The integration of a solar septic tank (SST) system with an economically viable solar water-heating apparatus comprising polyethylene (PE) and copper (Cu) pipes, as well as a lightweight structural framework, has been conceptualized and implemented. Two variants of low-cost SSTs were subjected to empirical scrutiny within authentic settings, encompassing public toilets and residential establishments in Cambodia. Both the PE pipe and Cu pipe solar water-heating devices demonstrated efficacy in consistently maintaining temperatures within the system above ambient levels. Although the parameters do not conform to Cambodia's effluent requirements, the results of this study suggest that the SST is effective in reducing organic loads and provides a significant improvement over conventional septic tank effluent. This underscores the potential applicability of the developed system for the treatment of toilet wastewater, thereby mitigating pollution concerns and public health risks. Harnessing solar energy to elevate septic tank temperature, this system's cost varies but includes materials, tools, and miscellaneous components, with a total estimated cost ranging from $600 to $2500, depending on size and complexity.

  • An innovative solar water heating system, using PE and Cu pipes, efficiently raises water temperature and maintains it above ambient levels, ensuring optimal functionality.

  • Tested in Cambodia's diverse environments, the low-cost tanks provide consistent heating and adhere to standards, showing potential for eco-friendly treatment and reducing public health risks.

Water, sanitation, and hygiene (WASH) are significant global concerns and have become priority areas in the research and international development sectors. About 2.4 billion people worldwide live without improved sanitation systems. Waterborne diseases, including diarrhea caused by inadequate sanitation facilities and drinking water, have resulted in the deaths of about 760,000 children annually around the world (United Nations Children's Fund (UNICEF) & World Health Organization (WHO) 2019). Additional deleterious consequences encompass diminished life expectancy, water contamination, reduced daily activity time, and missed prospects for converting human excreta into energy or fertilizer. Cambodia, situated among the developing nations in Southeast Asia, grapples with the significant challenge of inadequate sanitation, which has a profound impact on public health. Despite certain strides made in enhancing sanitation initiatives within Cambodia, there is a compelling need for further demonstrations of efficacious sanitation technologies. Cambodia stands out as one of the developing nations in Southeast Asia that is grappling with the pervasive issue of inadequate sanitation. This problem has a profound impact on public health. In recent years, there has been a surge in initiatives aimed at improving sanitation in Cambodia. Kampong Chhnang City, serving as the capital of Kampong Chhnang Province and situated northeast of Phnom Penh City, has embraced onsite sanitation practices to an extent of approximately 100%. This city, traversed by the National Highway 5 and the National Railway, spans an expansive area of nearly 47 km2. The population stands at 42,916, exhibiting an annual growth rate of approximately 2.04%. Located in the extensive plain of the Tonle Sap Basin-Mekong lowland region, Kampong Chhnang City rests at an elevation of about 17 meters above sea level. The annual precipitation in this region varies between 800 and 20,000 mm. Pursat City, located in Pursat Province, Cambodia, covers a vast area of 271.5 km2. The city has a resident population of 66,621, and an additional floating population estimated at 288,185 living in the Tonle Sap Basin annually. This contributes to an overall population growth rate of approximately 2.49%. About 98% of the township primarily relies on onsite sanitation systems for toilet wastewater. Positioned equidistantly between the Tonle Sap Lake in the east and the Cardamom Mountains in the west, Pursat town is situated along the banks of the Pursat River (ADB 2014). The cities close to the Tonle Sap River, a vital natural resource in Cambodia, are crucial for minimizing pollution and represent areas of focus for improving environmental and sanitation conditions.

In the context of primary treatment for toilet wastewater, onsite sanitation (OSS) emerges as a system that involves storage and treatment within the confines of individual households or small communities. It employs commonly used septic tanks and cesspool systems (Bari et al. 2018). The cesspool system, which is characterized by an open bottom that facilitates the percolation of liquid into the adjacent soil, is susceptible to receiving water from the surrounding area (Schouw et al. 2002; Geels 2006). The septic tank serves as a holding tank or primary treatment to separate solids and liquids from toilet wastewater. The accumulated solids, known as septage sludge or fecal sludge (FS), require regular emptying or desludging and further treatment to ensure safe sanitation management (Harada et al. 2008). Most conventional septic tanks serve as the primary treatment or the baseline scenario for toilet or wastewater treatment in many countries across Asia. Operating within a temperature range of 25–30 °C, these systems often demonstrate relatively low removal rates due to design limitations and operational factors. Studies indicate that organic matter removal rates typically range from 50 to 60%, nutrient removal from 40 to 60%, and pathogen reductions of less than 1 log. These findings emphasize the necessity for improvements in the treatment systems (Connelly et al. 2019; Koottatep et al. 2020a, 2020b). Additionally, septic tanks necessitate a designated drainfield as per the proposed design. However, in urban or certain rural settings, detailed specifications for the drainfield may still be lacking. Consequently, effluents are often discharged directly into nearby canals or surrounding soil. This poses challenges for septic tanks to meet effluent quality requirements, particularly in developing countries (Koottatep et al. 2014a, 2014b). In the absence of proper management, fecal sludge tends to accumulate within inadequately designed OSS systems. It is commonly disposed of by being released into stormwater drains or surface water bodies or indiscriminately dumped into the surrounding environment, wetlands, and unsanitary landfill sites (Giri et al. 2005; Blackett et al. 2014). It was found that only a minor proportion of fecal sludge was managed and treated properly (Peal et al. 2015). Furthermore, in certain urban areas of low- and middle-income countries, it has been estimated that less than half of the daily generated fecal sludge is collected, and only approximately half of the collected sludge undergoes appropriate treatment (Blackett et al. 2014).

The solar septic tank (SST) represents an innovative iteration within OSS systems and is a modified version of the conventional septic tank that has been enhanced with a solar-heating component. Previous studies indicate that the operating temperature of SST can be enhanced to a range above 30 °C up to 50% (Koottatep et al. 2014a, 2014b, 2020a, 2020b; Pussayanavin et al. 2020). By harnessing solar energy, this system raises temperatures within the septic tanks, effectively deactivating pathogens. As a result, there is a reduction of more than 2 logs when the temperature increases to 40 °C during operation. Furthermore, it facilitates the conversion of organic wastes into methane (biogas), offering a dual benefit of pathogen inactivation and bioenergy production by using an airbag. The integration of solar heating in the SST contributes to address environmental issues associated with the management of fecal sludge in traditional septic tank systems (Polprasert et al. 2018; Connelly et al. 2019; Pussayanavin et al. 2020). Koottatep et al. (2020a, 2020b) reported that the removal efficiencies of SSTs operating at temperatures of 40–70 °C were more than 80% for COD and BOD5. Typically, the SSTs utilized commercial solar-heated water systems such as evacuated vacuum tubes or other flat plate solar-heated water systems for energy harvesting, which are then employed in SST systems. The cost of solar-heated water could range from 1,000 to 5,000 US$. The high installation costs of commercial versions, implementing low-cost options of solar-heated water systems, are proposed as a solution for low-income contexts.

Hence, the creation of a do-it-yourself (DIY) solar water heater that utilizes the design and implementation of SST with polyethylene (PE) or copper (Cu) pipes results in a low-cost structure that is both cost-effective and readily accessible. To mitigate both initial investment and subsequent maintenance expenses, a low-cost SST has been devised, incorporating widely available materials such as PE and Cu pipes as solar water-heating components. This study aims to scrutinize the treatment performance and assess the technical feasibility of two distinct variants of low-cost SST in actual field conditions, specifically deployed in public toilets and households throughout Cambodia.

Prototype description

The SST system was integrated with low-cost solar water-heating devices made from PE and Cu pipes, along with a lightweight structure. The SST structure, consisting of a septic tank and a disinfection chamber (Koottatep et al. 2020a, 2020b), harvests free energy from the sun to heat the water inside the pipes and circulate hot water to increase the temperature inside the septic tank. The SST unit comprises two compartments. The initial compartment facilitates solid settling and anaerobic digestion of collected solids. Liquid from the first chamber overflows into the second compartment, known as the disinfection chamber, which is constructed from plastic. This chamber acts as a polishing unit, aiming to minimize the impact of short-circuiting from impulse flow, while enhancing pathogen inactivation in the effluent.

The low-cost SST, which is 650-L in size and equipped with solar collectors made from the PE pipe, was designed for a one-family unit. Its cost is approximately 810 US$. The low-cost SST, which is 1,000 L in size and equipped with solar collectors made from the Cu pipe, was designed for installation at community levels such as health centers and schools. Its cost is approximately 2,130 US$. Area requirements for the solar collector made from PE and Cu pipes were 5.8 and 8.9 m2, respectively. The PE pipe version consists of two panels (Figure 1), while the Cu pipe version is composed of three panels (Figure 1). To support the investigation and facilitate the ease of operation, the installed solar collectors were installed on the ground. Five units of the low-cost SST were installed and tested at Kampong Chhnang and Pursat Provinces in Cambodia.
Figure 1

Solar water-heating device (a) made from the PE pipe (with an SST size of 650 L) and (b) made from the Cu pipe (with an SST size of 1,000 L).

Figure 1

Solar water-heating device (a) made from the PE pipe (with an SST size of 650 L) and (b) made from the Cu pipe (with an SST size of 1,000 L).

Close modal

The investigation involved testing five units of the SST in Cambodia's Kampong Chhnang and Pursat Provinces. Specifically, two units were installed at the Kbal Teuk health center and a nearby household situated in the Kbal Teuk commune, Tuek Phos district, Kampong Chhnang Province. Additionally, three units of the SST were installed in the Kbal Trach commune, Krakor district, Pursat Province, with two units located at Trapaeng Smach primary school and one unit at a nearby household. The representation of community units at healthcare facilities and schools may vary depending on user activity and periods of non-use observed during monitoring. The installation duration for each SST unit ranged from 2 to 3 days (the groundwork was supported by each community, while the technical aspects were handled by the research team from the Asian Institute of Technology), contingent upon site conditions and weather considerations. The installation process comprised various steps, including land preparation, pipe connection, septic tank placement, and installation of solar water-heating devices. The septic tank installation followed a structured procedure, encompassing three principal steps including the excavation of a hole with dimensions approximately 120 × 140 cm (depth × width) and the application of piles and foundation preparation using sand and concrete (approximately 7 cm), or alternatively, the use of cement ring covers at the bottom for foundation purposes, with the cement ring employed to control landslides. The septic tank is placed in the prepared hole, settled down and subsequently filled with water. To facilitate the connection between the toilet and the septic tank, various pipes and accessories were employed, including PVC joints (45 and 90° bends), T-way pipes, PVC pipes, flexible rubber pipes, PVC glue, galvanized steel U bolts, and stainless clamps. The proper placement of the SST on the pre-fabricated foundation within the pit was imperative. The inlet of the septic tank was linked to the toilet outlet using PVC drainage, while the outlet hole of the septic tank was connected to a PVC pipe drainage. The septic tank was filled with water to prevent damage before surrounding it with sand. Subsequently, the tank was backfilled with soil, and the top was insulated using an elastomer material.

The installed units in Cambodia are succinctly presented (Figure 2) as follows:
  • SST 01: SST operational at a primary school in Pursat

  • SST 02: SST operational at a household in Pursat

  • SST 03: SST operational at a high school in Pursat

  • SST 04: SST operational at a healthcare center in Kampong Chhnang

  • SST 05: SST operational at a household in Kampong Chhnang

Figure 2

Pursat and Kampong Chhnang sites ((a) SST 02, (b) SST 03, (c) SST 01, (d) SST 04, and (e) SST 05).

Figure 2

Pursat and Kampong Chhnang sites ((a) SST 02, (b) SST 03, (c) SST 01, (d) SST 04, and (e) SST 05).

Close modal

Monitoring

The SST was in the operational phase for a duration of six months (November 2018 to the end of March 2019), during which it was subjected to the dynamic conditions of fluctuating flow rates, ambient temperatures, and the characteristic composition of black water. This period was crucial for assessing the performance of the SST. The elevation of temperatures within the septic tank was achieved through the circulation of hot water, which was generated by the solar water-heating device through a heat transfer apparatus (stainless coil). A pumping mechanism was employed to ensure a continuous flow of hot water, thereby maintaining the desired temperature within the septic tank. To monitor temperature variations, PT100 sensors were strategically placed in the disinfection chamber and the liquid within the septic tank (Figure 3). Concurrently, ambient temperature readings were recorded for comparative analysis. It is noteworthy that the operation of the SST necessitates an AC power supply to support the water pump and the temperature sensor controller. Additionally, a continuous supply of tap water or rainwater is essential to facilitate the functioning of the solar water-heating device, specifically the solar collector.
Figure 3

Components for operating (1) SST, (2) temperature recorder, and (3) sampling points of the SST in Cambodia.

Figure 3

Components for operating (1) SST, (2) temperature recorder, and (3) sampling points of the SST in Cambodia.

Close modal

To evaluate the treatment performance of the low-cost SST, grab samplings were done once a month from November 2018 to March 2019. Uniform samples were collected from both the influent and effluent points of each SST unit (Figure 3) for analyses of total suspended solids (TSSs), volatile suspended solids (VSSs), biochemical oxygen demand (BOD), and chemical oxygen demand (COD) concentrations, according to the Standard Methods (APHA 2017). Furthermore, the perception of sanitation technology was conducted by interviewing users' opinions.

The SST units were actively operational from November 2018 to the end of March 2019. The primary objective during this operational period was to comprehensively assess the treatment performance of the system and evaluate the effluent quality of the SST units in accordance with relevant discharged standards. The estimated design flow rate for the proper usage of the SSTs ranged from 80 to 2,000 L/d. The average ambient temperature ranged from 21 to 32 °C, with observations showing that the average surface water temperature consistently remained below 28–30 °C during the day. The influent concentration ranges for COD, BOD, TSS, and VSS in wastewater span from lower to upper limits: COD of 2,600–19,653 mg/L, BOD of 898–5,448 mg/L, TSS of 973–5,489 mg/L, and VSS of 492–3,747 mg/L. This evaluation was conducted under real-world conditions characterized by varying flow rates, ambient temperatures, and the distinct characteristics of black water.

The overarching goal was to ascertain the system's capacity to align with and adhere to the wastewater standards established by Cambodia, as delineated in Table 1.

Temperature profile

PE pipe

Figure 4 shows temperature profiles in 24 h of the SST with the PE pipe located at Pursat Province. The temperatures inside the 650-L SST and in the disinfection chamber were higher than the ambient temperature, reaching the maximum temperature in the disinfection chamber of 47 ± 3 °C during the period of 10.00 am–06.00 pm. Figure 4 shows temperature profiles of the SST over 24 h. The PE pipe, located in Kampong Chhnang Province, reaches the maximum temperature of 41 ± 3 °C. This peak temperature is observed in the disinfection chamber during the period of 12.00 pm–06.00 pm. A similar study reported that the 40-solar vacuum tube heat pipe (Compact Pressure Model, Shanghai Ruty Energy Co., Ltd, China) could cause temperatures inside a 600-L up-flow SST to be in the range of 26–37 °C, while the average ambient temperature was about 32 °C (Koottatep et al. 2020a, 2020b). The results of Figure 4 indicate that the low-cost SST with the PE pipe could achieve higher temperatures in the system than the 40-solar vacuum tube system.
Figure 4

Temperature profile of SST with the PE pipe at (a) Pursat and (b) Kampong Chhnang.

Figure 4

Temperature profile of SST with the PE pipe at (a) Pursat and (b) Kampong Chhnang.

Close modal

Cu pipe

Figure 5 shows the temperature profile in 24 h of the 1,000-L SST with the Cu pipe located at Pursat, which was similar to that of Figure 2 with the maximum temperature reaching 43 ± 1 °C in the disinfection chamber during the period of 11.00 am–06.00 pm. Figure 5 shows temperature profiles in 24 h of the 1,000-L SST with the Cu pipe at Kampong Chhnang, in which the maximum temperature in the disinfection chamber of 49 ± 3 °C occurred during the period of 09.00 pm–06.00 pm. In general, variations in the ambient and SST temperatures would depend on climatic factors such as cloudy or rainy and daytime and nighttime. The data of Figure 5 suggested that the low-cost SST with the Cu pipe could reach the maximum temperature in the thermophilic phase and promote microbial reactions for organic degradation, CH4 production, and pathogen inactivation. Polprasert et al. (2018) reported that a 1,000-L SST with 36-solar vacuum tubes (6 m2) could result in average temperatures of the SST liquid of 41 ± 4 °C in the daytime and 40 ± 3 °C in the nighttime. The solar system's performance was effective in the nighttime because there was a 200-L hot water storage tank. A previous study by Pussayanavin et al. (2020) found the increased temperatures in the SST to be effective in inactivating the fecal bacteria, thus minimizing potential health risks to the local people.
Figure 5

Temperature profile of SST with the Cu pipe at (a) Pursat and (b) Kampong Chhnang.

Figure 5

Temperature profile of SST with the Cu pipe at (a) Pursat and (b) Kampong Chhnang.

Close modal

The integrated SST system, when combined with a low-cost solar collector, reduces costs by minimizing structural expenses. The need for separate or additional infrastructure is minimized, leading to cost-savings and enabling mass production with local materials. Composed of PE and Cu pipes and featuring a lightweight structure, this system has undergone rigorous testing for validation. The system harnesses solar energy to heat water within the pipes and circulates the resultant hot water to elevate the temperature inside the septic tank, specifically designed for a single-family unit. The cost details of the low-cost SST can vary, contingent upon factors such as system size, materials utilized, and geographical location. A comprehensive breakdown of potential costs is outlined below:

  • – Materials:

    • o Solar collector: $100–$2,000 or more, depending on the type and the size.

    • o Pipes or hoses: $20–$50, varying based on length and diameter.

    • o Insulation: approximately $20–$50 for insulation materials.

    • o Frame and cover: $50–$200 for the frame and $20–$100 for the cover material.

  • – Tools: basic tools such as a drill, saw, and screwdriver: $50–$100 if not already available.

  • – Water stage tank: cost varies significantly based on size and material, ranging from $100 to $500 or more.

  • – Circulating pump: estimated cost of $50–$200.

  • – Miscellaneous: additional fittings, connectors, valves, and plumbing materials: $50–$100.

The total estimated cost is expected to range from $600 to $2,500 or more, contingent upon the complexity and size of the system. The routine maintenance for solar collector, piping system, and circulating pump is required twice a year for optimal performance. In Cambodia, SSTs with solar collectors made of PE pipes, coupled with 650 L septic tanks, were installed to demonstrate the feasibility of SST performance in households in Kampong Chhnang and Pursat. Additionally, SSTs with solar collectors made of Cu pipes, paired with 1,000 L septic tanks, were installed at community levels, such as health centers and schools, in Kampong Chhnang and Pursat, respectively.

Treatment performance

During the operational phase of the SST, several challenges were encountered, impacting the sampling process and system functionality. In the initial sampling period, SST units 01 and 03 were inactive with toilet clogging issues, pre-existing toilet problems, and low rate of wastewater flow resulting from the low activities in the communities in some periods, rendering sample collection unfeasible. Subsequently, during the subsequent sampling, SST units 01 and 03 faced clogging issues attributed to pre-existing toilet problems, while SST unit 04 exhibited no effluent for collection.

Despite these operational challenges, the wastewater characteristics of influent and effluent from the five pilot-scale testing units in Cambodia were analyzed. Throughout the testing period, a marginal variation in wastewater characteristics was observed across all testing sites. The pH values of influents ranged from 6.12 to 8.16, whereas effluents exhibited values between 7.26 and 8.62 for all five SST units. This observed variation is likely attributed to differences in sampling times and temperature fluctuations. However, when compared to Cambodia's wastewater standards, the pH values of the SST effluents consistently met the prescribed standard values. The dissolved oxygen (DO) levels demonstrated seasonal fluctuations, with slight changes influenced by temperature variations and the decomposition of organic matter. The recorded DO effluent concentrations for all SST units exceeded 1 mg/L, aligning with Cambodia's wastewater standards for discharge into the surrounding environment. The DO in the effluent is likely affected by the conditions of the disinfection chamber, as it is connected to the effluent pipe, allowing mixing and turbulence with the outside air. Oxidation–reduction potential (ORP) serves as an indicator of a wastewater system's ability to support specific biological processes. Monitoring ORP aids in understanding process efficiency and identifying treatment issues before they impact effluent quality. For instance, specific ORP value ranges correspond to various biological activities, such as sulfide formation, biological phosphorus release, and acid formation. Most notably, the observed ORP values for the effluents of each SST unit predominantly fell within the range of −50 to −250 mV, indicative of sulfide formation.

Table 1

Wastewater discharge standard in Cambodia (public water area and sewer)

No.ParametersUnitAllowable limits for pollutant substance discharging to the public water area and the sewera
Temperature °C <45 
pH  5–9 
DO mg/L >1.0 
ORP mV – 
BOD5 (5 days at 20 °C) mg/L <80 
COD mg/L <100 
TSS mg/L <80 
VSS mg/L – 
No.ParametersUnitAllowable limits for pollutant substance discharging to the public water area and the sewera
Temperature °C <45 
pH  5–9 
DO mg/L >1.0 
ORP mV – 
BOD5 (5 days at 20 °C) mg/L <80 
COD mg/L <100 
TSS mg/L <80 
VSS mg/L – 

Solid parameter

The effluent concentrations of TSS from the low-cost SST exhibited a range of 16–1,740 mg/L. This variability in TSS concentrations can be attributed to the elevated temperatures within the SST, which likely led to a reduction in liquid density and viscosity. This, in turn, facilitated the improved sedimentation of incoming TSS particles. Furthermore, the incorporation of a disinfection chamber within the SST structure (as depicted in Figures 1 and 2) played a role in preventing the leaching of small particles into the effluent. In this study, the solid parameters under consideration include both TSS and VSS (Table 2). Notably, a substantial decrease in both TSS and VSS concentrations was observed after release from all SST units. The inclusion of the disinfection chamber proved instrumental in preventing the leaching of small particles into the septic tank effluent, thereby averting shock loading and reducing flow short-circuiting and particle sedimentation (Koottatep et al. 2014a, 2014b; Haydar et al. 2018). The recorded effluent values for TSS ranged from 16 to 1,740 mg/L, while VSS values spanned from 30 to 1,740 mg/L.

Table 2

TSS and VSS values of wastewater from each site of SSTs

DateSST 01
SST 02
SST 03
SST 04
SST 05
INFEFFINFEFFINFEFFINFEFFINFEFF
TSS (mg/L) 
04-Nov-18 N/A N/A 2,467 100 68 N/A 1,733 153 367 127 
14-Dec-18 918 16 5,645 58 102 N/A 1,107 N/A 9,500 168 
06-Jan-19 21 N/A 4,270 42 81 N/A 327 N/A 5,010 1,860 
02-Feb-19 12 N/A 7,560 18 26 N/A 1,090 N/A 7,580 40 
03-Mar-19 36 N/A 8,740 47 70 N/A 610 N/A 4,990 55 
VSS (mg/L) 
04-Nov-18 N/A N/A 2,342 153 64 N/A 867 300 447 113 
14-Dec-18 824 30 4,400 18 33 N/A 463 N/A 890 32 
06-Jan-19 46 N/A 4,410 90 111 N/A 280 N/A 4,860 1,740 
02-Feb-19 29 N/A 7,160 37 40 N/A 515 N/A 7,470 26 
03-Mar-19 45 N/A 8,350 57 74 N/A 334 N/A 5,070 52 
DateSST 01
SST 02
SST 03
SST 04
SST 05
INFEFFINFEFFINFEFFINFEFFINFEFF
TSS (mg/L) 
04-Nov-18 N/A N/A 2,467 100 68 N/A 1,733 153 367 127 
14-Dec-18 918 16 5,645 58 102 N/A 1,107 N/A 9,500 168 
06-Jan-19 21 N/A 4,270 42 81 N/A 327 N/A 5,010 1,860 
02-Feb-19 12 N/A 7,560 18 26 N/A 1,090 N/A 7,580 40 
03-Mar-19 36 N/A 8,740 47 70 N/A 610 N/A 4,990 55 
VSS (mg/L) 
04-Nov-18 N/A N/A 2,342 153 64 N/A 867 300 447 113 
14-Dec-18 824 30 4,400 18 33 N/A 463 N/A 890 32 
06-Jan-19 46 N/A 4,410 90 111 N/A 280 N/A 4,860 1,740 
02-Feb-19 29 N/A 7,160 37 40 N/A 515 N/A 7,470 26 
03-Mar-19 45 N/A 8,350 57 74 N/A 334 N/A 5,070 52 
Table 3

COD and BOD values of wastewater from each site of SSTs

DateSST 01
SST 02
SST 03
SST 04
SST 05
INFEFFINFEFFINFEFFINFEFFINFEFF
COD (mg/L) 
04-Nov-18 N/A N/A 2,900.0 360.0 150.0 N/A 1,300.0 88 2,100.0 200.0 
14-Dec-18 44.0 85 2,600.0 104.0 360.0 N/A 880.0 N/A 9,600.0 256.0 
06-Jan-19 34.5 N/A 5,172.0 206.9 34.5 N/A 965.4 N/A 7,930.0 3,017.0 
02-Feb-19 34.5 N/A 19,653.0 103.4 34.5 N/A 344.8 N/A 10,688.0 431.0 
03-Mar-19 75.9 N/A 15,516.0 289.6 179.3 N/A 1379.2 N/A 10,344.0 275.8 
BOD (mg/L) 
04-Nov-18 N/A N/A 922.5 50.1 58.8 N/A 363 50.1 898.5 40.5 
14-Dec-18 8.7 25 801.0 53.0 45.0 N/A 726.0 N/A 1,821.0 77.0 
06-Jan-19 73.5 N/A 2,172.0 30.0 21.8 N/A 365.4 N/A 2,703.0 469.8 
02-Feb-19 23.1 N/A 748.5 22.9 52.9 N/A 322.1 N/A 5,448.0 44.4 
03-Mar-19 44.8 N/A 2,779.0 64.8 91.7 N/A 642.0 N/A 4,201.0 44.9 
DateSST 01
SST 02
SST 03
SST 04
SST 05
INFEFFINFEFFINFEFFINFEFFINFEFF
COD (mg/L) 
04-Nov-18 N/A N/A 2,900.0 360.0 150.0 N/A 1,300.0 88 2,100.0 200.0 
14-Dec-18 44.0 85 2,600.0 104.0 360.0 N/A 880.0 N/A 9,600.0 256.0 
06-Jan-19 34.5 N/A 5,172.0 206.9 34.5 N/A 965.4 N/A 7,930.0 3,017.0 
02-Feb-19 34.5 N/A 19,653.0 103.4 34.5 N/A 344.8 N/A 10,688.0 431.0 
03-Mar-19 75.9 N/A 15,516.0 289.6 179.3 N/A 1379.2 N/A 10,344.0 275.8 
BOD (mg/L) 
04-Nov-18 N/A N/A 922.5 50.1 58.8 N/A 363 50.1 898.5 40.5 
14-Dec-18 8.7 25 801.0 53.0 45.0 N/A 726.0 N/A 1,821.0 77.0 
06-Jan-19 73.5 N/A 2,172.0 30.0 21.8 N/A 365.4 N/A 2,703.0 469.8 
02-Feb-19 23.1 N/A 748.5 22.9 52.9 N/A 322.1 N/A 5,448.0 44.4 
03-Mar-19 44.8 N/A 2,779.0 64.8 91.7 N/A 642.0 N/A 4,201.0 44.9 

Chemical parameters

The influent COD and BOD concentrations of the low-cost SST displayed fluctuations, ranging from 44 to 19,653 mg/L for COD and 25 to 5,448 mg/L for BOD (Table 3). These variations were attributed to the diverse composition and quantity of feces, as well as differences in the volume of flushing water used across the testing sites. Despite the influent variability, the settling and biodegradation processes within the SST led to significant reductions in effluent concentrations. Effluent COD values ranged from 34.48 to 3,017 mg/L, and BOD values ranged from 22.86 to 469.8 mg/L. Notably, the effluent values of BOD for SST 01, SST 02, SST 03, and SST 04 remained within the prescribed effluent standard values of Cambodia during the monitoring period. However, there was an overshoot value in the effluent of SST 05, exceeding the standard. Despite influent variability, as indicated by our findings where samples from 11 septic tank effluents in this study met BOD < 80 mg/L while one exceeded it, two met COD < 100 mg/L while 11 exceeded it, and seven met TSS < 80 mg/L while five exceeded it, our study sheds light on the challenges associated with meeting effluent quality standards in OSS systems. Although these parameters do not completely conform to Cambodia's effluent requirements, our results suggest that SST presents a viable solution for mitigating organic pollution. The observed variability in effluent characteristics underscores the complex nature of wastewater treatment processes within septic tank systems. Despite efforts to optimize treatment efficiency, challenges persist in achieving comprehensive removal of organic pollutants. However, our study highlights the effectiveness of SSTs in addressing these challenges. Long-term monitoring will be necessary for future studies to investigate the long-term effluent quality and performance of certain units. The sedimentation and biodegradation processes inherent to the low-cost SST contribute to the reduction of effluent BOD concentration, confirming the system's capacity to produce treated effluent that aligns with public acceptability. Continued operation over an extended period is expected to further enhance the system's efficiency, underscoring its potential to effectively treat toilet wastewater.

Table 4

Acceptance of technologies for the sanitation treatment system

TechnologiesBenefitsAcceptance of technologies
1. Alarm signal for FS storage system Able to know before the FS storage system is full Desirable 
2. FS storage system can produce electricity Used as energy for household electrical application Desirable 
3. Material for the FS storage system to help digestion Able to disinfect for the FS storage system and reduce desludging Desirable 
4. Add the microorganisms that help to digest Able to increase digestion efficiency and reduce desludging Desirable 
5. SST Disinfect the FS storage system using renewable energy from sunlight Desirable 
TechnologiesBenefitsAcceptance of technologies
1. Alarm signal for FS storage system Able to know before the FS storage system is full Desirable 
2. FS storage system can produce electricity Used as energy for household electrical application Desirable 
3. Material for the FS storage system to help digestion Able to disinfect for the FS storage system and reduce desludging Desirable 
4. Add the microorganisms that help to digest Able to increase digestion efficiency and reduce desludging Desirable 
5. SST Disinfect the FS storage system using renewable energy from sunlight Desirable 

User attitude

To a preliminary and highly participatory discussion to explore the challenges associated with introducing SSTs and to gather additional views or opinions regarding perceptions toward SSTs, the interviewers included local community representatives, a representative from the Pursat Provincial Department of Rural Development (DPRD), a representative from the local consultant agency, directors of high schools and primary schools, and household users of SSTs. The interviews conducted with seven users of the low-cost SST in Pursat Province revealed that over three of the users have agreed to pour squat toilets equipped with two cement rings to treat toilet wastewater. This preference is attributed to the affordable pricing and ease of installation associated with the low-cost SST. The users' perceptions regarding the comprehension of wastewater and fecal sludge management are detailed in the Supplementary material, Table S1. The interviews were further done to seek the acceptable level of sanitation technology for further development of innovative technologies for actual application. From the interview results (Tables 4 and 5), the requirements for innovative technologies were increased treatment efficiency using more durable and cheaper materials, reduced septic tank size reduced desludging frequency, and ability to produce useful byproducts from FS (electricity, fertilizer, etc.).

Table 5

Byproduct from the sanitation treatment system

ByproductApplicationsAcceptance of byproduct
1. Water Watering or washing the floor (outside) Desirable 
2. Electricity Used as energy for household electrical appliances (from the biogas as a byproduct) Desirable 
3. Fertilizer Used as a fertilizer Desirable 
ByproductApplicationsAcceptance of byproduct
1. Water Watering or washing the floor (outside) Desirable 
2. Electricity Used as energy for household electrical appliances (from the biogas as a byproduct) Desirable 
3. Fertilizer Used as a fertilizer Desirable 

Regarding the applicability of the low-cost SST based on the interview results, about four users thought the SST maintenance was at a moderate price. More than half of the users said that using the SST with less pollution was comfortable. Over four of them mentioned that the SST was expensive and not affordable. About six said that the SST maintenance was more accessible than the old system of two cement rings for toilet wastewater treatment. They also stated that the SST was very safe for the environment and the people with better hygiene and produced less odor than the old system. Moreover, they require the SST price to be affordable (200–300 US$ per person) and need more training on improving sanitation service.

In summary, the low-cost SST equipped with both PE and Cu pipes has proven effective in consistently maintaining temperatures above ambient levels. This study reveals influent variability in septic tank effluents, with 11 samples meeting BOD < 80 mg/L, two meeting COD < 100 mg/L, and seven meeting TSS < 80 mg/L, indicating challenges in meeting effluent quality standards. Despite not fully conforming to Cambodia's requirements, our findings suggest that septic tank treatment (SST) effectively reduces organic pollution. The complexity of wastewater treatment processes in septic tank systems is underscored by the observed variability in effluent characteristics. SSTs show promise in addressing challenges associated with organic pollutant removal, offering a viable solution for OSS systems. User interviews highlighted the potential of the low cost. However, challenges in adoption include the need for an affordable price and considerations regarding system maintenance. The conclusions are summarized as follows:

  • – Effluent concentrations of TSS from the low-cost SST ranged from 16 to 1,740 mg/L. The incorporation of a disinfection chamber within the SST structure contributed to preventing the leaching of small particles into the effluent.

  • – Despite influent variability, the SST's biodegradation processes resulted in significant reductions in effluent COD and BOD concentrations. Effluent COD values ranged from 34.48 to 3,017 mg/L, and BOD values ranged from 22.86 to 469.8 mg/L.

  • – Users find the maintenance of the low-cost SST to be moderately priced and more than half of users express comfort in using the SST, emphasizing reduced pollution. Users highlight the SST's safety for the environment, improved hygiene, and reduced odor compared to the existing OSS system.

  • – An integrated SST system, incorporating PE and Cu pipes along with a lightweight structure, undergoes rigorous validation. This system harnesses solar energy to elevate septic tank temperature. Cost details vary but include materials, tools, and miscellaneous components, with a total estimated cost ranging from $600 to $2,500, depending on system size and complexity. This showcases its potential as an alternative option for minimizing pollution in certain areas in Cambodia through installations with PE pipe solar collectors in households and Cu pipe solar collectors in community-level facilities.

This research was financially supported by the Asian Development Bank (ADB) and the Royal University of Phnom Penh was supported the laboratory analysis.

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

The authors declare there is no conflict.

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Supplementary data