ABSTRACT
A sufficient water supply is important for human health; however, it results in the generation of large amounts of domestic wastewater. This study aimed to determine the impact of domestic wastewater on the surrounding environment as well as residents' opinions on the issue. Domestic wastewater consists of blackwater (toilet wastewater) and greywater (miscellaneous domestic wastewater), each of which is discharged in different ways. Blackwater was treated using a septic tank, without the required desludging, despite policy goals, owing to a lack of awareness among residents as well as structural problems. However, the targeted septic tanks efficiently removed microbiological indicators. It is assumed that the larger capacity and tropical savanna climate, with consistently high temperatures, were beneficial for the treatment process. Greywater was discharged into public waters without any treatment and caused degradation of the waterside environment. More than 70% of respondents thought there was a problem with the canal that flows close to houses; however, there was variation in the awareness. It is necessary to reduce the greywater load from each household at the source as much as possible, and this goal requires residents to have an accurate understanding of greywater emissions.
HIGHLIGHTS
In developing Asia, blackwater was treated using a septic tank, but greywater was discharged without treatment.
The large capacity, tropical climate, and high temperatures support the septic tank system.
Untreated greywater discharged into public waters causes environmental degradation.
Reducing the household greywater load requires an accurate understanding of greywater emissions.
INTRODUCTION
In developing Asia, the availability of a sufficient water supply, following the United Nations (UN) Millennium Development Goals, set as common goals for the international community from 1990 to 2015, and economic development led to the spread of lifestyles based on a stable water supply and the generation of large amounts of domestic wastewater (Cabeza et al. 2018; Apanga et al. 2020). However, as domestic wastewater is often not properly treated, developing Asia faces numerous management and infrastructure challenges (Gallego-Schmid & Tarpani 2019). Traditional centralized sewers, such as those in developed countries, were built in some large cities; however, their widespread implementation is impractical owing to costs, maintenance difficulties, and the inability to keep pace with increasing urbanization and wastewater generation (Swan et al. 2023). This requires a decentralized approach, that is, processing on a community or household basis, where many problems arise (Ryals et al. 2019; Muzioreva et al. 2022).
Domestic wastewater consists of blackwater (toilet wastewater) and greywater (miscellaneous domestic wastewater from kitchens, bathrooms, and laundry). As blackwater poses a high burden and direct health risks, it can have negative impacts on the environment and human health (Nansubuga et al. 2016) if not properly managed. Therefore, the UN's sixth Sustainable Development Goal, which ensures access to water and sanitation for all, targets treatment before discharge. Most developing Asian countries have adopted simple decentralized technologies to treat blackwater adjacent to the point of discharge (Chirisa et al. 2017). On-site sanitation systems (OSSs), such as septic tanks and pit latrines, are used by more than 38% of the world's population, predominantly in developing countries (WHO & UNICEF 2017; WHO & UNICEF 2019). However, there are many unexplored aspects of manure treatment within OSS, and it is unclear whether contaminants in blackwater are adequately removed. Based on analyses of faecal contamination indicators, such as Escherichia coli and nitrogen, the World Health Organization (WHO) states that a minimum horizontal distance of 15 m and a vertical distance of 1.5 m between the OSS and wells is sufficient to prevent contamination (WHO 2018). However, viral indicators of human faecal contamination have been detected in well water even when these conditions were met (Otaki et al. 2021). Simply installing an OSS does not solve the fundamental problems and may cause contamination. Very few studies have focused on faecal-derived viruses, although endemic pathogenic bacteria and noroviruses have been evaluated in a limited number of regions (Amin et al. 2020). Problems caused by a lack of adequate operation and management have also been identified (Starkl et al. 2013). For example, pit latrines, which are widely used in low-income countries in Asia, were not originally intended for flush toilets; however, they are often used in connection with flush toilets and are likely to have insufficient toilet effluent treatment (Odey et al. 2019). Septic tanks, which are widely used as OSS in middle-income countries in Asia, require maintenance (i.e. desludging) every few years. However, this maintenance is rarely done and may result in negative environmental and health impacts on local populations (Mehta et al. 2019). Although there are reports on the viral removal capacity of controlled septic tanks in developed countries, there are few studies on the treatment of septic tanks under such unmanaged conditions (George & Heufelder 2000; Nicosia et al. 2001). In addition, the varying sizes and types of septic tanks among houses, as well as different soil types and groundwater conditions across different regions, make it difficult to assess the treatment capacity of septic tanks (Bunsri et al. 2004).
Greywater, the wastewater component generated from bathing, showers, washbasins, laundry, and kitchens, excluding toilets, is often discharged directly into the environment without any treatment in developing countries where there is no centralized sewage system (Khanam & Patidar 2022). Although greywater has lower health risks and environmental impacts compared with those of blackwater, it is a major contributor to river surface water pollution (Mohamed et al. 2018).
This study determined the current status of black and greywater discharge and treatment in low- and middle-income households in Asia and their respective impact on the surrounding environment. This study also examined the direction of household wastewater discharge and treatment in low- and middle-income countries in Asia by investigating how local residents perceive the current state of household wastewater and the surrounding environment.
MATERIALS AND METHODS
Description of the study area
The study was conducted in Ho Chi Minh City, the largest economic city in Vietnam, which is a typical urban area of developing Asia in terms of water supply and wastewater handling; clean water is supplied through tap water and groundwater, and only toilet wastewater is treated. Ho Chi Minh City is located in southeastern Vietnam and has a tropical savanna climate according to the Koppen climate classification (Polo et al. 2015). Since the introduction of the Doi Moi policy in 1986 in Vietnam, rapid economic growth has led to serious environmental degradation and problems, such as water pollution from domestic and industrial wastewater (Duc & Truong 2003). The areas served by centralized sewers are, and will continue to be, limited owing to cost, technology, and maintenance. Therefore, it is necessary to consider decentralized wastewater treatment methods such as OSS.
In Vietnam, OSS installation is mandatory, and over 90% of households use septic tanks for sewage disposal. There are indications that there are poorly constructed septic tanks on the market (Conaway et al. 2023); however, the biggest challenge is that they are not properly managed (i.e. the sludge is not being drawn off). However, the legal system does not require desludging. Septic tanks show poor performance due to a lack of proper management, discharge, and insufficiently treated effluent, which causes water pollution and odours in public rivers and lakes. Decision No. 2149/QD-TTg refers to a national strategy for the integrated management of solid waste up to 2025, with a vision to set targets for septic tank sludge recovery and treatment rates of 100% in 2025 in urban centres of grade II and above and 50% in other cities.
Water quality of the canal and the main river in Ho Chi Minh City, Vietnam. Note: Observational data for 2022 were obtained from the South Centre of Environmental Monitoring (2022). The data for the canal are reported as average values for the three sites (Cau Tra Va, Cua Hu Y, and Cau Mong) and data for the river are the average values of the three sites (Cat Lai, Mui Den, and Long Phuoc).
Water quality of the canal and the main river in Ho Chi Minh City, Vietnam. Note: Observational data for 2022 were obtained from the South Centre of Environmental Monitoring (2022). The data for the canal are reported as average values for the three sites (Cau Tra Va, Cua Hu Y, and Cau Mong) and data for the river are the average values of the three sites (Cat Lai, Mui Den, and Long Phuoc).
Blackwater
Thirty-four households in Ho Chi Minh City were visited on 23–27 September 2022 and interviewed on the current status of blackwater treatment. The households were collected through snowball sampling, a viable method of recruiting research participants who are not readily accessible or known to the researcher (Goodman 1961; Parker et al. 2019). The households were asked (1) whether they perceived any problems in the treatment of blackwater and (2) what kind of management is applied to the blackwater treatment system.
Sampling process. (a) The supernatants were taken from the hole of the tank and (b) effluents of the tank were taken from the drainage.
Sampling process. (a) The supernatants were taken from the hole of the tank and (b) effluents of the tank were taken from the drainage.
Samples were collected in 1.0 L containers, placed in pre-sterilized polyethylene containers, refrigerated, and taken to the laboratory for the determination of E. coli, a bacterial indicator of faecal contamination, and Pepper Mild Mottle Virus (PMMoV), an indicator of viruses in human faecal contamination. E. coli was measured by the chromogenic colony formation method with Magenta-GAL and X-GULC using Compact Dry EC (Shimadzu Diagnostics Corporation), a simple dry medium (Mizuochi et al. 2016). PMMoV was measured by the quantitative polymerase chain reaction after primary concentration using a negatively charged membrane and secondary concentration using an ultrafiltration membrane. Each measurement was performed according to the method described by Otaki et al. (2021).
Greywater
To investigate the current status of domestic grey water, the perceptions of residents regarding greywater discharge and the surrounding water environment were evaluated. In total, 60 new households were surveyed using snowball sampling. The households were visited from 7 to 10 September 2023 to make observations and complete questionnaires. The questionnaire consisted of three sections: highest education (primary school or secondary school/high school/university and above), relationship with waterside spaces, and greywater discharge.
Regarding their relationship with waterside spaces, residents were asked about the canals and rivers located near their households. To evaluate physical proximity, participants were asked to indicate how often they saw the waterside space on a four-point scale (daily, once a week, once a month, or rarely). They were then asked what they thought about each waterside space on a five-point scale (major problems, problems, undecided, few problems, and no problems at all).
Greywater discharge was evaluated through observations and questionnaires. In the questionnaire, the options were arranged such that they could be followed from the first point of discharge to the final point of discharge to clarify whether residents were correctly aware of where their current greywater was flowing. They were then asked to indicate on a five-point scale (major problem, problematic, undecided, few problems, and no problem at all) what they thought about the current greywater discharge method.
RESULTS AND DISCUSSION
Blackwater
Current situation
Currently, around Ho Chi Minh City, blackwater is treated in septic tanks before being discharged into a drainage system. Most households noted that they had no problems with the current treatment methods, with only 2 of 60 households reporting an odour problem. For the proper operation of septic tanks, it is necessary to remove sludge every few years; however, most households have never managed it as they have not felt it necessary. In addition, for the few households (only 3 of 60) that did manage the tanks, the reason was that the tanks overflowed. It is clear that people were unaware of the need to manage their septic tanks. Most lids of the septic tanks are sealed with concrete and were not designed for management. Households that had to pull out sludge due to overflows had to break the concrete cap to open the lids.
Thus, although blackwater treatment was carried out in the septic tank, the system was not adequately managed owing to a lack of awareness of the need and structural problems. Therefore, it is uncertain whether treatment in the septic tank was properly carried out. To realize the national goal of proper management of septic tanks, it is necessary to create regulations and change the structure; however, it is also necessary to improve awareness of this need among the residents.
Pathogenic microbial removal capacity of septic tanks
Concentrations of (a) E. coli and (b) PMMoV in blackwater. Note: Error bars indicate standard deviation.
Concentrations of (a) E. coli and (b) PMMoV in blackwater. Note: Error bars indicate standard deviation.
The septic tank removed approximately 2.9 log10 E. coli and 2.9 log10 PMMoV. Compared with the reduction of E. coli in activated sludge at 2.0–3.0 log10 and PMMoV reduction of 1.7–3.7 log10, this is a sufficient removal rate. Therefore, the septic tanks surveyed maintained sufficient treatment ability, despite never having been maintained. It is possible that the capacity of the septic tanks was greater than that specified in the guidelines. Therefore, the treatment process was sufficiently advanced given the long residence times. Furthermore, as the study area has a tropical monsoon climate with consistently high temperatures, it is assumed that microbial activity is high, thereby promoting processing (Koottatep et al. 2020). It should be noted that standards for the design of septic tanks in tropical regions may need to be reconsidered. Kihila & Balengayabo (2020) indicated that septic tanks often do not treat wastewater well by using Chemical Oxygen Demand (COD), Total Suspended Solids (TSS), and phosphate as indicators. However, different from the present study, in addition to toilet effluent, greywater flowed into the septic tank in this previous study in Tanzania. In many low- and middle-income countries in Asia, only blackwater flows into septic tanks. The low water volume and high load entering the septic tank are likely the major factors in effective treatment. However, greywater is discharged untreated; therefore, measures to deal with it need to be considered.
Greywater
Ordinal logistic regression analysis
. | Estimate . | z-value . | p-value . |
---|---|---|---|
Correct understanding of greywater discharge routes | −1.34 | −2.22 | 0.03** |
Highest level of education | 0.03 | 0.17 | 0.86 |
Frequency to see waterside spaces | −0.37 | −1.10 | 0.27 |
. | Estimate . | z-value . | p-value . |
---|---|---|---|
Correct understanding of greywater discharge routes | −1.34 | −2.22 | 0.03** |
Highest level of education | 0.03 | 0.17 | 0.86 |
Frequency to see waterside spaces | −0.37 | −1.10 | 0.27 |
***p < 0.01, **p < 0.05, *p < 0.1.
The current state of untreated greywater discharge into the environment is considered problematic by residents, as it has led to the deterioration of water quality in the canal, which is a familiar waterside environment. Although it is difficult to improve the situation immediately and significantly in the absence of a sewerage system, it may be possible to take steps to reduce the load before discharge by correctly understanding the flow of greywater discharged into the environment.
CONCLUSION AND LIMITATIONS
Developing countries in Asia are moving up the sanitation ladder. The transition from the pit latrine (basic OSS) to a septic tank is under way; however, septic tanks are not being managed as recommended. Analyses of effluent from septic tanks that have never been desludged indicated that the treatment process was effective from a microbiological perspective. One reason for this was that only blackwater was added to the septic tank. In Asia, priority is given to treating blackwater, and greywater is discharged into the environment without any treatment. This may contribute to the good condition of septic tanks. Additionally, the large capacity of the septic tank allowed for a longer retention time and high microbial activity due to high temperatures led to increased decomposition. However, greywater discharged without treatment degrades the waterside environment; in particular, the canal that flows close to houses has become a drainage channel. More than 70% of respondents in this study thought there was a problem or a slight problem with the canal; however, awareness that it is caused by household greywater varied. The difference lies in whether respondents are aware of how their greywater is discharged. To address the current challenge, it is necessary to reduce the greywater load from each household at the source as much as possible, which requires a correct understanding of each individual's greywater emissions.
This study had several limitations. First, owing to the difficulty in sampling effluent from the septic tank, it was only possible to sample black and greywater mixed effluent from two houses. To obtain a more accurate picture, sampling from a large number of septic tanks under different conditions is necessary. Moreover, it is necessary to collect samples of black and greywater separately. Second microbial indicators were the focus of the study. Organic indicators and nitrogen should also be measured to determine the treatment conditions. Third, the questionnaire survey was undertaken using snowball sampling and the number of households surveyed was limited. More extensive surveys are required to determine the views of residents.
Despite these limitations, this study provides a new perspective on the treatment conditions of septic tanks and clarifies how residents perceive greywater discharge.
FUNDING
This research was funded by the JSPS (Japan Society for the Promotion of Science), Grant-in-Aid for Scientific Research (b) (grant number JP20H04391).
AUTHOR CONTRIBUTIONS
Y.O. was responsible for conceptualization and contributed to data curation, formal analysis, investigation, methodology, project administration, validation, visualization, and writing the original draft. M.O. was responsible for conceptualization, funding acquisition, investigation, methodology, project administration, supervision, validation, and writing the review & editing. Q.T.T.D. handled project administration and resources. E.S. and H.N. contributed to data curation and formal analysis. C.T. also participated in data curation and formal analysis.
DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.
CONFLICT OF INTEREST
The authors declare there is no conflict.