. The main objective of this paper was to investigate the causes of the sewer system poor performance. This was achieved by assessing the sewer system infrastructure as well as the operation of the system. The impact of the behaviour of residents on the performance of the system was also discussed. The main components of the infrastructure (manholes and sewers) were physically checked for soundness. The operation of the system was assessed by a hydraulic analysis of discharge, depth of flow and velocity of flow in the sewers. Questionnaires were used to investigate the impact of the behaviour of residents on the performance of the sewer system. It was found that 4% of the components of the sewer system infrastructure were below standard and the collector main sewer was the one mainly affected. 68% of the sewers along the collector main had velocity of flow below 0.6m/s and all the sewers had depth of flow below 50%. Of the residents interviewed, 90% dumped solid wastes in the sewers and 43% did so because they were unaware of the impact. It was concluded that the sewer system failed to meet standard because the sewer system fails to self-cleanse the solid waste dumped by residents in the sewers due to water shortages. It is recommended that the municipality raises awareness among residents and some of the infrastructure should be rehabilitated.

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Urban Planning and Design Research (UPDR) Volume 1 Issue 3, September 2013 www.seipub.org/updr
45
Understanding the Poor Performance of
Urban Sewerage Systems:
A Case of Coldstream High Density Suburbs, Chinhoyi, Zimbabwe
Annatoria Chinyama
*1
, Tendai Toma
2
1, 2 Department of Civil and Water Engineering, National University of Science and Technology, P.O Box AC939,
Ascot, Bulawayo, Zimbabwe
*1 anchinyama@gmail.com; 2tndtoma@gmail.com
Abstract
The population of Coldstream High Density Suburbs in
Chinhoyi, Zimbabwe is increasing and the sewerage system
shows signs of poor performance. The frequency of reported
sewerage system blockages in the suburb increased from 43
per month to 65 per month between April 2012 and March
2013. The suburb has also been experiencing water shortages.
An increasing population and low water supply versus
overflowing sewage is a potential health risk. The main
objective of this paper was to investigate the causes of the
sewer system poor performance. This was achieved by
assessing the sewer system infrastructure as well as the
operation of the system. The impact of the behaviour of
residents on the performance of the system was also
discussed. The main components of the infrastructure
(manholes and sewers) were physically checked for
soundness. The operation of the system was assessed by a
hydraulic analysis of discharge, depth of flow and velocity
of flow in the sewers. Questionnaires were used to
investigate the impact of the behaviour of residents on the
performance of the sewer system. It was found that 4% of
the components of the sewer system infrastructure were
below standard and the collector main sewer was the one
mainly affected. 68% of the sewers along the collector main
had velocity of flow below 0.6m/s and all the sewers had
depth of flow below 50%. Of the residents interviewed, 90%
dumped solid wastes in the sewers and 43% did so because
they were unaware of the impact. It was concluded that the
sewer system failed to meet standard because the sewer
system fails to self-cleanse the solid waste dumped by
residents in the sewers due to water shortages. It is
recommended that the municipality raises awareness among
residents and some of the infrastructure should be
rehabilitated.
Keywords
Sewer Blockages; Sewerage Hydraulics; Sewer System
Performance
Introduction
Rapid urbanisation in developing countries creates
massive demand for basic infrastructure in cities
(Schouten and Mathenge, 2010). Conventional urban
water management struggles to deliver water and
sanitation services and dispose of wastewater, without
adversely impacting on the quality of life of urban
populations and the downstream environment
(Verhagen, et.al, 2008). Sewer systems play a very
significant role in urban sanitation. Their structural
quality, operational efficiency and response to the
imposed conditions by the users of the system are key
parameters to guarantee the transfer of domestic,
commercial and industrial wastewater to wastewater
treatment plants (WWTP) without any back up flows
(Cardoso et al., 2004). Increasing population and poor
maintenance of the sewer system are some of the
factors that affect the performance of sewer systems.
When a sewer system malfunctions the first visible
signs are; the backing up of the sewage, pipe bursts
and a very high level of blockage frequency rate. A
study by Beattie and Brownbill (2007) on Bendigo, an
urban area in Australia found that the rate of sewer
blockages averaged 107 blockages per month between
the years 2002 and 2005. The study showed that the
performance of a sewerage system is affected by the
piping material, pipe diameters and the maintenance
of the systems.
Water borne conventional sewage reticulation is the
preferred wastewater management system in
Zimbabwe (Chinyama et al., 2012). Existing sanitation
infrastructure are in disrepair resulting in untreated
wastewater polluting water courses, through burst or
blocked sewer pipes thus creating serious health and
environmental risks (Tanyanyiwa and Mutungamiri,
2011). Now with unreliable water supplies, an
increasing population, aged infrastructure, over
loading of systems and poor maintenance, urban

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46
sanitation provision in the country, poses a challenge
of efficiency of service and not access.
Currently, high density suburbs in Chinhoyi town
show visible signs of a malfunctioning sewer system,
in the form of overflowing sewage which also pollutes
the nearby Manyame River. Causes of poor
performance of sewer systems vary from place to
place; therefore, this paper’s main objective is to
investigate the causes of the poor performance of the
sewerage system particularly in Chinhoyi town by
assessing the sewer system infrastructure as well as
the operation of the system. The impact of the
behaviour of residents on the performance of the
system was also investigated.
Materials and Methods
Study Area
Chinhoyi town is the regional capital for Mashonaland
West Province in Zimbabwe. The town has six (6) high
density areas namely Chikonohono, Coldstream,
Chemagamba, Gadzema, Gun-hill and White-City
located on the western banks of Manyame River, 115
km North West of the capital city Harare. The town is
built on rugged and hilly terrain bisected by many
streams that drains into Manyame River. According to
the municipality officials, the hilly terrain posed major
problems in constructing sewage reticulations in the
area.
FIG 2.1 COLDSTREAM HIGH DENSITY SUBURB (GOOGLE
EARTH, 2012)
The study was conducted in Coldstream high density
suburb (see figure 2.1); which according to
municipality officials was established between the late
70s and the early 80s with a total of 900 residential
units. Currently, the number of residential units has
increased by about 100 new units. At that time, the
suburb was serviced with a gravity sewer reticulation
system which feeds into a gravity trunk sewer that is
then pumped to the treatment works.
Some of these new units are not on the municipal
water reticulation system but are connected to the
municipal sewer system. The population in the town is
increasing mainly due to rural-urban migrations since
the town is surrounded by farms. The suburb is also
located close to Chinhoyi University of Technology
therefore during the university academic year; most of
the students seek accommodation in the suburb
thereby contributing to the population increase. The
suburb is located next a stream (named Coldstream)
that flows into Manyame River and currently the
stream is heavily polluted by solid waste and sewage
overflowing from the residential area. Figure 2.2
shows how polluted the stream is with solid waste as
well as sewage.
FIG 2.2 POLLUTED COLDSTREAM
The municipality officials indicated that the suburb is
under a 12 hours water rationing programme every
day with a bulk water allocation of 18000 m3/month
and a household allocation of approximately 2 m
3
/
month.
Assessing the Sewerage Infrastructure
Generally, sewer infrastructure consists of manholes,
sewers, pipe vents, sand traps, and pump stations.
Due to limited resources, data on sewer infrastructure
was only compiled on manholes and sewers, which
according to Cardoso et al. (2004) was enough for
sewerage system infrastructure assessment. In order to
make a physical assessment of the infrastructure, 121
manholes were physically checked at random for;
cracks, foreign objects, signs of overflow, covers and
spacing. The patterns of manholes with sign of
overflow were used to identify the sewer line route
that was mainly malfunctioning. Sewer-line routes,
sewer slopes and sewer diameters were checked

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47
physically and using long section drawings. Chinhoyi
municipality officials were then interviewed on how
the sewer system infrastructure was initially designed
and its age. The collected data was then compared to
the standards which all sewer reticulation design in
Zimbabwe has been based on: Sanitation Manual
Design Procedures. (Swedish Association of Local
Authorities, 1990).
Assessing the Operation of the Sewerage System
The operation of a sewer system is based mainly on
the hydraulic function of the system. According to
Swedish Association of Local Authorities (1990), the
hydraulics of the sewer system is governed by the
velocity of flow and depth of flow of the sewage in the
sewers. Data was collected to enable calculation of the
system hydraulics of the identified malfunctioning
sewer line from the infrastructure assessment. In order
to assess the current operating conditions,
questionnaires were used to collect data on water
consumption per residential unit on the following
activities bathing, laundry, dish washing and toilet
usage, so as to determine the approximate wastewater
generated. The residential units were randomly
selected and the questionnaires were administered to
100 (10%) residential units. The current population of
residents in the suburb was also estimated from
information obtained from the questionnaires. The
information obtained was compared to municipality
records for verification. As built drawings of the
suburb sewer system which were provided by the
Chinhoyi municipality were used to compile data on
sewer slopes, material, diameter, and length, this data
was important for the hydraulic analysis of the system.
Information on the design water consumption per
residential unit and the official population estimates
was obtained through interviews with the
municipality officials. MS Excel spread sheets were
used in analysing the questionnaires. Standard
hydraulic calculations based on the Manning’s
equation were used to estimate the different
parameters required for a hydraulic assessment of the
sewer system, assuming 100% return flow. AutoCAD
Civil 3D 2011 Imperial software was used as the
simulation model to cross check the manual
calculation. The obtained values were then compared
to the data collected on the initial design criterion of
the sewer system and the standards in Swedish
Association of Local Authorities (1990).
Investigating the Impact of the Behaviour of
Residents on Sewer System
The questionnaires administered to residents were
also used to collect data on material used as toilet
paper, what the residents’ solid waste consist of, how
the residents disposed of their solid waste and if they
report any sewage overflows to the authorities. The
municipality officials were interviewed and a list was
compiled of materials which were usually cleared
from the blockages in Coldstream. MS Excel spread
sheets were used in analysing the questionnaires.
Results and discussion
Assessment of Sewer System Infrastructure
The sewer system in Coldstream high density suburb
was 28 years old at the time of study and all the
sewers are asbestos concrete (A.C) pipes. This means
the sewer reticulation under investigation has
surpassed its design life. The results of the assessment
of the infrastructure are as summarised in Table 3.1.
From the results of the physical checks on manholes, it
was noted that the sewer collector main is the one
which is malfunctioning. Figure 3.1 shows the Google
map of Coldstream high density suburb, indicating the
collector main sewer (with the arrows) which was
observed to exhibit the most sewage overflows.
FIGURE 3.1 GOOGLE MAP OF COLD STREAM HIGH DENSITY
SUBURB SHOWING THE COLLECTOR SEWER MAIN
The results of the current condition of the sewer
system infrastructure in Coldstream high density
suburb were then compared to standards in Manual
design procedure 5 as shown in the table.
According to the municipality officials the frequency
of blockages in the suburb increased from 43 per
month in to 65 per month between April 2012 and
March 2013. Figure 3.2 shows the results of the
blockages along the sewer line.

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48
FIGURE 3.2 OVERFLOWING MANHOLE
The 12% of the manholes spaced greater than 75 m are
likely to face problems due to bending stresses as a
result of increased length to diameter ratio (Chughtai
and Zayed, 2007). A sewer line under large bending
stresses is subjected to large deflection hence breaking
leading to sewer leakages. According to Swedish
Association of Local Authorities (1990) foreign objects
in sewers are not permissible because they become
obstacles to the flow of the sewage causing settlement
of solids leading to blockages. The effect of a manhole
without any cover is that the manhole will be
subjected to intrusion from surface water runoffs or
abuse by the residents. Surface water runoff usually
carries twigs, plastics and vegetation of which these
cause blockages in the sewers and it can also lead to
overload of the sewer system during times of floods. It
is expected that the effect of discharge from surface
water runoff will be directly related to the number of
uncovered manholes in the sewer line. Odds of
groundwater infiltration are minimal because the
manholes in the suburb have no visible cracks and the
sewer line route is restricted to dry land. Generally,
about 4% of the sewer system components inspected
failed to meet the required standard, and this is a very
small fraction of the system.
The Operation of Sewerage System
From the results of the questionnaire survey, it was
found that the average population in Coldstream was
10 people per residential unit. Using this average of 10
people/residential unit, the approximate population of
Coldstream was found to be 10000 people and a peak
factor of 3.7 was calculated. Although the monthly
bulk allocation to the suburb of 1800 m
3
translated to
65 litres/residential unit/day, it was found that the
average water consumption was 290 litres/residential
unit/day. This could be attributed to the fact that
residents use water from other sources such as
boreholes to augment the rationed supplies from the
municipality. Assuming 100% return flow, water
consumption of 290 litres/residential unit/day and a
peak factor of 3.7 the wastewater generated in
Coldstream was estimated at 1073 litres/residential
unit/ day. The initial design of the system was based
on a water allocation to residents of 1000
litres/residential unit/day with a return flow of 85%
and the population per residential unit was 6 people.
The system was designed based on self cleansing
criterion. Since the collector main was found to be the
one showing the symptoms of malfunctioning sewer
system, the hydraulic analysis of the system was
focused on the collector main sewer line. Table 3.2
shows the summary of results for the operating
conditions of the collector main sewer shown in figure
3.1. The estimated operating conditions were
compared to the initial design conditions and the
design standards used in Zimbabwe.
TABLE 3.1 CONDITION OF THE SEWER SYSTEM INFRASTRUCTURE
Structure
Parameter
checked
Current observed condition
Manual Design Procedure 5 Standards (Swedish
Association of Local Authorities, 1990)
Manholes
spacing
12% of the manholes are spaced greater than 75m.
The rest are spaced 75mm apart.
Maximum manhole spacing is 75m.
covers 7% of the manholes have no covers.
All manholes are to be provided with covers strong
enough to resist vandalism.
cracks
No signs of cracks were observed inside all the
manholes inspected.
Manholes should be water tight to avoid infiltration of
ground water.
Signs of
overflow
11% of manholes have signs of overflow (see
Figure 3.2).
At all cost sewage overflow should be avoided.
Foreign
material
75% of the non-covered manholes had stones,
twigs and plastic in them.
No foreign material should be allowed to enter
manholes.
Sewers
slope
All sewers are laid at a slope greater than 1 in pipe
diameters.
Minimum slope provided is 1 in pipe diameter.
diameter
Minimum sewer diameters of the reticulation are
150mm.
Minimum pipe diameter for collector mains is 150mm,
and 100mm for sewers at start of a reticulation.
Sewer line
route
The collector main route is free from trees; it is
only covered with grass. The sewer line route does
not pass through any wetlands or swampy areas.
Sewer-line route should be clear from all deep
penetrating tough rooted vegetation.

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However considering the estimated peak flow of 1073
litres/residential unit/day all the sewers have a depth
of flow below the minimum design flow depth of 50%.
This could be attributed to the intermittent supply of
water due to water rationing that is currently in place.
About 68% of the sewer lines had a velocity of flow
less than 0.6 m/s. According to Ackers et al (2001) a
velocity of flow below 0.6m/s is subjected to settlement
of suspended solids resulting in accumulation of
solids in the sewers hence clogging the system.
As shown by the results in table 3.2 the sewer system
in Coldstream high density suburb was initially
designed to self cleanse at peak flows in accordance to
the required standards. The depth of flow in the
sewers was initially designed to be in the range of
50%-70% under peak conditions. According to
Cardoso et al (2004) sewer systems perform best at a
depth of flow between 50%-75%.
The Impact of the Behaviour of Residents on the Sewer
System
The impact of the behaviour of residents on the sewer
system was investigated by finding out how they used
the system and their perceptions on the responsibility
of maintaining the system. When residents were asked
what material they use as toilet paper, 34% said they
use tissue paper, 38% said they use newspaper and
28% said they use any form of paper, as represented in
figure 3.3.
TABLE 3.2 CURRENT OPERATING CONDITIONS OF SEWER SYSTEM VERSUS DESIGN STANDARDS
Parameters
Estimated current
operating conditions
Initial design parameters
Manual Design Procedure 5 design
standards (Swedish Association of Local
Authorities, 1990)
Population per household
10 people
6 people
6 people
Water consumption
290L/residential unit/day
1000L/ residential unit /day
700L/ residential unit /day
Design Peak discharge
(P.F=3.7)
1073 L/ residential unit
/day
3230.4L/ residential unit
/day
Minimum of 2160 L/ residential unit /day
Depth of flow in the sewer 27%-35% 50%-69% 50%-75%.
velocity of flow
Per sewer(m/s)
0.412-1.126
(66.7% below 0.6m/s)
0.706-1.515 self-cleansing velocity 0.6m/s - 3m/s
FIG 3.3 COMMON MATERIALS USED AS TOILET PAPER
A small quantity of newspapers in the sewers has no
effect on the system. But because newspapers do not
dissolve quickly, Ackers et al (2001) suggest that a
large concentration of such suspended solid will settle
in sewer regardless of velocity of flow in the sewers
leading to blockages. Tissue papers unlike news
papers quickly get wet and break into smaller pieces
thereby reducing the chances of blocking the sewers.
On the other hand, any form of paper for example
magazine paper or bond papers takes more time than
newspapers to dissolve in water and once dissolved
they do not disintegrate quickly (Heimlich and
Howard, 1990) hence having the most probability to
cause blockages. Therefore, from results in figure 3.3,
38% of the residents use newspapers and 28% use any
form of paper, hence basically 66% of the residents use
material which is likely to cause blockages as toilet
paper.
Besides using materials that can cause sewer blockages
as toilet paper, it was found out that residents have
got a tendency to dump some of their solid waste in
sewers. The solid waste was dumped in sewers in
three different ways namely through the manhole,
down the drains and flushing in toilet. The majority of
the residents (70%) agreed that solid waste collection
was rare or never in the suburb. Therefore, 13% said
they flush their solid waste down the toilet, 57% said
they let it down the drains, and 23% said they put in
manholes in their yards and 7% said they use other
means for example burning. When the residents were
asked about what materials constituted their solid
waste, their response is shown in figure 3.4.

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FIGURE 3.4 COMMON SOLID WASTES DUMPED IN SEWERS IN
COLDSTREAM HIGH DENSITY SUBURB
Generally, 90% of the residents dump their solid waste
in sewer regardless of how they do it. The list
provided by the municipality of common materials
cleared from blockages in the suburb was as follows;
stones, plastics, condoms, maize cobs, diapers, female
sanitary wear, twig, mopping rags, swiping brooms
and sand. The list was very similar to what the
resident say they put in the sewers for example maize
cobs, condoms, diapers plastic and female sanitary
wear suggesting that what the residents are dumping
in the sewers is contributing to the blockages.
The effects of the solids dumped in sewers vary; and
some of them can cause instant blocking of the sewers
once settlement occurs while other solids need to
accumulate first to cause a blockage and some have an
aging effects on the sewers. Food leftovers are the
most frequently dumped solid in sewer followed by
vegetable cuttings and maize cobs have got an aging
effect on the sewer system. This is mainly because
sulphur oxidising bacteria feed on these organic
materials producing sulphuric acid which corrode the
walls of the sewers. The effect of these materials on
sewer is long term. Instead of the sewers reaching
their design life they will quickly start to burst causing
overflows. Diapers and female sanitary wear absorb
water and expand but do not breakdown hence
reducing the sewer diameter and can also cover up the
whole sewer diameter. A combined percentage of 73%
of residents who dumped diapers and female sanitary
wear is a large population such that the concentration
of these solids might end being too much for self
cleansing velocity as suggested by Ackers et al. (2001).
Once self cleansing velocity is compromised
permanent settlement of the solid occurs clogging the
sewers. Plastics and condoms take long to biodegrade
or may never biodegrade hence end up accumulating
when they are trapped with something in the sewers.
Once these accumulate in the sewers they can lead to a
permanent blockage of the sewers which cannot be
self cleansed by the sewage flowing.
When asked why they dump their solid waste in
sewers, 9% responded that it was the right thing to do
while 34% wanted to avoid piling up of solid wastes
since they were rarely collected, 40% were unaware of
the effect of the action and 17% just did it for no
reason. On the issue of who was responsible for the
sewer system 61% of the respondents said it was the
municipality and 39% said both the municipality and
the residents both have a role to play in the upkeep of
the system. These results reflect a low level of
awareness of the impact of users on a sewer system
and hence possibly explain why they indiscriminately
dump solid wastes which have no place in a sewer
system. This behaviour was compared with that of
residents of Dzivarasekwa a high density area in
Harare (Tanyanyiwa and Mutungamiri, 2011). In
terms of the application of the one of principles of
sustainable sanitation as suggested by Mara et al. (2007)
of institutional appropriateness, this sanitation system
can be said to be unsustainable as the community
failed to partner with the municipality in the operation
and maintenance of the system.
Conclusion
The municipality does not have a planned routine to
maintain the sewer system infrastructure and they
only respond to blockage reports due to a shortage of
labour. The sewer system infrastructure, however, is
still in good condition despite the fact that it has
surpassed its design life. The low wastewater
discharge in the sewer system resulting from the
intermittent water supply fails to generate enough
velocity of flow to self cleanse the system leading to
settlement of most of the suspended solids. Therefore,
the operation of the sewer system in cold stream high
density performs below standard because the system
is not self cleansing as initially designed. It would be
expected that the system should be overloaded given
the increased population, however, this can only hold
true if there is a corresponding increase in water
consumption. In the case of Coldstream, water
consumption has greatly reduced due to water
rationing, therefore, the current malfunctioning of the
system cannot be attributed to hydraulic overloading.
The residents seem unaware of the impact of dumping
their solid waste into the sewers and they feel that
they do not have anything to do with the operation
and maintenance of the sewer system. Therefore, the
sewer system fails to satisfy operating standards
mainly because the residents dump solid waste into
the sewers; and at the same time the system fails to
self cleanse due to the fact that the peak wastewater
generated is too little to generate the required velocity
of flow to self cleanse. Further robust studies on the
hydraulics of the system are required to inform the
municipality on ways to improve the performance of
the system.
It is recommended that the residents should be made
aware of their role as partners in the operation and
maintenance of the sewer system through awareness

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51
campaigns by the municipality, which would reduce
the incidents of solid waste dumping into the sewer
system. Although only 4% of the sewer system
infrastructure was found inadequate, it is
recommended that the municipality rehabilitate that
section of the infrastructure to prolong the life of the
system. Efforts should also be made to improve the
water supply situation in the suburb.
ACKNOWLEDGEMENTS
The authors would like to acknowledge the assistance
provided by the Chinhoyi Town Municipality employees in
the Engineering Department. The cooperation of the
residents of Coldstream high density suburb is also hereby
acknowledged. The results presented here are part of a BEng
research project by Tendai Toma in the Department of Civil
and Water Engineering at National University of Science and
Technology, Bulawayo, Zimbabwe.
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