Integrated assessment of decentralized wastewater treatment plants in a semi-arid region in Bolivia

The rapid urban and industrial growth has intensified sanitation challenges, diminishing access to water resources. This scarcity adversely impacts economic growth, living standards, and environmental sustainability. The agricultural sector, in particular, has resorted to using treated and untreated domestic or municipal wastewater to fulfill its needs (Dalezios et al. 2018).

Globally, around 360 km³ of domestic wastewater is produced annually. Approximately 11.4% is treated and reused, 41.4% is treated and discharged, while 47.2% is released untreated (Jones et al. 2021). Effective wastewater treatment is essential for safe reuse and reducing environmental impacts (Rodriguez et al. 2020).

Worldwide, a variety of technologies have been implemented for the treatment of municipal wastewater. Effective wastewater management is well-established in developed countries but is still limited in developing countries. In developed countries like the USA, WWTPs generally follow a standard design comprising primary clarifiers, aeration processes, and secondary clarifiers, with slight variations (Mason et al. 2016). In China, the most common treatment processes include the anoxic–anaerobic–oxid (AAO) and its variants, covering 61.88% of WWTPs, followed by oxidation ditches and sequencing batch reactors (SBRs) with a share of 22.27%. The third most popular process is the SBR and its modified processes, with a share of 8.99%. Only 6.86% of WWTPs in China use other operating processes (Zhang et al. 2021).

Latin American WWTPs predominantly use lagoon treatments and activated sludge processes, with 38 and 26% utilization, respectively. Other methods include up-flow anaerobic sludge blanket, UASB (17%), aerated lagoons, wetlands, and trickling filters (Noyola et al. 2012). In Bolivia, stabilization ponds and various anaerobic processes like Imhoff tanks and septic tanks are prevalent (MMAyA 2020). Recently, trickling filters have been adopted for centralized municipal wastewater treatment.

Decentralized sanitation systems have gained traction in Bolivia's rural and peri-urban regions for populations of 2,000–20,000. In the Cochabamba department, decentralized WWTPs with anaerobic–aerobic configurations have been implemented, such as UASB followed by horizontal gravel filters (Saavedra et al. 2019), UASB combined by biological filters and stabilization ponds (Cossio et al. 2018; Mercado Guzmán et al. 2020), UASB-activated sludge and ABR followed by hybrid biofilters (Echeverría et al. 2022; Aguatuya 2023a). Similar plants are also operational in the Tarija department (Aguatuya 2023b).

Choosing the best technology for a specific context is challenging. This selection goes beyond the consideration of a cost-effective technical solution (Kalbar et al. 2012) requiring consideration of technical, economic, environmental, and social factors. The decision-making process also involves geographical and technological complexity aspects, with the aim of selecting a reliable technology that meets water quality standards and is socially acceptable (Leverenz & Asano 2011). Adopting a suitable WWTP requires evidence-based decisions from similar full-scale systems operating under real conditions in specific contexts.

To ensure the sustainability of treatment systems over time, it is fundamental to adopt a holistic approach in the technology selection process (Arroyo & Molinos-Senante 2018). The consensus among various scholars underscores the importance of integrating technical, economic, social, and environmental dimensions in the planning and sustainability assessment of treatment systems (Castillo et al. 2017). Additional dimensions like institutional robustness (Cossio et al. 2020), and legal considerations (Flores-Alsina et al. 2010) are also recognized as vital to sustainability.

Globally, numerous studies on full-scale treatment plants employ diverse approaches and metrics. For instance, Kalbar et al. (2012), Fighir et al. (2019), and Brault et al. (2022) analyze both technical (reliability, durability, replicability) and environmental aspects (global warming potential, eutrophication potential, land use, effluent, and sludge quality). Popovic et al. (2018) focus on social dimensions, including acceptability and promotion of sustainable practices among stakeholders. Economic evaluations, considering capital and operational costs, are also integral to these studies (Arroyo & Molinos-Senante 2018).

In Bolivia, technical evaluations of such systems have concentrated on efficiency and operational and climatic factors (Saavedra et al. 2019; Echeverría et al. 2022). Reports have also linked economic factors, like treatment costs, to system efficiency, particularly in management terms (Escalante et al. 2023). Furthermore, studies exploring the relationship between treatment system performance and operational management have been conducted (Cossio et al. 2018). However, comprehensive assessments of Bolivian WWTPs that encompass all these dimensions have not been reported to date.

This study aims to conduct an integrated assessment of four different WWTP configurations located in a semi-arid region with low to moderate temperatures, where untreated water reuse is common due to scarcity. These WWTPs, co-financed through international cooperation and municipal contributions, are currently operational with subsidy support. The assessment covers environmental, technical, economic, and social aspects to identify the most suitable technology for the context using the Most Appropriate Technology Index (MATTI) proposed by Soares et al. (2022). In addition, a hypothetical scenario was evaluated, where WWTPs operate without subsidies, with operation and maintenance (O&M) managed by water associations, and without a specific reuse destination.

The focus of this study encompasses four distinct wastewater treatment plants located in the provinces of Germán Jordán and Punata, each featuring a unique configuration, as detailed in the following.

For the ABR + HGF + VGF, UASB + HGF, and ABR + PONDS systems, efficiency and effluent quality data were sourced from existing literature (Saavedra et al. 2019; Echeverría et al. 2021, 2022). The UASB + AS system underwent nine monitoring campaigns to evaluate its efficiency and effluent quality concerning chemical oxygen demand (COD), total suspended solids (TSS), ammonia nitrogen (NH₃-N), and phosphorus (P). In all instances, influent and effluent samples were analyzed for anions and cations to calculate the sodium adsorption ratio (SAR), a critical parameter in evaluating irrigation water quality, particularly for soil sodium accumulation risks.

For the estimation of the amount of sludge produced in the four configurations studied, reference values from Brault et al. (2022) and Soares et al. (2022) were used. The land requirement was measured based on the current area occupied by the WWTPs, excluding potential expansion space.

From a technical perspective, the assessment included operational characteristics, specialized labor requirements, and the complexity level of O&M. The complexity level was rated by WWTP operators on a scale from 1 to 5, with 1 being the most complex and 5 the simplest.

Economically, the study considered investment costs, O&M costs, and the cost of wastewater treatment.

Social impacts were evaluated through semi-structured interviews with operators, focusing on odor perception at each WWTP. Odor levels were rated on a scale from 1 to 5, with 1 indicating the strongest and 5 the weakest odor perception.

When the analyzed characteristics become less desirable with increasing numerical values, such as implementation costs, the values are subtracted from 1, expressed as (1-normalized value). This approach serves to minimize the impact of negative characteristics (Soares et al. 2022).

The variables selected for evaluating the MATTI include COD removal efficiency, NH₃-N removal efficiency, GHG emissions, sludge production, land requirements, power consumption, capital expenditures (CAPEX), operational expenditures (OPEX), simplicity of O&M, and esthetics.

The MATTI quantifies the level of criteria compliance on a scale from 0 to 1, with increasing values indicating greater suitability in meeting local demands. The levels of suitability are classified as follows:

Greater than 0.75 to 1.0: Highly recommended

Greater than 0.5 to 0.75: Adequate

Greater than 0.25 to 0.5: Poorly adequate

0 to 0.25: Inadequate

Weights are generally assigned by expert judgment.

Application of MATTI additionally requires the definition of the scenarios where it is applied. Two scenarios were considered for this assessment and are described below.

This scenario reflects the current state of the wastewater treatment systems in the studied region. WWTP construction has been partially funded through international cooperation, and a foundation manages, operates, and maintains these facilities with subsidies. As a result, the significance of CAPEX and operational (OPEX) costs is diminished. In addition, the complexity of operations is not a major concern because trained personnel ensure proper O&M. Furthermore since the treated water is primarily intended for crop irrigation, nutrient removal becomes a secondary consideration.

In this scenario, it is anticipated that water associations will eventually manage the WWTPs. Given Bolivia's status as a medium- to low-income country, treatment costs assume greater importance. The complexity of O&M also becomes crucial to ensuring the effective functioning of the WWTPs. Moreover, in different contexts where these systems might discharge into natural water bodies, more efficient nutrient removal becomes essential.

This section details the outcomes of the evaluation, focusing on various critical aspects of the WWTPs.

The analysis of the efficiency shows that the UASB + AS and ABR + HGF + VGF configurations achieve higher COD removal efficiencies. As can be seen, the UASB in this evaluation achieves a removal rate of nearly 45%, and the overall efficiency reached in combination with the AS is 88%. This coincides with the results of various researchers who indicate that the UASB + AS combination is very effective, as the anaerobic stage proficiently removes a large amount of organic matter, and the activated sludge facilitates the removal of the remaining organic matter and suspended solids while aiding in nitrification (Von Sperling et al. 2001). Research indicates that integrating an up-flow anaerobic reactor with an activated sludge reactor can result in COD removal efficiencies of up to 87% (Von Sperling et al. 2001; Díaz-Gómez et al. 2022).

Furthermore, the ABR + HGF + VGF system is also recognized as a robust alternative, addressing certain drawbacks of up-flow anaerobic sludge blankets, such as fluidized bed expansion and biomass loss (Manariotis & Grigoropoulos 2006). In this study, this combination achieves a removal efficiency of 90% of total COD, with more than 50% occurring in the anaerobic stage. This combination is also efficient for suspended solids removal thanks to the addition of the hybrid biofilter, which effectively complements the removal of organic matter and suspended solids (Echeverría et al. 2022).

In comparison, the UASB + HGF and ABR + PONDS systems attain lower efficiencies of 81 and 68%, respectively. These disparities are largely due to the distinct treatment methodologies implemented.

Regarding the removal of ammonia nitrogen, the highest efficiencies are achieved with the UASB + AS combination, around 60%. However, none of these WWTPs have been designed for nutrient removal since the final destination of these effluents is agricultural irrigation reuse.

Table 1 compares the effluent characteristics of the four evaluated WWTPs against national and international standards pertinent to agricultural reuse.

Globally, different countries have set varied COD thresholds for wastewater reuse in agriculture. For instance, Italy and Israel mandate a maximum COD value of 100 (Jeong et al. 2016), while France stipulates a limit of 60 for irrigation, except in the case of raw-consumed crops (Paranychianakis et al. 2016). In the local Bolivian context, although specific COD standards for agricultural reuse are absent, environmental regulations (Law 1333) prescribe stringent discharge criteria at the WWTP outlet. The UASB + AS and ABR + HGF + VGF systems align with these international standards, unlike UASB + HGF and ABR + PONDS. Notably, ABR + PONDS fails to meet even the local discharge requirements, likely due to its expansion beyond the designed capacity in 2020.

Typical recommendation values for irrigation water according to the SAR index as sodium hazard to soil are 0–10 (low risk), 10–18 (medium risk), 18–26 (high risk), and >26 (very high risk) (Mohammadi-Moghadam et al. 2015). The effluents of the WWTPs evaluated according to their SAR index represent a low risk to the soil.

The results of estimating GHG emissions, sludge production, land requirements, and specific energy consumption are presented in Figure 8.

GHG emissions were relatively uniform across all systems, while sludge production was notably higher in the UASB + AS configuration.

The ABR + PONDS system demands the largest area per capita for wastewater treatment. Traditional waste stabilization ponds and constructed wetlands generally require substantial land, with standard pond systems needing about 4 m² per capita. In contrast, UASB combined with waste stabilization ponds reduces this demand to approximately 1.5 m² per capita (Brault et al. 2022). The results obtained from this evaluation suggest that a combined ABR + PONDS system can further reduce the area requirement to 1 m²/cap.

The UASB and ABR systems are often categorized as intermediate-level treatment technologies, given their requirement for subsequent treatment stages, such as filters or ponds, to achieve optimal efficiency. These additional steps are important in removing residual contaminants, thereby producing high-quality effluent. UASB and ABR systems, as standalone processes, typically require a moderate amount of land, ranging from 0.1 to 0.4 m² per capita. Besides, processes like constructed and hybrid wetlands (both horizontal and vertical flow) demand considerably more space, approximately 2–4 m² per capita (Brault et al. 2022). In this evaluation, the combined UASB + HGF system occupies 0.87 m² per capita, while the ABR + HGF + VGF setup requires 0.47 m² per capita. These findings suggest that integrating compact anaerobic pretreatment with these systems can significantly reduce the area typically needed for processes like wetlands. Moreover, activated sludge technology, known for its space efficiency in wastewater treatment, further exemplifies this efficiency. For instance, extended aeration systems generally demand about 0.2 m² per capita (Brault et al. 2022). The UASB + AS combination in this study aligns with these standards, requiring only 0.19 m² per capita, underscoring its effectiveness in space utilization.

The energy consumption ranges from 0.06 to 0.94 kWh/m³ of treated water. Energy consumption is not only one of the most critical factors to consider in the operational cost of wastewater treatment, which accounts for 15–50% of total operating costs (Buchauer et al. 2015) but also constitutes a significant concern in terms of environmental impacts due to its association with GHG emissions. In fact, there are several studies that classify this as an environmental indicator rather than an economic one, even though they are directly related (Popovic et al. 2013; Arroyo & Molinos-Senante 2018). The energy needed to operate a treatment plant can vary significantly, depending on various factors such as the energy efficiency of the devices, the quality of the raw water, and the size of the plant, among others.

Among the treatment configurations analyzed, the UASB + AS system exhibits the highest energy consumption, largely due to the energy-intensive nature of the secondary treatment processes such as aeration, sludge mixing, and recirculation. Aeration, which is essential for supplying oxygen to microorganisms, is particularly energy-demanding. Furthermore, the mixing and recirculation of activated sludge are essential for maintaining microbial populations but also contribute to energy use. Average energy consumption for conventional activated sludge plants varies internationally: 0.46 kWh/m³ in Australia, 0.269 kWh/m³ in China, 0.33–0.60 kWh/m³ in the United States, 0.30–1.89 kWh/m³ in Japan (Bodík & Kubaská 2013), and 1.02 kWh/m³ in Italy (Ranieri et al. 2021). Decentralized plants similar in scale to those assessed in this study have energy consumptions ranging from 0.35 to 0.65 kWh/m³ (Gu et al. 2017). The UASB + AS system evaluated here records a higher consumption of 0.94 kWh/m³, suggesting potential areas for optimization to reduce energy use.

UASB and ABR systems, known for their anaerobic processes, are typically more energy-efficient than aerobic systems. This efficiency stems from their ability to decompose organic matter without the need for energy-intensive air blowers, thereby reducing operational costs (Mainardis et al. 2020). For instance, full-scale anaerobic systems in Italy report an average energy consumption of 0.43 kWh/m³ of treated wastewater (Ranieri et al. 2021). The UASB + HGF system evaluated in this study demonstrated notably low energy consumption at 0.06 kWh/m³. However, the ABR + HGF + VGF system exhibited a higher energy consumption of 0.48 kWh/m³, despite a lower treatment capacity, which can be attributed to the energy required for pumping in the hybrid wetland system, particularly for the horizontal biofilter.

Waste stabilization ponds are another energy-efficient treatment option, often operating without energy input by utilizing gravity for flow between process units. The ABR + PONDS system in this study recorded an energy consumption of 0.21 kWh/m³, primarily due to the energy required for pumping wastewater from the pretreatment stage to the ABR and the size of the facility.

Operational characteristics of the WWTPs, such as staffing requirements, operational hours, frequency of various maintenance activities, and complexity of operations as assessed by the operators, are detailed in Supplementary material (Table S1). Key findings reveal that the UASB + AS system is the most operationally complex, while the UASB + HGF is less complex but still requires consistent attention due to its high flow rate, which may generate suspended solids washout from the anaerobic reactors to the filters, as reported by Saavedra et al. (2019). In all the WWTPs, routine operational activities predominantly focus on cleaning the pretreatment units to prevent clogging and ensure efficiency. Despite its larger treatment capacity, the ABR + PONDS system is simpler in terms of O&M.

According to Brault et al. (2022), treatment costs are categorized into high (above 20 USD/cap-yr), moderate (approximately 3–20 USD/cap-yr), or low (less than 3 USD/cap-yr) operating costs. The ABR + HGF + VGF and UASB + AS systems incur the highest operating and maintenance costs, falling within the moderate range at around 5 USD/cap-yr. In contrast, the UASB + HGF and ABR + PONDS systems demonstrate lower operational and maintenance costs, ranging from 1 to 3 USD/cap-yr. Furthermore, the AEC per volume of treated wastewater estimated for design flow were as follows: 0.33, 0.26, 0.32, and 0.13 USD/m³ for ABR + HGF + VGF, UASB + AS, UASB + HGF, and ABR + PONDS, respectively.

When deploying a WWTP, several critical factors come into play, such as the size of the population served, public engagement, esthetic considerations like noise and odor, and the overall benefit to the community. In the rural areas surrounding the evaluated WWTPs, agriculture is the primary livelihood, with about 60% of the residents, or approximately 27,528 individuals, relying on it. The provision of treated water for crop irrigation by these WWTPs has been a significant boon, especially in a region grappling with resource scarcity. This support has been instrumental in bolstering the economic sustainability of these communities.

A key aspect of WWTPs' acceptability relates to esthetics, particularly odor management. Odor perception was evaluated on a scale of 1–5, with 1 representing the strongest odor and 5 the weakest. The ABR + HGF + VGF system received a score of 4, indicating relatively low odor perception, while UASB + AS was rated 5. Both the UASB + HGF and ABR + PONDS systems were rated 2, suggesting stronger odor emissions.

Utilizing the gathered information, the MATTI methodology was applied to identify the most suitable technology among the four configurations under review, particularly for regions in Bolivia or globally with similar socio-economic, climatic, and environmental contexts.

Table 2 outlines the variables selected for the MATTI calculation across all dimensions. These data come from the comprehensive assessment of the WWTPs discussed in the preceding sections. The column ‘Direction’ indicates that a variable with a positive direction (increasing) means that it is better the higher its value (e.g., treatment efficiency), while a variable with a negative direction (decreasing) means that it is better the lower its value (e.g., implementation costs). For ordinal values, the direction depends on the established scale. For example, in the case of odor perception, a higher odor perception is rated as 1 on a scale of 1–5, while a lower odor perception is rated as 5, making the direction of this variable positive.

Since the variables selected for the estimation of the MATTI are of different natures, both quantitative and qualitative, normalization was performed based on Equation (1). The results of the normalized values are provided as Supplementary material (SM-1 and SM-2). Subsequently, the estimation of the MATTI requires calculating the product of the normalized value by the weight assigned to the variable in question, as specified in Equation (2).

The weight assignment for these methods is generally based on the expertise of those involved in decision-making for WWTP implementation, and the criteria can vary by region. In this study, the work of Cossio et al. (2020), a local study, was used as a reference, gathering opinions from technical experts. According to their results, the importance of the four evaluation criteria follows this order: Social > Technical > Economic > Environmental. This indicates that the social factor is essential for the selection and sustainability of a technology, while the environmental factor is considered less important. However, in the present study, based on the authors' own judgment, greater weight was given to the environmental factor, considering that an effluent of adequate quality is more visually and esthetically acceptable, directly influencing the social acceptance of WWTPs.

Under these criteria, weights were assigned values from 1 to 5, where 1 represents the least importance and 5 is the greatest. For example, the importance of CAPEX is assigned a value of 3 when there are economic subsidies for the installation of WWTPs (Scenario 1). However, when the municipality relies on its own resources, CAPEX becomes more important, and a weight of 5 is assigned (Scenario 2).

The values of the assigned weight, the product of normalized values by the weight of each variable for each scenario, and the results of the estimation of MATTI are presented in Table 3. Additional details regarding the calculations for estimating MATTI are provided in the Supplementary material (SM-1 and SM-2). This information is presented in an Excel spreadsheet and has been customized for the purpose of a comprehensive evaluation, based on the original tool proposed by Soares et al. (2022).

Table 3

Results for MATTI assessment in Scenarios 1 and 2

Parameter .	Land requirement (m2/cap) .		GHG emissions (kg-CH4/cap-yr) .		Sludge to be disposal (l/cap-yr) .		Power consumption (kWh/m3) .		Odor presence .		Simplicity O&M .		COD removal (%) .		NH3-N removal (%) .		CAPEX (USD/cap-yr) .		OPEX (USD/cap-yr) .		MATTI .
Scenarioa	1	2	1	2	1	2	1	2	1	2	1	2	1	2	1	2	1	2	1	2	1	2
Weights	4	4	2	2	3	4	3	4	5	5	3	5	3	5	2	5	3	5	3	5
Normalized value × weight:
ABR + HGF + VGF	2.60	2.60	0.00	0.00	0.00	0.00	2.01	2.68	3.33	3.33	3.00	5.00	3.00	5.00	1.37	3.43	0.67	5.00	0.45	0.75	0.53	0.54
UASB + AS	4.00	4.00	2.00	2.00	3.00	4.00	0.00	0.00	5.00	5.00	0.00	0.00	2.73	4.55	2.00	5.00	0.00	4.55	0.00	0.00	0.60	0.56
UASB + HGF	0.60	0.60	1.70	1.70	0.41	0.55	3.00	4.00	0.00	0.00	1.50	2.50	1.77	2.95	1.41	3.53	0.89	2.95	1.66	2.77	0.42	0.46
ABR + PONDS	0.00	0.00	1.91	1.91	1.03	1.38	2.59	3.45	0.00	0.00	3.00	5.00	0.00	0.00	0.00	0.00	3.00	0.00	3.00	5.00	0.47	0.49

Parameter .	Land requirement (m2/cap) .		GHG emissions (kg-CH4/cap-yr) .		Sludge to be disposal (l/cap-yr) .		Power consumption (kWh/m3) .		Odor presence .		Simplicity O&M .		COD removal (%) .		NH3-N removal (%) .		CAPEX (USD/cap-yr) .		OPEX (USD/cap-yr) .		MATTI .
Scenarioa	1	2	1	2	1	2	1	2	1	2	1	2	1	2	1	2	1	2	1	2	1	2
Weights	4	4	2	2	3	4	3	4	5	5	3	5	3	5	2	5	3	5	3	5
Normalized value × weight:
ABR + HGF + VGF	2.60	2.60	0.00	0.00	0.00	0.00	2.01	2.68	3.33	3.33	3.00	5.00	3.00	5.00	1.37	3.43	0.67	5.00	0.45	0.75	0.53	0.54
UASB + AS	4.00	4.00	2.00	2.00	3.00	4.00	0.00	0.00	5.00	5.00	0.00	0.00	2.73	4.55	2.00	5.00	0.00	4.55	0.00	0.00	0.60	0.56
UASB + HGF	0.60	0.60	1.70	1.70	0.41	0.55	3.00	4.00	0.00	0.00	1.50	2.50	1.77	2.95	1.41	3.53	0.89	2.95	1.66	2.77	0.42	0.46
ABR + PONDS	0.00	0.00	1.91	1.91	1.03	1.38	2.59	3.45	0.00	0.00	3.00	5.00	0.00	0.00	0.00	0.00	3.00	0.00	3.00	5.00	0.47	0.49

^aScenario 1: with subsidies, with effluent reuse; Scenario 2: without subsidies and no effluent reuse.

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The outcomes of the MATTI methodology indicate that both the ABR + HGF + VGF and UASB + AS configurations are considered adequate technologies for both scenarios, as they achieved the highest scores. Conversely, the UASB + HGF and ABR + PONDS sequences were rated as poorly adequate, scoring the lowest in both scenarios.

The empirical application of composite multi-criteria indicators for comparing the sustainability or suitability of WWTPs is still limited. The analytical hierarchy process (AHP) methods are possibly the most suitable for developing a multi-criteria indicator for decision-making purposes. There are some specific examples where these indicators have been applied. For instance, Soares et al. (2022) proposed the MATTI, a tool developed from AHP concepts as a composite indicator tailored to regional needs and the criteria of its users. Soares employed technical, economic, and environmental evaluation criteria in assessing seven treatment technologies across different scenarios in Brazil. The study found that UASB combined with high-rate plastic media percolating filters and UASB combined with biological aerated filters are the most suitable technologies for wastewater treatment, particularly in scenarios where the majority of the population resides in metropolitan areas with space constraints, stringent quality standards, and adequate tariffs to cover construction, operation, and maintenance costs. Conversely, rock media percolating filters and anaerobic ponds combined with percolating filters were found to be the most suitable technologies for small municipalities with fewer than 50,000 inhabitants, where ample space is available for WWTP implementation.

These findings contrast somewhat with those of this study, as the study area encompasses intermediate cities combining rural and urban activities, where space has become a limiting factor despite having municipalities with populations ranging from 2,000 to 20,000 inhabitants.

Another study conducted by Molinos Senante et al. (2014) focused on evaluating the sustainability of small WWTPs serving 1,500 inhabitants. This research assessed seven secondary treatment technologies commonly applied in small WWTPs. The study describes a hypothetical but typical context where the sewerage system is mixed, including both wastewater and stormwater, resulting in standard effluent concentrations, and where wastewater is discharged into non-sensitive areas without reuse. Up to seven scenarios were considered for weighting the dimensions evaluated in the study – technical, economic, and social. In the scenario where weights were assigned by experts, extended aeration emerged as the most sustainable technology for small communities. When equal weights were assigned to each dimension, constructed wetlands were found to be the most sustainable technology. Depending on the weight allocation in the other scenarios, the most sustainable technologies varied between constructed wetlands and extended aeration. However, lagoons also scored highly in all scenarios.

There are only a few examples of applying multi-criteria analysis for selecting the best treatment alternative or evaluating the most sustainable option in a specific scenario. These include studies by Castillo et al. (2017) for selecting the best technology for industrial effluent treatment and Bringer et al. (2018) for optimizing water quality in watersheds under management principles, in addition to the aforementioned studies by Soares et al. (2022) and Molinos Senante et al. (2014); Soares et al. (2022) and Molinos Senante et al. (2014).

Since selecting the best technology depends on specific contexts, gathering evidence-based information is essential.

This comprehensive assessment of four decentralized WWTPs in a similar geographical and social context encompassed technical, environmental, economic, and social aspects.

The ABR + HGF + VGF and UASB + AS configurations exhibited very good performance, with COD removal efficiencies exceeding 87% and TSS removal over 85%. The UASB + HGF system showed reasonable efficiency in COD and TSS removal, but less than the aforementioned technologies. ABR + PONDS, while adequate in TSS removal, was less efficient in COD removal. UASB + AS and ABR + HGF + VGF achieved superior effluent quality (<85 mg-COD/L and <35 mg-TSS/L), aligning with agricultural reuse standards. In terms of the SAR index, all WWTPs presented a low risk, indicating suitability for irrigation purposes. UASB + AS was the most space-efficient with a land usage of 0.19 m²/cap, and UASB + HGF and ABR + PONDS were more energy-efficient. GHG emissions and sludge production were also considered, with UASB + AS having the highest sludge disposal requirements.

ABR + HGF + VGF and ABR + PONDS were noted for their ease of O&M. The UASB + AS system, despite being more complex operationally, justified this with its high efficiency. UASB + HGF, due to its larger size, had greater operational complexity.

CAPEX was fairly uniform across all WWTPs ranging from 6.2 to 6.8 USD/cap-yr. However, OPEX varied, with ABR + PONDS having the lowest (1.17 USD/cap-yr), followed by UASB + HGF (2.96 USD/cap-yr). ABR + HGF + VGF and UASB + AS had slightly higher OPEX, 4.58 and 5.18 USD/cap-yr, respectively. The treatment costs per volume of water treated also followed this trend: ABR + PONDS (0.13 USD/m³) < UASB + AS (0.26 USD/m³) < UASB + HGF (0.32 USD/m³) < ABR + HGF + VGF (0.33 USD/m³).

The reuse of treated water for irrigation was significant, especially considering the agricultural dependency of about 60% of the population (27,528 inhabitants). The esthetic factor related to the acceptability of WWTPs in terms of odors reflects that these are more noticeable in ABR + PONDS and UASB + HGF.

The application of the MATTI in two scenarios – with and without economic subsidies, and with and without agricultural reuse as the end goal – showed that ABR + HGF + VGF and UASB + AS are adequate technologies. In contrast, UASB + HGF and ABR + PONDS were rated as poorly adequate. The selection of technology should consider local criteria and the specific context.

In short, the methodology used in this study is highly recommended for selecting appropriate technology for decentralized WWTPs in developing countries with similar climatic, geographic, and social conditions.

The authors wish to thank the PERIAGUA program at GIZ for their support of this study.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.