Investment, operation and maintenance costs (2012) for natural wastewater treatment systems in small communities in Colombia

Abstract

A comparative analysis of investment, and operation and maintenance (O&M) costs, for natural wastewater treatment systems for small Colombian communities (500–30,000 capita) (Colombia, Ministerio de Desarrollo Económico, 2000) was performed, and a set of tools (equations, graphs and tables) were developed to estimate these costs as a support for decision making. This was achieved by a simulation where the climatic variables, water quality parameters, material, man labour and land costs were generated randomly, varying between the researched ranges for Colombia. These input variables were used to design 15 different combinations of natural treatment technologies, such as septic or sedimentation tanks, anaerobic, facultative and maturation ponds, free water surface wetlands and horizontal subsurface flow wetlands. For each one of these scenarios, an investment and O&M budget was estimated, uploading this information into a database performing one million iterations. Comparative cost graphs show the differences between combinations of technologies, influence of inlet principle variables on investment and O&M costs. The variation of O&M costs in natural treatment systems estimated for the different combinations was not significant, but the investment cost varies a lot. Land cost is one of the most important variables. Excavations and waterproofing were two construction activities with important influence on all the combinations. Conformation of wetlands was another construction activity that was significant in those that included wetland. Disinfection with maturation ponds is expensive especially in areas of high land cost.

Full text

European Water 40: 19-30, 2012.
© 2012 E.W. Publications
Investment, operation and maintenance costs (2012) for natural
wastewater treatment systems in small communities in Colombia
J.M. Patiño Gómez1 and J.A. Lara-Borrero2
1 Research group Science and Engineering of Water and Environment, Department of Civil Engineering, Javeriana University,
Bogotá D.C., Colombia, email: patino.jose@javeriana.edu.co
2 Department of Civil Engineering, Javeriana University, Bogotá D.C., Colombia, email: laraj@javeriana.edu.co
Abstract: A comparative analysis of investment, and operation and maintenance (O&M) costs, for natural wastewater treatment
systems for small Colombian communities (500–30,000 capita) (Colombia, Ministerio de Desarrollo Económico,
2000) was performed, and a set of tools (equations, graphs and tables) were developed to estimate these costs as a
support for decision making. This was achieved by a simulation where the climatic variables, water quality
parameters, material, man labour and land costs were generated randomly, varying between the researched ranges for
Colombia. These input variables were used to design 15 different combinations of natural treatment technologies,
such as septic or sedimentation tanks, anaerobic, facultative and maturation ponds, free water surface wetlands and
horizontal subsurface flow wetlands. For each one of these scenarios, an investment and O&M budget was estimated,
uploading this information into a database performing one million iterations. Comparative cost graphs show the
differences between combinations of technologies, influence of inlet principle variables on investment and O&M
costs. The variation of O&M costs in natural treatment systems estimated for the different combinations was not
significant, but the investment cost varies a lot. Land cost is one of the most important variables. Excavations and
waterproofing were two construction activities with important influence on all the combinations. Conformation of
wetlands was another construction activity that was significant in those that included wetland. Disinfection with
maturation ponds is expensive especially in areas of high land cost.
Keywords: domestic wastewater, investment costs, operation and maintenance costs, natural systems, Colombia
1. INTRODUCTION
According to the Vice Ministry of Water and Sanitation, in 2008 Colombia was only treating 9%
of the wastewater, because over 50% of constructed treatment plants were not working
properly (Guerrero, 2010). Some of the problems related to the malfunction of the treatment
systems are that technologies do not follow a logical sequence, many plants are incomplete, there is
very poor maintenance and control of the processes taking place in plants and/or recommendation
of experts are only on familiar technologies (Guerrero, 2010). In Colombia, there is a need to
develop a support tool for decision making, based on the most efficient and economical
technology that best suits the characteristics of each community; the resources of small
communities in Colombia are limited, and a poor choice of technology would have a major impact
on the budget. Decisions on the more efficient use of the appropriate technology, are often made not
on the basis of the general benefit but are dictated by various ‘factors’, which often result in wrong
directions and decisions and of course high cost installation (Tsagarakis et. al., 2003). In the design
of small wastewater treatment plants, the most important aspect is the choice of the best solution
with respect to limited financial resources considering the capital, O&M costs and the effluent
quality requirements which have to be met (Siber, 1991). A decision making support tool would
help significantly at the feasibility stage of a project to know the magnitude of the initial investment
costs and thus select the technology that best adapts to the financing scheme and the needs of each
population.

20 J.M. Patino Gómez & J.A. Lara-Borrero
2. MATERIALS AND METHODS
2.1 Bibliographic research
Complete bibliographic research was performed on wastewater generated by small communities,
Colombian regulations framed under the care of the environment and sustainable use of water
resources, natural systems for domestic wastewater treatment, intervals of the input variables in
Colombia, such as temperature, net evaporation, suspended solids, BOD, fecal coliform, Helminth
eggs, organic load per capita, and materials, machinery, labor and land costs.
2.2 Selection of technologies and combinations
To estimate costs it was necessary to define the combination of technologies in what is called
treatment trains. All trains were designed using preliminary treatment with screening grids, sand
trap and Parshall flume. Primary treatment varies between anaerobic pond (AP), septic or
sedimentation tank (ST) and primary facultative pond (pFP).Secondary treatment varied between
facultative pond (FP), free water surface wetland (FWS) and horizontal subsurface flow wetland
(HSSF). Maturation pond (MP) was used for tertiary treatment, as recommended by the World
Health Organization (WHO), when the effluent is to be used in agriculture or aquaculture, as it is
effective in removing helminth eggs (WHO, 2006). The 15 defined combinations were: 1.pFP
2.AP-FP, 3.AP-FWS, 4.AP-HSSF, 5.AP-FP-FWS, 6.AP-FP-HSSF, 7.AP-FWS-HSSF, 8.AP-HSSF–
FWS, 9.TS-FP, 10.TS-FWS, 11.TS-HSSF, 12.TS-FP-FWS, 13.TS-FP-HSSF, 14.TS-FWS-HSSF,
and 15.TS-HSSF-FWS. The effluent from ponds is filtered through a rock bed before entering
wetlands to prevent clogging.
2.3 Developing simulation algorithm
The simulation consists of an algorithm in computational language PHP (Free Software and open
source license www.php.net). PHP is an open source server-side language designed for web
development to produce dynamic web pages. The code is interpreted by a web server with a PHP
processor module which generates the resulting web page.
The simulation contains different functions that generate the input variables randomly, designed
the treatment trains, estimate investment and O&M costs with and without disinfection. When the
iteration finished the output variables were uploaded to a MySQL database (Free Software and GPL
license www.mysql.com). The SQL phrase stands for Structured Query Language. MySQL is a
popular choice of database for use in web applications, and can be connected with programs to
analyze data.
In order to perform a comparative analysis between technologies for Colombia, it was necessary
randomly generate a dataset that represent most of the possible combinations for the Country. To
establish the amount of data needed, was necessary to iterate and to analyze the dispersion of the
generated data. The sum of the total costs of treatment trains was calculated for each iteration.
These results were recorded in the database, and the standard deviation of the data was checked, in
order to determine where it began to vary in a small percentage. This was stabilized at a value lower
than 0.1% after 300.000 iterations, which means that from this point the dispersion between the data
began to be stable and have an important content of data between their ranges. With this in mind it
was decided to make 1 million iterations.
The generation of random numbers for input variables was considered between researched
intervals applied to Colombia, as shown on Table 1. Water quality parameters were generated with
a normal distribution as the others with uniform distribution, because typical values are normally in
the center of the range, as the others with uniform distribution because any value has the same

European Water 40 (2012) 21
probability. Designs were performed with general models of design, considered the most
appropriate for the conditions of the country. These are the models used in each technology:
preliminary treatment (Rolim 2000), septic tank (EPA, 2002; Romero, 2004), ponds (Mara, 2003)
and wetlands (Kadlec & Wallace, 2009).
It was necessary to consider a standard scenario in order to estimate quantities of work,
materials and land. There were several considerations: The terrain level must be below the sewer
level to operate by gravity. To avoid excesses in excavations the terrain was considered relatively
flat. The plant is located in a terrain with an organic layer of 30 centimeters, which needs to be
removed. Treatment units are considered to be excavated in the terrain under the topsoil, using the
removed material to build dikes around each treatment component, as well as protective barriers
around the property. The material below the top soil allows slopes 1: n (Vertical: Horizontal), with
n varying randomly between 0.5 and 3. The project site was considered to have a tertiary road
within 30 km from the urban area allowing proper machinery and materials transport to the project
site. In all cases, waterproof geomembrane was used, which requires a compacted soil layer of at
least 10cm, except septic tanks for which waterproof reinforced concrete was used.
Table 1. Inlet variables and limits.
Variable Un Min Max Distribution Reference
Population IE 500 30,000 Uniform
Average temperature of coldest month oC 7 26* Uniform (IDEAM, 2005)
Precipitation mm/year 500 12,000 Uniform (IDEAM, 2005)
Evaporation mm/year 600 3,700 Uniform (IDEAM, 2005)
Land cost €/m2 1.30 77.82 Uniform (ASOLONJAS, 2011)
Organic load per capita q/capita.day 39 54 Normal (Mara, 1976)
Suspended solids mg/l 100 350 Normal (Metcalf & Eddy, 1991)
BOD5 mg/l 110 400 Normal (Metcalf & Eddy, 1991)
Fecal coliforms FC/100ml 106 109 Normal (Metcalf & Eddy, 1991)
Helminth eggs No/l 300 600 Normal (Metcalf & Eddy, 1991)
% effect of construction activity % 0 100 Normal
* Two regions in Colombia present average temperature in the coldest month higher than 25oC (La Guajira and San Andres islands)
For O&M were considered 20 years at constant prices. In O&M activities, mechanical sludge
removal was considered only for facultative and anaerobic ponds. The work models quantities were
developed independently for each component of the train in activities chapters such as preliminary
works, excavations, concretes, metal carpentry, waterproofing, landfills, interconnections, external
works and wetlands conformation.
The unit costs of each of the activities of these chapters were researched for the different regions
of the country, as shown on Figure 1. This research was developed using different sources, such as
construction companies, oil companies, construction costs software, government data, hardware
stores in different towns, among others.
With all of this information, costs limits for all construction activities were estimated; this
allowed the algorithm to generate a random cost for each activity between research ranges covering
all regions of the country.
Costs were calculated considering two scenarios, with and without disinfection in order to
analyze the economic impact in each technology when it is applied with maturation ponds.
O&M costs were divided into three categories, personnel direct costs, other activities and
material direct costs and indirect administration costs. For each of these categories tables were
made for the cost calculation. This was prepared for different scenarios of BOD, temperature,
organic load per capita and population. In these tables, graphs were generated for the different cost
categories in function of the population, in order to develop regressions that represented these costs
for different trains, considering the variation in the population, per capita organic load and BOD.
In the next figure one can see the general process of data generation by the algorithm.

22 J.M. Patino Gómez & J.A. Lara-Borrero
Figure 1: Map of Colombian Regions and principal cities
Sta
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Random values generator for the input variables
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Sizing of systems without disinfection
a
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cost
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Investment and O&M costs calculation
for systems without disinfection
Sizing of disinfection systems
and unit cost calculations
Investment and O&M costs calculation
f
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Figure 2: Diagram of the algorithm
3. RESULTS AND DISCUSSION
Once the entire methodology and processes were exposed the results analysis were presented.
This analysis intended to perform a comparison between treatment trains without giving the exact

European Water 40 (2012) 23
total cost of the plant, since factors like guard house/office, indirect construction costs such as
administration, unforeseen and utility, and auditing were not included in the analysis considering
them inherent to the technology.
The median of the investment, O&M costs of treatment trains without disinfection are presented
in Figure 3. This shows that treatment trains 9 to 15 with septic tank in primary treatment, present
the higher costs in contrast to trains with anaerobic pond due to the usage of reinforced concrete, the
general model design considered a minimum of one day of hydraulic retention time for both
tecnologies and anaerobic pond present a higher depth. Train 4 presents the lower costs. O&M costs
are very similar for all treatment trains.
All the data set of the initial investment costs of treatment trains without disinfection is shown in
Figure 4. These correspond to the sum of direct construction and land costs. This confirms the
higher costs that present trains with septic tanks as shown in Figure 3. The disadvantage of
anaerobic pond is the generation of odors in the surrounding area, making the septic tank an option
for scenarios where the plant can only be located near the population.
Land cost is a very important variable, since it represents an important percentage of the initial
investment costs, and become very high when land cost is higher than € 30 per m². Other study
support this hypothesis, arguing that waste stabilization ponds are cost effective when cost of land is
less than US$ 30 per m² subjected to minor earthworks (Tsagarakis et. al., 2003).
The figure also shows that the train 1, which only has primary facultative pond, presents high
costs, resembling the behavior of systems with septic tank on primary treatment. This behavior is
due to the design of the primary facultative pond, because it must maintain a BOD surface charge to
prevent this from becoming anaerobic, thus increasing size and costs. Train 2 is more efficient than
train 1, a similar conclusion also presented by Mara (2003), where examples of calculation show
that anaerobic-facultative pond train is more efficient than primary facultative pond, since it
requires less area to present the same removal efficiency of BOD.
Trains with anaerobic ponds in primary treatment and wetlands on secondary have the lowest
average and 25 percentile. One can generally conclude that these trains are more economically
viable, since they also have the lowest 75 percentile and maximum limit among all treatment trains.
The same behavior is seen in trains with septic tanks in the primary treatment and wetlands on
secondary, but as mentioned above, trains with anaerobic ponds in the primary treatment have lower
costs in comparison to the ones with septic tanks.
0
1,000,000
2,000,000
3,000,000
4,000,000
5,000,000
6,000,000
7
,
000
,
000
CostsinEuros
Treat men tTrain s
O&Mcosts20years InitialInvestmentcosts
1234567891011 12131415
Figure 3: Median of Investment, O&M costs of treatment trains without disinfection
Trains that have two series systems on secondary treatment were designed according to the
organic load that must be removed (70% faculty and 30% wetland). These generally show a lower
cost compared to the treatment train that only has secondary facultative ponds, showing that
wetlands are systems that can improve efficiency and reduce initial investment costs. Similarly, we
see that the trains that have two serial wetlands present lower costs than those that include pond and

24 J.M. Patino Gómez & J.A. Lara-Borrero
wetland when primary treatment is an anaerobic pond, while the costs are similar when primary
treatment is a septic tank. Train 4 presents the smallest initial investment costs of all trains. For
trains that have septic tanks as primary treatment, the combination with subsurface flow wetland is
the one with the lowest costs among them. This indicates that the subsurface flow wetland is the
most efficient secondary treatment, analyzing all the data set.
Comparing Figure 4 and Figure 5, we can conclude that including disinfection increases the
magnitude of the costs in average for all trains in approximately 3 times. This shows that including
this phase with maturation ponds can increase costs in large proportions, although this is required
when the effluent from the plant is used for example in agriculture and the benefits may outweigh
the costs. Note that there are other disinfection systems and it is important to assess them when it is
better to use other technologies. In Figure 5, one can also see that differences between trains with
septic tanks in primary treatment are not as big as trains without disinfection.
Figure 4: Initial investment costs of treatment trains, without disinfection.
Figure 5: Initial investment costs of treatment trains, with disinfection.

European Water 40 (2012) 25
Figure 6 shows the median percentages of cost structure in the chapters of construction activities,
without disinfection. Here, one can see the importance of construction activities in the total
construction cost. The preliminary chapter has less than 5% of influence for all trains, indicating
that this activity is important.
0
10
20
30
40
50
60
70
80
90
100
Chapterrate(%)
TreatmentTrains
1.Preliminaries 2.Excavations 3.Concretes
4.Metalcarpentry 5.Waterproofing 6.Fillings
7.Interconnections 8.Externalworks 9.Wetlandsconformation
1234567891011 12131415
Figure 6: Costs structure - Chapters construction activities, without disinfection
Trains 1 and 2 have significant percentages in the chapter excavations, it reaches 55%
approximately, where mechanical excavation and stripping activities are the most important;
generally for all trains this activity represents an important percentage. This is one of the most
important chapters in the trains that have anaerobic pond in primary treatment, as concrete is the
most significant in the ones with septic tanks. Waterproofing is an activity that must be taken into
account as it is presented on all trains at a rate greater than 5%, reaching 30% in train 1 and 2. Metal
carpentry, fillings and interconnections are activities that represent less than 5% together, indicating
that are not considerable activities. External works, such as closure of the plant, rainwater gutters,
road material and compacting and planted grass is a chapter that is relatively important. The
wetland conformation is an important chapter in the trains that contain it, but these have reduced
percentage in excavations, because wetlands are designed with a lower depth.
The median percentages of cost structure for the chapters of construction activities are presented
in Figure 7, with disinfection. In comparison with Figure 5, the inclusion of disinfection increases
the incidence percentage of activities. Preliminary works percentage approximately increases by 2%
in all trains, excavations and waterproofing increase in an important percentage in all trains, while
concrete, metal carpentry and wetlands conformation decrease. Fillings, interconnections and
external works increase in small percentages.
As mentioned previously O&M costs were similar between trains, because direct and indirect
costs are equal between trains and other direct costs of materials and activities vary in a small
percentage. The differences found are sludge removal activities and some materials and tools. It is
important to mention that generally in wetlands sludge removal is not realized between O&M
activities, but do not have a greater impact.
Per capita costs for treatment trains without disinfection are presented in Figure 8 and with
disinfection in Figure 9. The cost of domestic wastewater treatment per person increases quite

26 J.M. Patino Gómez & J.A. Lara-Borrero
sharply as one deals with population smaller than 50,000 (Siber, 1991). As mentioned before the
inclusion of disinfection increases the total costs in an important percentage, so cost per capita also
increases in a high rate as can be seen comparing Figure 8 and 9.
0
10
20
30
40
50
60
70
80
90
100
Chapterrate(%)
TreatmentTrains
1.Preliminaries 2.Excavations 3.Concretes
4.Metalcarpentry 5.Waterproofing 6.Fillings
7.Interconnections 8.Externalworks 9.Wetlandsconformation
1234567891011  12131415
Figure 7: Costs structure - Chapters construction activities, with disinfection
Figure 8: Costs per capita for treatment trains without disinfection

European Water 40 (2012) 27
Figure 9: Costs per capita for treatment trains with disinfection
In Figure 9 one can see that including disinfection with maturation ponds, the train 2 presents the
lower median cost per capita and train 14 the highest. We can also see that dispersion between data
is higher in all trains.
Figure 10: Total surface area required for the plant without disinfection

28 J.M. Patino Gómez & J.A. Lara-Borrero
Figure 11: Total surface area required for the plant with disinfection
Figure 12: Per capita surface area without disinfection

European Water 40 (2012) 29
Figure 13: Per capita surface area with disinfection
Figure 10 and 11 shows information related to the area required for the treatment for each train.
One can see that without disinfection train 4 (AP-HSSF) presents the lower average land
requirements with important differences with other trains. Furthermore, to include disinfection
equals costs between trains and increase them in a significant way.
Likewise, Figure 12 and 13 shows the per capita area required for each train. One can see
important differences between trains being train 4 (AP-HSSF) the one that needs less area per capita
in average by not including disinfection. But as before, to include disinfection equals the per capita
land requirements for all the trains.
An estimating costs tool was developed in PHP. This allows the user to enter the inlet variables
and calculate the investment, O&M costs of all trains and see them on a bar graph. The tool can be
found on the webpage: http://www.somosunservivo.org
4. CONCLUSIONS
The presented analysis revealed important differences between natural systems on initial
investment costs, while O&M costs were very similar. This shows that the O&M cost is not a
determining factor in decision making on which natural system is more economically viable.
The most important variation found in the initial investment costs between tested trains, is the
increase in costs that have for primary treatment a septic tank designed in reinforced waterproofed
concrete and with a design model that considers one day of retention time. This shows that the
septic tanks are to be used only when environmental conditions require, for example, when the plant
can only be located near the town and cannot generate odors.
Trains 1 (pFP) and 2 (AP-FP) presents the lowest costs for disinfection for the entire population
range under analysis, suggesting that if disinfection is required, because the effluent from the plant
is to be reused, for example in agriculture, this is the most economically viable train.

30 J.M. Patino Gómez & J.A. Lara-Borrero
Trains with anaerobic ponds and wetlands present the lower costs, being train 4 (AP-HSSF) the
one with the lower median costs. This train presents also less total land requirement and less per
capita area requirement.
Trains which have two systems in secondary treatment present lower costs than the ones with
only one system, especially when both are wetlands. This makes wetlands a very good secondary
treatment option, because of its efficiency, landscape improvement and creation of animal habitat.
Disinfection with maturation ponds is expensive, because their costs can be three times greater
than without it. Construction chapters such as preliminary works, excavations, waterproofing and
external works increase percentage of influence in construction costs structure when disinfection is
included. Other technologies must be considered when land costs, materials and labor are high.
The two categories of initial investment cost are very important; since land cost is a very
important variable and natural systems are extensive systems with large area requirement. Direct
construction costs are also very important, representing a significant percentage of the total cost for
30% of the activities.
Excavations and waterproofing are activities that have a high influence percentage in direct
construction costs, especially for trains with anaerobic pond in primary treatment. They remain
important for those with septic tanks, but in these the most significant is concrete. For systems that
have wetland, the conformation of this represents a significant cost. The percentage of external
works and preliminary works is similar to the different trains and are important. Costs of machinery
for excavations, costs of materials and man labor for concretes, waterproofing and external works
are very important variables in total costs.
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ASOLONJAS, 2011. Colombian Association of Colleges and estate auction (ASOLONJAS), Bogotá D.C., Colombia.
Colombia, Ministerio de Desarrollo Económico, 2010. Resolución No 1096 de 2000 por la cual se adopta el reglamento técnico para
el sector de agua potable y saneamiento básico - RAS. Diario Oficial No. 44.242, Bogotá.
EPA, 2002. Onsite Wastewater Treatment Systems Manual, U.S. Environmental Protection Agency (EPA), United States.
Guerrero, J., 2010. Sanitation in Colombia and Latin America, International Conference: Constructed wetlands for wastewater
treatment: Knowledge transfer to Latin America, Technological University of Pereira, Colombia.
IDEAM, 2005. Colombia Atlas Climatological, Institute of Hydrology, Meteorology and Environment Studies of Colombia
(IDEAM), Colombia.
Kadlec, R. & Wallace, S., 2009. Treatment Wetlands, CRC Press, London.
Mara, D., 1976. Sewage Treatment in Hot Climates, John Wiley & Sons, London.
Mara, D., 2003. Domestic Wastewater Treatment in Developing Countries, Earth scan, London.
Metcalf & Eddy, 1991. Wastewater Engineering: Treatment, Disposal and Reuse, McGraw Hill, United States.
Rolim, S., 2000. Systems Stabilization Ponds, McGraw Hill, Bogotá D.C., Colombia.
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