Study of the characteristics of pollutants in rural domestic sewage and the optimal sewage treatment process: a Chengdu Plain case study

In China, rural areas cover 94% of the national land area, and the rural population size is approximately 600 million (Wei et al. 2020; Zhang et al. 2021). However, the treatment of rural domestic sewage has not yet been solved, and most domestic sewage is discharged directly into nearby surface waters without treatment. With the development of the rural economy, the treatment of rural scattered domestic sewage has become an important issue affecting the sustainable development of rural areas. The annual discharge of rural domestic sewage in China exceeds 8 × 10⁹ m³, and the amount of sewage treatment is increasing each year (Chen et al. 2022). Although 34.87% of villages in China have sewage treatment plants, very few of them are used regularly, and only 4.5% of the overall sewage is treated each year (Ministry of Housing & Urban‒Rural Development of the People's Republic of China 2020). The direct discharge of rural scattered domestic sewage containing considerable amounts of nitrogen, phosphorus, organic matter, and other pollutants into rivers can contribute to the eutrophication of water bodies (Wang et al. 2011, 2019; Yan et al. 2013; Song et al. 2018; Yu et al. 2019; Li et al. 2020; Mao et al. 2021). A study by Huang et al. (2019) showed that untreated rural domestic sewage can also interfere with the natural development and reproductive processes of aquatic animals, and thus, sewage can be harmful to the aquatic organisms and local residents in a watershed. For this reason, the Chinese government has proposed a strategic plan for ‘new rural construction’, which stipulates that rural domestic sewage must be treated before it is discharged into rivers or soil.

Rural domestic sewage is the domestic wastewater produced by rural residents, including kitchen sewage, toilet sewage, and washing sewage, which is called ‘the rural three waters’. It is characterized by wide dispersion, multiple sources, randomness, large daily and seasonal variations, low pollutant concentrations, and difficulty to regulate and control (Sun et al. 2013; Mao et al. 2021; Wang 2021; Yi et al. 2021). In response to increasing concerns about environmental pollution and human health, the Chinese government aims to develop and implement advanced wastewater treatment plants in rural areas to improve the rural environment, water quality, and lives of rural people (Ai et al. 2020; Bo & Wen 2022). Accelerating the research on rural domestic sewage treatment technologies and treating rural domestic sewage as scientifically and effectively as possible is a very important task, and it is of great significance for the overall improvement of the rural environment in China (Li et al. 2022); furthermore, it is an important step in the implementation of rural revitalization strategies (Chang et al. 2018; Zhang et al. 2020b). Selecting appropriate wastewater treatment methods to improve environmental conditions in rural areas has become a top priority for the State Council of China (Liu et al. 2013; Yu et al. 2019). The number of wastewater treatment plants in Chinese townships increased from 763 in 2007 to 3,437 in 2015 (Cheng et al. 2017). These wastewater treatment plants can be divided into centralized wastewater treatment systems and decentralized wastewater treatment systems; however, centralized wastewater treatment systems are more expensive to build and maintain (Libralato et al. 2012). Decentralized wastewater treatment methods are clearly more suitable for rural areas in China that have a low population density and scattered households (Chen et al. 2019a; Hong et al. 2019). There are various decentralized treatment methods for treating rural domestic wastewater. Examples include constructed wetlands (Chen & Yao 2014; Lam et al. 2015; Saumya et al. 2015; Lu et al. 2016; Chen et al. 2022), septic tanks (Chen & Yao 2014; Singh et al. 2019), contact oxidation (Li & Zhou 2011; Chen et al. 2022), membrane bioreactors (Chen et al. 2022), hydrolytic acidification (Cokgor et al. 2009; Feng et al. 2009; Cheng 2020), combined anaerobic and aerobic processes (Li et al. 2016; Zheng et al. 2019; Chen et al. 2022), land use systems (Wang et al. 2011; Sun 2017), aeration basins (Sun 2017; Xie et al. 2019), oxidation ditches (Wang et al. 2011), and high-efficiency algal ponds (Xie et al. 2019). Decentralized wastewater treatment technologies have been explored as cost-effective and promising viable options for reducing pollutant loads in rural areas (Chen & Yao 2014). Each option has advantages and disadvantages and a scope of application (Sun et al. 2013; Chen et al. 2022), and the removal efficiency is influenced by factors such as temperature, wetland plants, and pollutant concentration (Wang et al. 2019). According to the characteristics of rural domestic sewage in China, wastewater treatment systems require certain characteristics, including simple operation and maintenance, low investment and cost, and a high efficiency of organic matter and nutrient removal (Chen & Yao 2014). Therefore, exploring decentralized wastewater treatment methods applicable in rural areas in China is both an inevitable requirement for promoting sustainable social development and a necessary path for building beautiful and livable villages.

To explore the wastewater treatment methods applicable in rural areas in China, the characteristics of rural domestic sewage in the watershed must be studied. However, most of the current studies on rural environmental pollution in watersheds by Chinese scholars focus on the perspective of surface source pollution, and there are fewer studies that directly address the characteristics of pollutants in rural domestic sewage in watersheds. In addition, with the advancement of new rural construction and changes in farmers' lifestyles, the previous source census data do not reflect the rural areas situation in real time, and this shortcoming may limit guidance for the construction of rural wastewater treatment demonstration projects. Therefore, it is necessary to conduct research on rural domestic sewage (Wang et al. 2020). In this paper, the concentration of each characteristic pollutant in the influent and effluent of 16 typical rural domestic wastewater treatment facilities in the Chengdu Plain of Southwest China was studied. The results of this study initially reveal the discharge pattern and characteristics of rural domestic sewage in Southwest China, thus providing a reference for the control and treatment engineering of rural surface source pollution in Southwest China as well as for the control and treatment of surface source pollution in watersheds similar to the Chengdu Plain in Southwest China. In addition, this paper analyzed the effects of the treatment process, season, and hydraulic retention time on the removal efficiency of each characteristic pollutant and discussed the elements that should be considered when selecting wastewater treatment facilities. Through this study, we hope to obtain the water quality characteristics of rural domestic wastewater in Southwest China, and obtain the most suitable wastewater treatment technology for removing each pollutant, thus providing an important reference for the application of various treatment processes in domestic wastewater treatment in rural areas.

The Chengdu Plain includes all districts and counties in Chengdu and parts of Deyang, Mianyang, Ya'an, Leshan, and Meishan, and it is the largest plain in three provinces of Southwest China. The Chengdu Plain is home to the world-famous Dujiangyan Irrigation Project, which has developed agriculture, rich products, and a dense population, and it has been known as the ‘Land of Heaven’ since ancient times. There are many rivers in the Chengdu Plain, and the west side is composed of the surface water system inlet, giving birth to two major water systems, the Minjiang River and the Tuojiang River, which enter the plain and are fan-shaped diversions.

Samples were collected in strict accordance with the methods provided in the national standards (Ministry of Ecology and Environment of the People's Republic of China 2002b, 2009). Sampling points were established in each village before the point where domestic sewage entered the wastewater treatment plant and at a point after wastewater had been treated in the wastewater treatment plant. The domestic sewage before entering the sewage treatment plant was obtained at the outfall of each village as the inlet water quality, and the sewage treated by the sewage treatment plant was obtained at the sewage outfall as the outlet water quality. The domestic sewage at the inlet and outlet of each village sewage treatment plant was collected daily in summer and winter, and the average value was taken as the concentration of domestic sewage at the inlet and outlet of the sewage treatment plant in summer and winter, respectively.

The water quality at the inlet and the outlet of each sampling point was analyzed with reference to the Analytical Methods for Water and Wastewater Monitoring (Fourth Edition) (Ministry of Ecology and Environment of the People's Republic of China 2002a). The analytical indicators included pH, BOD₅, COD, NH₃-N, TP, TN, and SS. pH was determined by the glass electrode method; BOD₅ was determined by the dilution and inoculation method with a minimum detection limit of 2 mg/L; COD was determined by the dichromate method with a minimum detection limit of 10 mg/L; NH₃-N was determined by the nano-reagent spectrophotometric method with a minimum detection limit of 0.05 mg/L; TP was determined by the ammonium molybdate spectrophotometric method with a minimum detection limit of 0.01 mg/L; TN was determined by the ultraviolet spectrophotometric method with alkaline potassium persulfate elimination with a minimum detection limit of 0.05 mg/L; and SS was determined by the weight method. The indicators were labeled and tracked in the sample at the same time, and parallel double sample measurement was guaranteed during the measurement process to ensure accurate monitoring results.

All statistical analyses, such as the mean value and Pearson correlation coefficient, were performed by SPSS 26.0, and scatter matrix plots were drawn by SPSS 26.0. The distribution characteristics of each characteristic pollutant were tested by the Shapiro‒Wilk test. If the value of the Shapiro‒Wilk test p-value was greater than 0.05, it indicated that the distribution of each pollutant obeyed a normal distribution. The correlation between each characteristic pollutant was determined by Pearson correlation analysis.

The water samples of domestic sewage collected from 16 villages were statistically analyzed to obtain the overall characteristics of rural domestic sewage in the study area, and the Shapiro‒Wilk test was used to test whether pH, BOD₅, COD, NH₃-N, TP, SS, and TN satisfied a normal distribution. The results are shown in Table 2.

As seen in Table 2, the p-value of the S‒W test of each characteristic pollutant was less than 0.05, except for that of pH, where the p-value was greater than 0.05. The results indicated that the distribution of pH satisfied a normal distribution among villages, while the distribution of the remaining characteristic pollutants did not satisfy a normal distribution among villages. Therefore, the BOD₅, COD, NH₃-N, TP, SS, and TN were log-transformed, and the Shapiro‒Wilk test was still used to test the significance of each characteristic pollutant after log-transformation. The results showed that the significance of BOD₅, COD, NH₃-N, TP, SS, and TN after logarithmic transformation was greater than 0.05, so BOD₅, COD, NH₃-N, TP, SS, and TN satisfied the log-normal distribution. When the normal distribution is satisfied, the mean can be described by the arithmetic mean; however, when the log-normal distribution is satisfied, the mean can be described by the geometric mean. The average values of pH, BOD₅, COD, NH₃-N, TP, SS, and TN in the whole study area were 7.15, 23.5 mg/L, 82.4 mg/L, 9.5 mg/L, 1.6 mg/L, 42.6 mg/L, and 18.3 mg/L in summer and 7.15, 31.5 mg/L, 91.7 mg/L, 14.6 mg/L, 2.3 mg/L, 54.5 mg/L, and 26.82 mg/L in winter, respectively. The results were similar to those of Zhang et al. (2008), who studied the drainage of some rural domestic sewage in the Minjiang Basin within Sichuan Province. The pH values were stable near the neutral level in both summer and winter, indicating that domestic sewage did not cause acid-base pollution of water bodies, and this result was consistent with the results of Xie et al. (2018), who analyzed the water quality of rural domestic sewage in Southwest China.

As shown in Table 3 and Figure 3, there were significant or highly significant correlations between the concentrations of all characteristic pollutants except pH. In summer, there were highly significant correlations between the concentrations of BOD₅, COD, NH₃-N, TP, SS, and TN. In winter, there were significant or highly significant correlations between the concentrations of BOD₅, COD, NH₃-N, TP, SS, and TN.

According to the results of other scholars, the strong correlation between each characteristic pollutant is due to the adsorption of other pollutants during the flow of SS (Hu 2013) or the presence of each characteristic pollutant in the form of SS (Cao & Chen 2003). Based on the strong correlation between the SS and the remaining pollutants, in the process of wastewater removal, the concentration of each of the remaining pollutants was reduced when the SS pollutants were removed. Therefore, the concentration of each pollutant can be reduced to a greater extent if precipitation and filtration are carried out first in the wastewater treatment process, and this research result can provide a reference for establishing the removal procedure of the wastewater treatment process. In addition, NH₃-N, TP, and TN were highly significantly positively correlated, which was probably caused by most of the nitrogen and phosphorus coming from detergents, laundry detergents, and other cleaning products in rural domestic sewage and resulted in a certain homogeneity of nitrogen and phosphorus pollution.

With the continuous improvement of the living standards of rural residents, the load generated by rural domestic sewage will increase significantly in the future and pose great challenges to local sewage treatment. It is necessary to study the main exceedance indexes in current domestic sewage, which provides some reference for the future treatment of rural domestic sewage. In this study, the excess indexes of domestic sewage water quality in each village were analyzed, and the analysis results are shown in Table 4.

To draw conclusions that were unaffected by the extreme values in the data, the median was used to reflect the general level of exceedance of domestic sewage quality standards within the study area. As seen from Table 4, the relationship between the magnitude of exceedance of each pollutant was as follows: NH₃-N > TP > SS > BOD₅ > TN > COD (summer) and NH₃-N > TP > SS > TN > BOD₅ > COD (winter).

Table 4 shows that the most serious exceedances of pollutants in the study area were found for NH₃-N, TP, and SS in both summer and winter. The median exceedances of NH₃-N were 2.57 and 3.56 in summer and winter, respectively; the median exceedances of TP were 2.40 and 2.99 in summer and winter, respectively; and the median exceedances of SS were 2.35 and 2.60 in summer and winter, respectively. Some scholars (e.g., Huang et al. 2016; Chen et al. 2019b) have shown that the reason for the high concentrations of NH₃-N and TP may be due to the large number of poultry, such as chickens and ducks, and livestock, such as pigs and cattle, in rural areas, as these animals produce a large amount of manure containing a large amount of NH₃-N. In addition, more phosphorus-containing detergents and laundry detergents are used in the washing processes of residents and lead to a high concentration of TP in the washing effluent. The wastewater treatment process should focus on NH₃-N and TP to avoid eutrophication in the surrounding water bodies. The reason for the high SS concentration may be because the study area is in the warm and humid subtropical Pacific southeast monsoon climate zone with abundant rainfall, and the abundant rainfall brings large-particle pollutants into the outfall area due to the combined flow of rain and sewage.

For rural areas with specific conditions, the characteristics of domestic sewage discharge should be comprehensively investigated to select a targeted wastewater treatment process. For existing rural domestic wastewater treatment processes, it is important to evaluate the process applicability (Zhang 2020), and the removal efficiency is particularly important in the evaluation of process applicability.

Currently, most rural areas in China have not yet or are planning to build wastewater treatment plants. The removal efficiency of each pollutant by different processes was obtained in this study, and this information can provide an important reference basis for the selection of wastewater treatment processes in different types of areas.

The collected water samples were analyzed, and the removal efficiency of each process for each characteristic pollutant in winter and the reduction rate relative to the removal efficiency in summer are shown in Table 5. The table shows that for organic BOD₅ and COD, the smallest decrease in removal efficiency in winter compared with summer was from the CO + CW treatment process. For NH₃-N, the smallest decrease in removal efficiency in winter was observed with the MBR-integrated treatment equipment. For TP, the smallest decrease in removal efficiency in winter was observed in the CO + CW treatment process and CO + HA treatment process; however, for SS and TN, the smallest decrease in removal efficiency in winter was observed in the CO + HA treatment process. The removal efficiency of various processes for each characteristic pollutant was generally reduced in winter compared with summer, but the reduction rate was different. The appropriate wastewater treatment technology should be selected based on the actual level of local pollutants to minimize the influence of temperature on wastewater treatment.

The results of this study showed that the climatic temperature is an important factor affecting the wastewater treatment process, and temperature affects the activity of microorganisms, which is consistent with the results of other scholars in related fields (Zhang et al. 2009; Jóźwiakowski et al. 2020; Li et al. 2022). Therefore, the study of pollutant removal efficiency in different seasons can serve as a guide for the selection of wastewater treatment processes, and the process that is used should ensure a high pollution elimination effect in different seasons and ideally be less affected by season.

In this study, the concept of α was proposed, and a significant negative correlation between α and removal efficiency was obtained. The research results can provide new ideas for the design of wastewater treatment plants, e.g., estimate the amount of domestic sewage generated according to the local population and domestic sewage discharge coefficient or try to increase the design treatment scale of wastewater treatment plants within the controllable cost so that the hydraulic retention time can be increased, which could effectively improve the pollutant removal efficiency.

Rural domestic sewage treatment in the context of rural revitalization strategies is an important issue related to a country's livelihood (Deng & Wheatley 2016; Wang & Zhang 2019). At present, the common evaluation systems of rural domestic sewage treatment technology include the hierarchical analysis method (Xia et al. 2012), fuzzy integral model method (Shen et al. 2014), entropy value method (Shen et al. 2014), and relaxed DEA model method (Cheng et al. 2020). Rural domestic sewage treatment methods should be integrated with local geographic conditions, economic conditions, environmental conditions and management levels in rural areas, and feasible rural domestic sewage treatment technologies must have a simple design, high treatment efficiency, low sludge production, simple maintenance, and low operating costs (Liu et al. 2013; Li et al. 2022); therefore, it is of great significance to explore processes that have low energy consumption, high efficiency, high utilization, and simple operation and maintenance (Ladu & Lu 2014).

According to the results of the study reported in Section 3.2, the removal efficiency of rural domestic sewage was related to the treatment process, season, and hydraulic retention time. To obtain the most suitable wastewater treatment process for removing each pollutant, the influence of factors such as season and hydraulic retention time on the wastewater removal efficiency must be excluded, and the system removal efficiency (SRE) of each pollutant by each process was obtained.

The SRE of each pollutant for each process in summer and winter was obtained according to the formula, and the results are shown in Table 7. Considering the basic principle of sustainable development, the primary criterion for selecting the technology for rural wastewater treatment plants should be the ecological criterion, i.e., the removal efficiency of pollutants from sewage. Based on this, combined with the calculation results of the SRE and the results of the SRE in summer and winter, we can conclude that for areas with more serious BOD₅ pollution, the preferred wastewater treatment process is the CO + CW treatment process, and the removal efficiency in summer exceeds 80%. For areas with more serious COD pollution, the preferred wastewater treatment process is the CW treatment process, and the removal efficiency in summer reaches 69.57%. For areas with serious NH₃-N pollution, the preferred wastewater treatment process is the MBR-integrated equipment, and the removal efficiency exceeds 90% in both summer and winter. For areas with serious TP, SS, and TN pollution, the preferred wastewater treatment process is the CO + HA treatment process, and the removal efficiency of TP, SS, and TN in summer is 75.52, 73.93, and 76.83%, respectively. Previous studies have focused mostly on the removal efficiency of various pollutants by one treatment process; they did not propose the most suitable treatment process for each pollutant. The present study filled this gap and obtained the most suitable wastewater treatment process for each pollutant, and this information can provide an important reference basis for the selection of domestic sewage treatment technology in rural areas, especially in the Chengdu Plain.

In this study, the concentrations of the characteristic pollutants in the rural scattered domestic sewage of the Chengdu Plain in the southwestern region of China were obtained by monitoring and analyzing the rural domestic sewage. The concentrations of the pollutants in the domestic sewage in winter were higher than those in summer. Pearson correlation analysis showed that significant correlations existed between the concentrations of the characteristic pollutants except for pH. The results of this study initially reveal the discharge pattern and characteristics of rural domestic sewage in Southwest China and provide a basic reference for the control and treatment engineering of rural surface source pollution in Southwest China. In addition, this study found that the removal efficiency of rural domestic sewage was related to the treatment process, season, and hydraulic retention time. There was a significant negative correlation between α and the removal efficiency. Combined with the calculation results of the SRE and the research results of the SRE comparison in summer and winter, we can conclude that the preferred process for the removal of BOD₅ is the CO + CW treatment process; the preferred process for the removal of COD is the CW process; the preferred process for the removal of NH₃-N is the MBR-integrated equipment; and the preferred process for the removal of TP, SS, and TN is the CO + HA treatment process. The research results provide new ideas for the construction of rural domestic sewage treatment technology in China and valuable references for rural domestic sewage treatment planning and process selection.

This work was supported by the project of rural receiving water bodies water quality improvement and guarantee key technology research (CX2019Z05).

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

The authors declare there is no conflict.