Septic tank usage and its faecal sludge management in Cuenca, Ecuador

The framework of the 2030 Agenda for SDG, and particularly SDG 6 ‘Clean water and sanitation’, promotes ensuring safe sanitation for all (UN-Water 2021). However, around 2.8 billion individuals in low- and middle-income countries still rely on their sewage disposal on sanitation systems known as on-site, with nearly half of those on-site sanitation systems (OSSs) failing to adequately process human excreta (Jakariya et al. 2023). Specifically in Latin America only one-third of the faecal sludge (FS) of OSSs are securely managed (UNICEF/WHO 2021). Thus, it is essential to improve the secure and effective operation of OSSs, by regularly emptying and transporting the accumulated FS to an external treatment facility before the final disposal (Conaway et al. 2023).

OSSs are predominantly employed in rural and peri-urban areas, particularly where conventional sewage infrastructure is absent or where challenging topography hinders its implementation. Septic tanks are the prevailing choice for on-site treatment and disposal of domestic FS (Richards et al. 2016). FS refers to the mixture of undigested and partially digested slurry or solids that arise from the storage or treatment of blackwater or excreta (Peal et al. 2014). For FS, the US Environmental Protection Agency (2023) recommends to empty the septic tanks every 3–5 years or when biosolids exceed 25% of their volume. The mechanical method used for emptying septic tanks involves the use of a vacuum truck or tanker. The same truck serves both as an emptying device and a transportation vehicle for FS (Conaway et al. 2023). Additionally, solid–liquid separation is vital in FS treatment. Plants are designed for efficient settling and dewatering before further processing of liquid and solid components (Ward et al. 2021).

Removing FS from households is a matter of personal interest, however, the whole FS management chain is a public interest (Strande et al. 2014). Therefore, the entire service chain, i.e. collection, transportation and treatment unit sizing (Shukla et al. 2023), needs to be accounted for effective management of FS. Nevertheless, one of the main obstacles to safely and effectively managing sanitation is the lack of data about the existing infrastructure, i.e., sewer networks and OSSs (Nasim et al. 2022). Then, quantifying the amount of FS produced, which is the initial step in developing an infrastructure (Shukla et al. 2023), as well as the emptying requirements is of the most relevance.

In Ecuador, which is a low-income country located in the northern Andean region of South America, sanitation and FS management are critical. Only around 68% of the Ecuadorian population have sewer connections (UNICEF/WHO 2022). Of the total population, 68.1% live in urban areas and 31.9% in rural areas. Among the population living in urban areas, 16.3% of households lacked a connection to the sewerage system, meanwhile, in rural areas, this figure increased to 75.8% (Viteri & Pozo 2019). This shows a significant contrast in sanitation infrastructure between urban and rural areas. The disparity arises due to the rural areas being designated for agriculture and ecosystem conservation, making them less consolidated and challenging to implement a sewerage system. To close disparities in access to enhanced sanitation, substantial financial investments and the implementation of sustainable technological solutions tailored to the specific context of each region, alongside political commitment, are indispensable (Adugna 2023).

The data provided by Viteri & Pozo (2019) evidenced that in Ecuador a considerable segment of the population requires an alternative approach to managing their wastewater. Among the various OSSs in use, septic tanks are a recognised secure alternative to centralised sanitation, especially in peri-urban and rural regions because of their simplicity for maintenance and construction. In fact, 24.3% of Ecuadorian households, without access to a centralised sewerage system, resort to discharging their wastewater into septic tanks. Furthermore, for those households using septic tanks, the majority (92.2%) do not empty their tanks but infiltrate the leachate into the soil, followed by those who report discharging it anywhere in the open (6.9%) (Pozo et al. 2016).

Ecuadorian regulation establishes that the local government should provide the sewerage service, while the households need to take responsibility for the end-user connection. Therefore, based on qualitative data collection, the local governments frequently assume that merely having sewerage coverage suffices as sanitation, leaving behind the end-user connections and the OSSs that are in use. A previous study by Córdova et al. (2023) examined that local governments showed efforts to address OSS, potentially linked to low levels of sewerage coverage. In the absence of strict regulations, there might be some households that besides being inside the sewer coverage area, are not yet connected to the network. This low connectivity to the sewerage system could carry out negative implications related to (1) unmaterialised health and environmental benefits of sewer infrastructure investments; (2) failure of the financial sustainability of the sewer system which depends on the user's contribution; (3) reduction in treatment plant's efficiency because of the low wastewater volume entering the plant; and (4) clogging of the sewer networks with sediments when very low volumes of wastewater enter the system (Sturzenegger et al. 2020).

In order to avoid these negative implications, a global understanding of the OSSs problem is essential. In Ecuador there is limited high-quality and representative data about OSSs usage, the end-user sewer connections and subsequent FS management. Therefore, research on OSSs and the assessment of the historical and current situation of FS management are imperative. This study is the first attempt to explore the temporal and spatial distribution of the use of OSSs in urban and rural areas, specifically septic tanks. Its objective is also to investigate whether there remain septic tanks in places where a sewer network is available and reveal the posterior FS management situation. As a study case, a middle-sized city in Ecuador, Cuenca was selected. The reason is that this city has implemented at least some level of FS management, although it may not be optimal and it has an extended sewer network with a wastewater treatment plant. However, it still highly relies on septic tanks.

Cuenca canton is divided into 36 districts, 21 in the rural area (light gray in Figure 1) and 15 in the urban area (dark gray in Figure 1). For the rural sector, the closest districts to the urban area that had a significant number of septic tanks were highlighted: Baños, Llacao, Nulti, Paccha, Checa, Chiquintad, Sidcay, Ricaurte, San Joaquín, Sinincay, Sayausí, Tarqui, Turi, and El Valle for deeper analysis.

Cuenca has a centralised wastewater treatment plant (WWTP) located at the west of the city (2°52′15.1″S, 78°56′30.8″ W) (Figure 1). It treats mainly urban wastewater through two waste stabilisation pond treatment lines. Each line includes an aerated, a facultative and a maturation pond. The system also has a sludge treatment unit that extracts and dehydrates sludge from the aerated and facultative ponds. The dehydrated sludge is finally disposed of in a landfill.

Three types of data were used for this research. First, records of septic tank desludging frequency; second, sewer network; and third, field visits with a household questionnaire. Specific data preparation was performed for each of them.

The frequency of septic tank desludging by vacuum trucks in the rural and urban districts of Cuenca was collected from 2009 to 2022. The data were obtained from the official records of ETAPA EP. For that, users in need of emptying a septic tank (based on their own perception of the service needed) contacted ETAPA EP. Subsequently, ETAPA EP processed this request based on the two available vacuum trucks to perform the cleaning at the designated location. During the study period, 19,731 service requests were recorded with an ID, cadastral code, geographical location and empty date.

The collected data were systematised, geographically mapped and classified according to their location in urban and rural districts. Throughout the analysed timeframe, a total of 5,636 operational septic tanks were documented as having been emptied. A septic tank was operational (receiving the wastewater from a household) when the owners required the emptying service regularly. Some septic tanks were emptied more than once during the study period. Recognising inactive units is crucial when assessing total septic tanks. Therefore, for temporal trends, we used desludging frequency, and for septic tank locations, we considered only 2022 data, ensuring the inclusion of exclusively active units.

The desludging frequency to district population ratio was calculated to assess population influence. For the urban average ratio, Machángara was omitted due to its distinct rural behaviour, preventing distortion of the urban average. Similarly, in establishing the rural average, Baños, Checa, and Turi were excluded for exhibiting urban-like characteristics.

The sewer networks had information regarding material, diameter, type of sewerage and year of construction. In this case, the sewer network from 2022 was used. The spatial coverage of the whole network and the length (km) for each district were calculated. Furthermore, the sewer network was contrasted with the active septic tanks in 2022. The shorter distance between the tanks and the network was obtained. The distance of 20 m was accounted for under the assumption that if the residence was within a 20 m distance, there was potential for a sewer network connection.

For the third data type, 12 septic tanks (purple dots in Figure 1) were randomly selected from nine districts of Cuenca. Field visits and household questionnaires were applied to the selected 12 septic tanks during July 2020. The questionnaire had 10 questions related to socioeconomic information and septic tank characteristics and maintenance, i.e, volume of the tank, number of inhabitants in the corresponding household and time since the last empty.

Furthermore, in order to quantify the height of the accumulated sludge in the septic tank, a white towel test was performed (Mara 1996). With these data, the per-capita sludge accumulation was estimated. Finally, technical field visits were conducted to gather data about the FS disposal location which is the Ucubamba WWTP.

Figure 2(a) showcases the spatial coverage of septic tank desludging frequency throughout the specified time frame, highlighting their presence in both urban and rural areas. Figure 2(b) displays the extent and coverage of the sewer networks in the year 2022. Similarly, as the septic tank desludging frequency coverage, the network had a presence in urban and rural districts. While the urban ones showed full coverage, the rural ones depicted a less extended coverage than the septic tank desludging frequency.

Furthermore, Figures 2(c) and 2(d) show the frequency of septic tank desludging and sewer pipeline length for the rural districts, and Figures 2(e) and 2(f) for the urban ones. By seeing the figures, patterns emerge in the utilisation and maintenance of septic tanks, revealing the trends in emptying frequency and geographic concentration. Rural districts showed a higher frequency of septic tank desludging and shorter lengths of sewer pipelines, demonstrating the increased necessity of FSM from OSSs in rural areas compared to urban ones. As the sewerage network will not reach the most remote areas, OSSs will continue to be used. Although, when considering the length of sewer pipelines, the area of each district must be noted. Smaller districts (central ones) presented fewer kilometers of sewerage but coverage of 100%.

The observed emptying patterns could be influenced by the district's population. Therefore, the ratio of septic tank desludging per inhabitant (ST/Inh) has been analysed as a comparative factor (Table 1). The urban sector exhibited the lowest ratio, while rural districts like Baños, Checa and Turi displayed similar ratios. Interestingly, Machángara, an urban district, demonstrated an ST/Inh ratio that was comparable to the average ratio observed in rural areas. This observation highlights that the urban/rural classification based on sanitation services does not always accurately reflect the situation of some districts.

It is important to note the lack of available data regarding septic tank emptying in the more remote areas of rural districts. Therefore, this study does not consider the operation of OSSs in those locations, however we expect they exist. Besides this limitation, understanding the evolving sanitation landscape in Cuenca offers insights into the coexistence of septic tanks and centralised sewer systems within the regiońs sanitation framework.

The pronounced decline in 2019 for urban and rural areas can be attributed to the removal of the subsidy that resulted in a more than 200% increase in the desludging cost (the hourly desludging rate increased from approximately 40 USD to around 105 USD), thereby making the service more expensive. With the new costs, users had the option to connect to the sewer network (if the sewer network were available and at an affordable cost), retain the FS in their septic tanks for an extended period or resort to illegal dumping or improper disposal practices. These choices impact public health and environmental sanitation since less desludging can result in overflowing septic tanks, contaminating groundwater and surface water and increasing the risk of diseases spread through faecal matter. Therefore, in urban areas with sewer infrastructure, it is recommended not to provide subsidies. Instead, policies should focus on encouraging sewer connections to improve sanitation effectively.

Prior to the removal of the subsidy, septic tank desludging was more cost-effective than connecting to the sewer network. Additionally, due to the lower cost before 2019, it is likely that users sought the service more frequently, even before the septic tank reached full capacity. In a research conducted in Dakar, Senegal, Dodane et al. (2012) discovered that the expenses per person per year associated with managing FS which includes the collection, transportation, and treatment of sludge from OSSs were notably lower compared to the costs of a conventional sewerage system. On the other hand, the capacity of septic tanks often correlates with the financial capabilities of households. Smaller tanks tend to fill up more frequently, requiring greater maintenance investment, conversely, larger tanks run the risk of collapsing if their structure is weak (Rojas 2012).

In Ecuador, there is no national law requiring households to connect to the sewerage system upon its construction. Each local government has its own legislation. In Cuenca, a regulation (Registro Oficial No. 222 1993) outlines the connection procedure to the sewerage system and the types of wastewater allowable to discharge into the system. The authority can prohibit connections to the sewerage. However, the regulation does not oblige households to close their OSSs and connect to the sewerage in areas where it exists. Thus, the absence of a regulatory structure poses enormous challenges for local governments to enforce end-user connections. This absence undermines sustainable FSM efforts and hinders progress towards achieving Sustainable Development Goals (SDGs) related to water, sanitation, and health.

The temporal evolution in rural districts (Figure 4(b)) is different than in the urban ones (Figure 4(a)). Most of the people who live in rural areas rely on septic tanks because they lack a sewer network nearby. Specifically, in El Valle, Ricaurte, Sidcay and Sinincay districts are suffering a rapid expansion of informal settlements which makes it more difficult to provide sewer service. However, there also was a reduction in emptying frequency as well for most rural districts during the last four years – 2019 to 2022 (Figure 4(b), also observed in Figure 3). Even if in rural areas there was an effort to build wastewater decentralised systems, this effort did not have a significant consequence in reducing septic tank emptying. This phenomenon may be attributed to the persistently low coverage of decentralised systems, stemming from the non-consolidated nature of rural areas in the region. Furthermore, funding is scarce and cultural preferences for traditional sanitation practices may have contributed to this scenario. The local administration built rural wastewater treatment plants in Tarqui (3), El Valle (1), Ricaurte (1) and San Joaquín (1). However, in 2019, one of the plants in Tarqui was shut down due to the implementation of a sewer network, directing the wastewater to the centralised WWTP in Cuenca. Nevertheless, in some other districts such as Nulti and Paccha, the frequency of septic tank desludging did not show any significant reduction in the last years. In those rural districts, the sewer network has not undergone expansion and the absence of other decentralised wastewater systems made the use of septic tanks the only and simplest sanitation option in these areas.

Every year, the sewer system coverage increases in Cuenca, in urban and rural areas. With this increase, a decrease in septic tank use is expected, as was analysed in the last section. Here, we contrast the placement of sewer networks and active septic tanks for urban and rural areas in the year 2022 within a maximum distance of 2.4 km.

Moreover, 381 septic tanks were situated within the proximity of 20–300 m from the sewer network, while 73 were located between 300 and 600 m, 40 between 600 and 1,000 m, five within the range of 1,000–15,000 m, and 16 between 15,000 and 24,000 m (Figure 5). These distance categories highlight areas where the challenge is not the end-user connection but rather the effective management of FS or the incorporation of decentralised wastewater systems.

Quantification of FS produced is the initial and crucial step in establishing an effective management system which includes infrastructure for collection, transportation, and appropriately sized treatment units (Shukla et al. 2023). Thus, the accumulation rates were individually calculated for each surveyed septic tank obtaining a median value of 142 L/person/yr (0.39 L/person/d) with an Interquartile Range (IQR) of 252 L/person/yr. This value was higher than the one reported by Englund et al. (2020) for Hanoi city (32 L/person/yr) and Méndez Novelo et al. (2007) for Mérida México (20 L/person/yr), but in the range of the ones reported for China (350 g/person/d), Kenya (520 g/person/d) and Thailand (120–400 g/person/d) (assuming a density of 1 g/cm³) (Strande et al. 2014). It is worth noticing that, due to significant variation in desludging frequency for each septic tank (i.e service is required based on households' perception), the gathered data exhibited also a large standard deviation (525.82 L/person/yr). The significant dispersion included extreme values extending up to 1,600 L/person/yr. Caution is advised in the application of these values for FSM purposes.

For the collection and transport of FS, ETAPA EP has two emptying trucks. According to Taweesan et al. (2015), a city should have at least one vacuum truck per 1,000 households to achieve an FS collection with more than 80% efficiency. But in the case of Cuenca (5,636 active septic tanks during the study period), the collection efficiency is below that number. Despite this deficiency, Cuenca is the only city in the country that reports this service for the entire urban and rural area.

Each septic tank owner bears the responsibility of requesting the emptying service when it is perceived as necessary. The time it takes for a septic tank to fill up depends on the size, lining type, soil type, rainfall and groundwater level (Sharada Prasad & Ray 2019). Thus, it could take from one month to years for a septic tank to fill up and consequently be emptied. ETAPA EP receives the requests, organises them based on location and dispatches vacuum trucks capable of collecting 8 m³ each. To reach that volume, the trucks often collect FS from multiple septic tanks resulting in septage with highly variable quality and varying degrees of stabilisation. This is especially important in order to propose an adequate treatment for the FS. Technologies that support an enormous variety of solids and hydraulic loads would be needed.

In Ecuador, as in other developing countries, there is a lack of policies regarding the FSM. A robust and comprehensive policy is urgently needed in this regard. In Cuenca, as outlined earlier, there is a management plan in place, but it lacks specific policies. The findings of this study suggest the formulation of effective policies in two critical directions. First, policies should motivate and, if required, mandate the connection of households where sewer systems already exist, and emphasising education and proper information dissemination regarding environmental and public health benefits. Second, FSM policies should prioritise decentralisation, particularly in rural areas, and should incorporate suitable technologies to support this objective, given the ongoing and expected use of OSSs. Then, a substantial investment in infrastructure and efficient collection systems will also be required to ensure proper FS extraction. A holistic, policy-driven approach is needed for a sustainable FSM, encompassing the entire process from extraction to treatment.

This study explored the temporal and spatial use of OSSs represented by septic tanks in urban and rural areas. It also revealed remaining septic tanks in places where there is an available sewer network and the FS management situation. As a study case, it was performed in Cuenca, Ecuador.

The study showed that some urban districts have a septic tank/inhabitants ratio comparable to the rural districts. This observation exhibited that the urban/rural classification based on sanitation services does not always accurately reflect the actual situation of some districts. Furthermore, of the 109 active septic tanks in the urban area, still, 14.6% of them were located within distances closer than 20 m from the sewer network. In the rural area, of the 469 operating tanks, 7.8% were located within the 20 m. Finally, 381 septic tanks were located between 20 and 300 m of the sewer network. This shows the challenge of end-user connection between dwellings and sewage pipelines. Governments need to address these connectivity challenges in a particular context, first by generating and collecting high-quality data to understand the magnitude and causes of the connectivity problem; and then by testing the effectiveness of different interventions. According to this study, a measure that effectively reduced the use of OSS in the urban area was the removal of the septic tank empty subsidy, showing that unfortunately, the economic factor is the only effective one in many cases. There are numerous reasons for promoting the end-user connection when there is sewage system coverage. There are important health and environmental benefits, as well as financial operability of the systems.

It results evident that the rapid population growth and informal human settlements, both in urban and rural areas, hinder the progress of centralised sewer system coverage from meeting the increasing year-to-year demand. The use of OSSs will continue to be a necessity. In our context, septic tanks stand as the most prevalent system in areas without access to sewer networks. Therefore, more extensive studies are required to quantify and characterise the FS from these systems, which is crucial for effective operation and consequent FSM.

Cuenca lacks a dedicated system solely for the treatment and final disposal of septic tank sludge. However, using the wastewater treatment plant for treatment purposes does contribute to some form of FSM, preventing it from causing environmental contamination. Knowing that the FS is highly variable, appropriate technologies should be studied. These technologies must be capable of handling the high hydraulic and solid loads associated with FS.

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

The authors declare there is no conflict.