Assessment of decentralized wastewater treatment systems in the rural area of Cuenca, Ecuador

There is a global trend for an annual increase in the percentage of the population with access to improved sanitation. The Millennium Development Goals (MDGs) for basic sanitation set for 2015 (WHO/UNICEF 2015) have been missed, however, unlike the drinking water target, which was achieved by 2010. Latin America missed the MDG for sanitation by a small amount, as a whole, but Ecuador met it by 2015. Nevertheless, in rural Ecuador, 25% of the population still has no access to improved sanitation facilities. Considering the limited financial resources available in developing countries, there is increasing demand for environmentally and economically sustainable wastewater treatment systems (WWTPs). Economically, the difference between centralized and decentralized systems is highly relevant (Singh et al. 2015). Moreover, decentralized wastewater treatment offers many other public health advantages, and brings the opportunity of resource recovery from wastewater as well as water recycling for selected agricultural and industrial purposes (Tchobanogious et al. 2004; Verstraete et al. 2009; Nansubuga et al. 2016), decreasing the demand for fresh water (Bakir 2001).

Cuenca, Ecuador's third largest city, leads in sanitation services in the country, operating the biggest WWTP – the Ucubamba waste stabilization pond system, 1.8 m³/s representing 90% urban area coverage. Nevertheless, since about 1990, the city has expanded hugely into the periurban areas, representing a serious challenge for ETAPA, the municipal institution in charge of water supply, sanitation and telecommunication services, to maintain their high standards across the areas served.

The rural settlements surrounding the city, also served mostly by ETAPA, are facing similarly rapid demographic growth and urbanization. Knowing the challenges of extending a sewerage network, which could take between 80 and 90% of the capital costs in a centralized approach, ETAPA decided to build 32 small, decentralized, wastewater treatment plants (DWWTPs). The many technical options available were balanced and analyzed in terms of treatment objectives, cost, energy demand, operation and maintenance (O&M) costs, etc. Finally, anaerobic and natural systems were selected with a view to their long-term sustainability. There were many difficulties from the start, especially with high overflows and primary treatment units clogging. However, most of the DWWTPs are still operating.

ETAPA acknowledges the systems’ decreasing efficiencies and some plant assessments have been performed. From both management experience and evaluation results, some constraints have been identified as major factors influencing the decline in efficiency in some systems. They include: (1) rapid population growth in the areas served, increasing flux-rates considerably, causing significant hydraulic retention time reductions as well as organic overloads in the systems; (2) limited economic resources combined with low user organization capacity in maintaining the systems, which contribute to system deterioration; (3) in some cases, the adoption of standard solutions applied successfully in other countries but not appropriate for the particular environmental conditions at a specific site. In this paper a full evaluation of the ETAPA systems is presented, their technical deficiencies are discussed and long-term sustainable options recommended for improving their efficiency and O&M.

A critical recompilation of the 32 DWWTP system evaluations was made. Past evaluations and characterizations, particularly those since 2013, were highlighted to obtain an objective overview of current system states. The recent evaluations have focused on a few systems with different technical approaches. Important information from two general evaluations has also been analyzed. These were performed in 2005 (six systems) (Neira 2005) and 2008–2009 (31 systems) (Ordóñez 2009). The evaluations differed in every case, but all (past and recent) have verified the current state of the structure and in many cases characterized the influent/effluent waters. Interviews with ETAPA staff (engineers and system operators), and system users were also carried out. The discussion is based on the quantitative and qualitative data obtained, and focuses on the systems’ technical aspects, their removal efficiencies for the main quality parameters, the treatment objectives, and the O&M problems. The discussion also covers problems in the sewerage network influencing system operation.

Table 1 is a summary of system technologies and includes some relevant information about them. All but one of the systems have a septic tank (ST) as the primary treatment unit, and 20 out of 32 are composed entirely of anaerobic units. The combination of ST and anaerobic filter, with vertical flow (AF/VF) and constructed wetland (CW) is the commonest technology, despite the sites’ differing climatic characteristics. It seems that the low construction and O&M costs were the selection relevant factors. Table 1 includes the altitudes and average temperatures of the DWWTPs, the latter varying from 11 to 18 °C in the highlands and being around 22 °C on the lowlands. The table shows that some high altitude systems have higher temperatures, because of the local microclimates in the valleys concerned. The areas served by the systems also differ significantly. Despite these substantial variations, the dimensions and configuration of the main structures are much the same. This could suggest some deficiencies in design – e.g., strict application of design-type guidelines without consideration of the characteristics of the wastewater or the climatic conditions – or that, in some cases, existing systems were used as a model for new ones.

Table 1

Infrastructure and Technology of the Decentralized Wastewater Systems

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In the rural areas of Ecuador, especially in the south Andean region, the main objective of any sanitation program remains preventing the spread of diseases, which means that a WWTP must effectively decrease the concentrations of pathogenic organisms in the effluent. However, small and compact anaerobic reactors do not remove pathogenic organisms efficiently (Chernicharo et al. 2015). Table 1 shows that no post-treatment is included in any of the full anaerobic systems, which implies that the disinfection objective, at least, was never met in any of them.

Table 1 also highlights the presence or absence of preliminary treatment in the systems. In most cases, an overflow structure, coarse screening and a small grit chamber are included in the configuration. Like the reactors, the preliminary treatment units are almost identical in size and configuration in all systems studied.

Finally, it is noted that almost 65% of the systems include a drying bed for excess sludge. However, none is currently operating, and few operated for more than a short period after start up (Ordóñez 2009). In a few cases the drying beds are used with minimal efficiency as filtration chambers for effluent polishing.

All DWWTPs are connected to sanitary systems, serving an average of 200 people (Neira 2005). The networks underlie both public and private land, making them vulnerable to illicit discharge connections via pipes and manholes. Moreover, the domestic networks were constructed illegally in some cases to allow rainfall drainage from yards and roofs.

The system overflows are controlled with bypass structures, consisting of a manhole containing a small baffle/weir, in which excess wastewater, flowing over the baffle, is discharged directly into the receiving body through a bypass pipe. This configuration does not, unfortunately, guarantee that particulate material goes through the system. After the first general evaluations, screen bars and settlers were added to many systems upstream of the STs, but these are insufficient to prevent the STs clogging during major rainfall events.

The STs commonly comprise a twin chamber reactor around 80 to 100 m³ volume (3.0 m deep and L/B around 2.5). The STs are in good order structurally after 10 to 15 years, and all include ventilation pipes, inspection manholes and inlet/outlet baffles, as recommended for good performance (Mara 1996).

CWs are present as secondary treatment in 25% of the systems. All CWs are of Free Water Surface Wetland (FWSW) configuration with inlet/outlet structures comprising perforated pipes to distribute/collect the influent/effluent from the reactor. The 8 secondary FWSWs use water hyacinths (Eichhornia crassipes) as the main aquatic plant species.

Used as secondary treatment for ST effluents, all comprise circular covered chambers with granular media (brick pieces and/or gravel) filling 70% of the reactor depth. There is no biogas recovery. The influent/effluent passes to perforated pipes at the bottoms and tops of the upflow configuration reactors. The one exception is at Bella Unión, where the VF/AF consists of a rectangular, open reactor. Despite the acidic environments within them, these reactors are in good structural condition.

Also used as secondary treatment for the STs. The easier and more economic construction, compared to the VF/AF, make this technology attractive for some sites. In this configuration, the ST and horizontal flow anaerobic filter (HF/AF) are constructed in a single enclosed chamber with a dividing wall between the two stages. The media used is gravel and typically fills 50% of the reactor's depth. A perforated wall distributes the filter influent. The physical infrastructure is in good condition in the systems visited, but considerable clogging was observed in the perforated walls and on the media surface.

Only one system (Quillopungo), out of 32, uses a upflow anaerobic sludge blanket (UASB) reactor as primary treatment. In the main reactors (3), the influent enters at the bottom via a single pipe and the effluent is collected by a perforated pipe. Biogas is not collected. The three-phase separator (De Lemos Chernicharo 2007) was not working properly during the several visits to this system. There were no baffles to prevent biogas entering the sedimentation chamber, which led to high suspended solids concentrations in the effluent. This DWWTP is the only one that incorporates a disinfection stage (chlorine contact chamber), although it was never used as such.

Table 2 presents the main characterization data for the systems. The very low concentrations of organic matter and other constituents in some system influents arise from the very high dilution of some wastewater influents, despite the exclusively domestic wastewater sources for the sewerage networks present in all service areas. This problem is acknowledged by ETAPA and the recurrent entry of illicit waters is identified as the cause. These illicit discharges cause the systems to overflow and bypass flows are often present in the receiving waters. They have several sources, the main ones are: (i) field crop drainage and excess irrigation water, which could incorporate high concentrations of pesticides and other farming products that could be toxic for some anaerobic biomass, thus inhibiting biological processes (Crites & Tchobanoglous 1998; Von Sperling & de Lemos Chernicharo 2005); (ii) runoff from roofs, courtyards, terraces, etc., and other household effluents, from which there is a high risk that uncontrolled particulate material and very high rainfall discharges will enter the system; and, (iii) runoff from roads and other public spaces, which usually enters the sewerage network via manholes. ETAPA often receives reports of manholes having been opened to allow stagnant flows to drain easily and quickly from unpaved roads, carrying large amounts of sand, clay and other particulate materials to clog the pre-treatment and even the STs, during and after high rainfall events. It is very difficult to solve these problems for various reasons described below.

Table 2

Characterization of the DWWTPs

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Tables 2 and 3 shown the organic removal efficiencies of some of the systems. These data correspond in some cases to only a single characterization performed, typically, a few weeks after full system maintenance. Such maintenance implies complete cleaning of the STs, and the removal, cleaning and reinstallation, or sometimes replacement, of the filter media in the anaerobic reactors. Because of this it is anticipated that, after maintenance, a substantial proportion of the particulate organic matter is retained in the STs and filter media during the first days/weeks of operation. Some removal efficiencies reported, therefore, in Table 2 (characterizations performed in 2016) should be treated cautiously, as they may not reflect the quality of the bioprocesses in the reactors. The almost negligible pathogenic organism removal in the fully anaerobic systems, should also be noted in Table 2. This was expected due to the typical low efficiencies of the anaerobic processes in this regard. It is emphasized that none of the systems characterized in terms of FC, achieve the Ecuadorian standards for either discharge to freshwater bodies or for water reuse in crop irrigation (<1,000 MPN/100 mL). Because of this, these effluents would require obligatory disinfection for any practical use downstream. Table 2 also shows the very high variability in the organic loads to the systems, which was expected due to the clear differences in system sizes and populations served. This could be also explained by the illicit discharges to the sewerage networks.

ETAPA, which has permanent and well trained personnel to operate Ucubamba WWTP, has evolved with time in relation to the O&M needs of the decentralized systems. Initially, in the mid-1990s, DWWTP maintenance was carried out by the staff in charge of the sewerage systems. The specific needs for personnel trained in wastewater treatment, obliged ETAPA to create new maintenance teams exclusively for the DWWTPs. This obviously implies permanent human and economic resources, and demands great effort since the systems are remote – see Figure 1.

A recurring O&M problem for the systems is the uncontrolled particulate material entering them during heavy rainfall. The clogging of pre-treatment units has been reported in past evaluations and still occurs now. The locations of and extensions to the sewerage networks have made it almost impossible for ETAPA to prevent the illicit discharges, despite continuous monitoring. Huge amounts of particulate material will continue to enter the sewers, if the communities served do not take care of the infrastructure, especially the manholes. The unpaved roads and natural steep slopes will affect system performance for many years to come, so efforts should also be made to educate the population served about the direct public health benefits, if wastewater is treated before discharge to streams.

It is also noted in Table 1 that 12 DWWTPs do not have access roads. This hinders maintenance, demanding more human and economic resources in the long run. It is no surprise to find that half the systems without maintenance plans are also without vehicular access. The availability of land and economic resources in the Andean region of Ecuador is certainly an important factor limiting the location of DWWTPs; nevertheless, for remote systems, the need must be emphasized for building capacity for O&M in local organizations and the related advantages of sustainable wastewater treatment.

The main conclusions are:

• Despite having separate sanitary systems, significant amounts of particulate material enter the DWWTPs during heavy rainfall, causing pre-treatment units to collapse and occasionally clogging the primary STs. Unfortunately, in some cases, the populations served by the systems contribute to the problem by opening manholes to enhance drainage from roads and public spaces.

• In some cases, when partial or full clogging of some units need urgent intervention, the remote location of some systems/DWWTPs and their inaccessibility by car, contribute directly to lack of maintenance, because only manual cleaning and minor activities are carried out. At times, this has led directly to reduced system efficiency and, occasionally, to system abandonment.

• The communities served by the DWWTPs have never been involved in their O&M, so full responsibility rests with ETAPA. On the basis of assessments of these and many other decentralized systems, it is strongly recommended real capacities should be created in the local communities with regard to system O&M. This can be done by providing education about the benefits of wastewater treatment.

• The pathogen concentration in the DWWTP effluents restricts the potential for use downstream. The anaerobic processes remove pathogenic organisms very inefficiently, and, despite some minor removal in the CWs, the FC concentrations remain very high, in those systems analyzed. There is room for improvement – e.g., by adding disinfection units – although, this should be considered carefully because the high effluent suspended solids concentrations may hinder disinfection.

• The frequent (and necessary) cleaning of the systems is causing the biomass to wash out and it grows poorly in such operating conditions. Thus, there is low biological activity in the reactors, and physical processes – sedimentation and filtration – are almost the only BOD-removal mechanisms in many of the systems studied. It is strongly recommended that the conditions for real growth of anaerobic biomass are maintained for the several months sometimes required at the temperatures of the Andean region. Characterizations of biomass activity over long periods should also be carried out during regular operation of the systems.

This work has been supported by ETAPA (Empresa Pública Municipal de Telecomunicaciones, Agua Potable, Alcantarillado y Saneamiento de Cuenca, Ecuador). The authors express their gratitude to the personnel of ETAPA's Sanitation Department, especially Ing. Patricio Rodriguez and Ing. Paul Torres. The authors also express their express gratitude to the personnel of the Sanitary Laboratory of the Engineering Faculty of the Universidad de Cuenca.