A conceptual model for decentralized municipal wastewater management

Wastewater pollution problems are associated with population growth and concentration in large urban centers. The historical growth trend on a global level indicates that the population doubled in size over a period of 40 years, from 1950 to 1990, going from 2.6 billion to 5.2 billion. The mean projection is that by 2050 the population of currently developed countries will be about 1.2 billion, while that of currently less developed countries will be about 7.8 billion (Daigger 2007).

The conventional approach allows use of drinking water for irrigation, laundry, and toilet flushing, even though these actions do not require high standards of physicochemical and microbiological quality. Regarding wastewater treatment and disposal, there is a high percentage (more than 90%) of the population in developing countries connected to centralized treatment systems (Metcalf & Eddy 1995). However, decentralized systems are gaining special interest in reducing treatment costs in the long term and the potential for reuse wastewater (Daigger 2009).

Although the three basic components of a wastewater management system are collection, treatment, and disposal where collection is less important than treatment and disposal. Nonetheless, collection costs represent more than 60% of the total budget for wastewater management in a centralized system (Massoud et al. 2009).

Understanding a decentralized treatment system as one in which raw wastewater is treated as close as possible to the source (Libralato et al. 2011), this approach becomes a viable, and necessary alternative for wastewater management, minimizing environmental impacts and facilitating resource recovery (Nhapi 2004). The focus of decentralization is on the development of more financially sound, more socially responsible and environmentally benign systems than conventional centralized systems (Burkhard et al. 2000; Nhapi 2004), filling the gap between on-site systems and conventional centralized systems.

The implementation of decentralized systems implies a different planning manner, where feasibility, design, and implementation must be performed by independent service areas, with particular contexts and whose solutions must respond to their individual needs, considering heterogeneity within the same urban center, where social, environmental, geographical, economic, and technological conditions can vary widely (Liang & van Dijk 2008). Decentralized systems possess the advantage of being easily adaptable to local conditions in urban areas, as well as of expanding their capacity in line with population growth. This decentralized approach has a greater potential for the reuse of water and treatment by-products such as nutrients, sludge, and energy.

For the purpose of identifying the key aspects of decentralization and the potential of conventional and natural wastewater treatment technologies in a decentralized or centralized scheme, a survey was conducted among professionals from universities, utility companies, environmental agencies, and consulting firms, and the results were prioritized using the Ranking and Rating methodology. In the first, a score is assigned to each item to reflect its importance degree on a scale of one to nine (Singhirunnusorn 2009). In the Rating method, the decision items are assigned a score of between 0 and 100. After assigning a score by both Ranking and Rating Methods, the relative weight and combined weight for the decision indicators were calculated (Macoun & Prabhu 1999).

In the second part of the methodology a minimum cost analysis was carried out of three case studies in municipalities in the Valle del Cauca Department (in the southwest region of Colombia) such as Buga, Jamundí, and Florida. Consideration was given to investment and operation and maintenance costs of main sewerage collectors and the wastewater treatment system.

Finally, using the previous outcomes, a conceptual model was proposed for selecting the centralization level for wastewater management, which encompasses a Hierarchical Analysis of Alternatives (Saaty 1994) including technological, environmental, social, and institutional criteria. This was followed by a Cost-Benefit Analysis of prioritized alternatives (Chen & Wang 2009) considering the benefits associated with a decentralization approach in each case.

From the perspective of economic and institutional aspects, it is evident that the success with the implementation of a decentralized management plan in the context of urban areas in Colombia will depend, to a large extent, on institutional support. Decentralization is potentiated for peri-urban and expansion areas that do not have any wastewater treatment coverage, and this factor goes hand in hand with the interest that companies may have on the administration, operation, and maintenance of the system.

Urban water management is visualized in a segmented way, and the relationship between sewage and treatment costs is not associated. One of the main competitive advantages of decentralization is that, by reducing the costs of investment in sewage, these can be transferred to investment in treatment, which reflects on higher treatment coverage, reduction of impacts by untreated sewage pollution, and a decrease in the risk of diseases associated with contaminated water. Three technological indicators were defined, namely, reuse potential, centralization level, and sewerage coverage. The relative weights for the three indicators were in the same proportion, indicating that they have the same importance for planning a project with a decentralized approach.

In relation to the reuse potential, it was found that the reuse for landscape/urbanistic purposes had the highest score, followed by the types of reuse in agriculture and energy recovery with a similar score (Caicedo & Bernal 2014). For social aspects, community participation in administration, operation, and maintenance of the system and reuse acceptance are key criteria that are consistent with what the findings from the review of experiences on a global level, where community participation plays a decisive role for the success of any decentralization project in wastewater management, as well as the implementation of reuse, which must be accepted by users. Table 1 shows the combined weights of the indicators included in the survey.

An assessment of the potential to implement conventional and natural technological alternatives in centralized and decentralized systems revealed that the most applicable technologies for a centralized scheme are: activated sludge (15.31), followed by UASB reactors (14.23), ponds (12.09), artificial wetlands (9.94), ponds with plants (9.58), septic tanks (7.07), and drain field systems (4.57). Likewise, the weighting for the same technological offering varies when its applicability for a decentralized scheme is evaluated, in accordance with the values, the natural treatment alternatives have a greater potential to be implemented in a decentralized wastewater management scheme, as there are septic tanks (15.11), UASB reactors (12.91), constructed wetlands (12.91), ponds systems (12.24), ponds with plants (11.22), trickling filters (10.72), drain fields (10.04), biodiscs (7.85), and activated sludge (7.00) (González & Bernal 2014). As the level of centralization increases, the potential for implementation of conventional treatment alternatives increases too.

Related to four centralization levels (i.e. on-site, decentralized, semi-centralized, and centralized), it was found that conventional technologies, such as activated sludge and UASB reactors, are more flexible to be located on different levels of centralization, taking into account that its optimal operating flow rate has a wider range. However, there is a trend for conventional technologies to be implemented in semi-centralized and centralized wastewater management schemes. This is contrary to what happens with natural treatment technologies, the application of which increases as wastewater management is decentralized, which is associated with the high area requirements for its implementation, the operational simplicity, and low costs associated with these systems.

An assessment of the reuse potential linked with three objectives shows that it primarily favors agriculture followed by urban reuse and energy recovery. Likewise, there is a more direct relationship among agriculture reuse and natural treatment alternatives than with conventional technologies. According to the results of the survey, the natural treatment technologies registered an acceptance rate for reuse in agriculture of 51%, urban uses of 38.85% and of only 10.07% for energy recovery. In the case of conventional alternatives, the potential for reuse in agriculture was 43.10%, for urban reuse 29.31%, and 27.59% for energy recovery.

The greatest potential for reuse in agriculture was presented by natural treatment systems, such as ponds (17.4%), artificial wetlands (14.9%), ponds with aquatic plants (11.6%), and drain fields and septic tanks (7.4%). To the conventional treatment alternatives, the same potential for reuse in agriculture was identified in the activated sludge, trickling filters and biodiscs (10.7%) followed by UASB reactors (9.1%). The same trend was observed for urban reuse, where the greatest application is also linked with natural treatment alternatives, ponds, and artificial wetlands (20.5%), ponds with plants (13.6%) and drain fields and septic tanks (3.4%). For energy recovery objective, the UASB reactor with 45.7%, followed by activated sludge with 15.2% and pond systems with 13.0%, is the most applicable technology. The other technologies were rated with applicability percentages below 8%.

An economic analysis was carried out of three case studies of the municipalities of Buga, Florida, and Jamundí, located in the Valle del Cauca department in the southwestern region of Colombia. Estimates were made of the investment costs of the treatment system (including land costs) and the cost of the main sewage collectors. These costs were used for the application of the conceptual model.

Table 2 shows the configuration options of wastewater management in the municipalities of Florida, Jamundí, and Buga. In all three cases there is a totally centralized option and other options with different centralization levels, such as decentralized, semi-centralized or in situ. As long as the level of centralization decreases, the need for large sewage collectors is also reduced. As a result of the economic analysis of the three cases, it was found that for the municipalities of Buga and Jamundí the centralized option exhibits the highest cost, with a NPV (Net Present Value) of USD 5,749,148¹ and USD 43,610,402¹, respectively. For Florida, the highest cost was for the most decentralized option, which proposes three treatment systems with an aggregate value of USD 6,982,152¹.

For the three municipalities, the lowest cost option involves decentralization. In the case of Buga the least expensive scheme was the semi-centralized one; for Jamundí, the lowest cost option is the most decentralized one (option 3); otherwise it was found for Florida, where the most decentralized option was the most expensive one (option 3), but the semi-centralized alternative (option 2) presented the lowest cost.

In the case of Buga, a cost analysis was performed by comparing systems with the same wastewater treatment technology, and in the case of Jamundí and Florida, the same treatment technology is also included for existing urban areas, but other technologies are considered to provide a solution in expansion areas based on land availability.

The conceptual model was developed to select the centralization level for municipal wastewater management. This model is based on the Hierarchical Analysis of Alternatives, considering the technological, environmental, social, and institutional criteria and 12 subcriteria to prioritize the alternatives.

In order to assign the weights to the criteria in the matrix, it was considered that 47.8% of the key defined aspects are technological, 21.7% social, 17.4% environmental, and 13.1% institutional in nature. Based on the combined weights obtained, it is defined that technological criteria have a strong importance above environmental criteria and moderate importance above social and institutional criteria. Social criteria have a greater importance to institutional criteria and a moderate importance over environmental criteria. The weights presented above for each sub-criterion were assigned according to the combined weights obtained from the expert surveys. The proposed conceptual model is shown in Figure 1.

The second part of the conceptual model consists of the Cost-Benefit Analysis. The costs and benefits included and quantified in the model are presented in Table 3 below.

A validation of the model was carried out using the three case studies. Table 4 lists the weights obtained with AHP method and results of the Cost-Benefit Analysis for a project period of 30 years. In the three cases, the lowest weights and the lowest cost-benefit ratios were always obtained for the centralized management options, which even in the cases of Jamundí and Florida are not economically viable. Options 2 and 3 that correspond to different configurations of decentralized schemes were rated feasible. In the case of Florida, the two decentralized alternatives were rated feasible.

With the application of the model, the importance of the main sewage collectors and the distance to the treatment system were identified. For the three cases, the centralized option with long pipe distances was rated last in the prioritization given by the model, some of them were not viable considering that the cost/benefit ratio was lower than 1. The above also defines that the main benefit for the conceptual model is associated with the savings in piping that is not installed when planning decentralized options.

As mentioned before, the operation and maintenance costs are included in the NPV obtained, being for this study in particular, that the decentralization is favored. However, it is necessary to study in depth the scale of economy of decentralized approach. It can be considered the centralized wastewater management corresponds to the best option from the point of view of per capita treatment costs. Nevertheless, when there are high population densities in peri-urban areas, located at great distances from centralized systems, this economy of scale disappears, making that the centralized systems require large investments that can be saved and regarded as a social benefit of decentralized water recycling system (Liang & van Dijk 2008).

Another important benefit to consider is the impact of the effluent of the wastewater treatment on the receiving body. For the case studies of Jamundí and Florida different discharges cause a lesser impact on the concentration of dissolved oxygen than only one discharge. This aspect is very important from an environmental point of view and is not commonly taken into account in wastewater management planning in urban areas. An examination of the results of the cost-benefit analysis makes it possible to identify that viable alternatives are those in which this benefit is considered, and do not necessarily constitute the options with higher investment and operating and maintenance costs.

The conceptual model has gained importance as an instrument for wastewater management planning in urban areas. This model includes the key aspects to be considered on the technological, environmental, social, institutional, and economic dimensions. It offers the possibility to select and plan, considering a wide range of relevant criteria (not only economic criteria) that can influence the selection of a centralized or decentralized scheme. There are two key criteria for the selection of the centralization level for wastewater management, namely, the distance of the treatment point and the impact of discharges on the receiving bodies. An assessment of the benefits associated with these two criteria shows that they are determining to the results provided by the conceptual model.

The proposed conceptual model is conceived as a dynamic model that can provide additional benefits depending on each particular case. However, when it is proposed that the second part of the conceptual model is a cost-benefit analysis, it is believed that there are benefits associated with decentralization that should not be overlooked and that are determining to the viability of a particular alternative, savings from piping length not installed, and the impact of discharges on the receiving bodies.

Although validation of the model was performed for three municipalities in the Valle del Cauca, the criteria and methodology used in the model can be used at other locations, presenting a great potential of application in medium and small municipalities; taking into account, the low wastewater treatment in Colombia and the high investment costs required in the traditional planning associated with implementation of sewage systems of large diameters.

Special thanks to COLCIENCIAS (Administrative Department of Science, Technology and Innovation of Colombia) for providing financial support to my doctoral studies.