1.1 Sources and Characteristics of Domestic Sewage in Rural Areas

1.1.1 Source and Composition

The domestic sewage (from bath, laundry, kitchen, toilet) flushing is the main source of domestic sewage in rural areas, with their proportions associated with climatic conditions, living standards, and living habits. According to the water quality of sewage, domestic sewage (from bath, laundry, kitchen, toilet) is classified as gray water, while the toilet-flushing sewage generated from excretion and feces flushing is known as black water (Paulo et al., 2013).

Compared with gray water, black water cannot be treated easily due to its high concentration of pollutants, but it features high-value resource utilization as a potential resource. Domestic sewage in rural areas is composed of complicated pollutants, including solid matters, organic matters, nitrogen compounds, phosphorus compounds, animal and vegetable oils, anionic surfactants, drugs, personal care products, bacteria, viruses and insect eggs. SS, COD, BOD, TN, NH3–N, and TP are critical water quality indicators that should be highly concerned in rural sewage treatment.

Specifically, SS is the physical measure of suspended solid inorganic particles (silt and clay), organic particles, and their loaded microorganisms for detecting water pollution. Note that the high concentration of SS might lead to murky water and reduced water clarity, consequently affecting the respiration and metabolism of aquatic organisms and destroying the aquatic ecosystem. More than that, organic suspended particulate matters are prone to anaerobic fermentation upon deposition, resulting in deterioration of water quality (Palleiro et al., 2013).

COD and BOD are the main evaluating indicators for the level of organic pollutants in water bodies. Excessively high COD and BOD contents indicate high contents of reductive organics and large DO consumption, prone to water environment problems such as black and odor water. Compared with the inflow of municipal sewage treatment plants, rural domestic sewage has higher organic contents (COD reaching up to 500 mg/L) with favorable biodegradability (BOD5/COD, 0.45–0.55), and the organics can be effectively removed by biochemical treatment (Fan et al., 2021; Xu et al., 2012).

TN is the sum of different nitrogen forms, including inorganic nitrogen (NH3–N, NO2−–N, NO3−–N, etc.) and organic nitrogen (proteins, amino acids, organic amines, etc.). Organic nitrogen and NH3–N are primary nitrogen forms in rural domestic sewage, and both are unstable. The former can be converted into NH3–N through ammonification, and the latter can be converted into NO2–N via nitrification and further oxidized to NO3–N. The consumption of water dissolved oxygen in the conversion process is a cause of deteriorated water quality (Fu et al., 2006; Ma et al., 2019). Moreover, excessively-high concentrations of NH3–N, NO2–N, and NO3–N have direct or indirect toxic effects on aquatic ecosystems, resulting in hazards such as eutrophication. Hence, TN and NH3–N are technical indicators that are essential for managing the discharge of domestic sewage in rural areas. Rural domestic sewage treatment has high nitrogen load and faces higher technical difficulties in treatment process design and engineering operation and maintenance than that in urban areas.

TP, including dissolved total phosphorus and solid total phosphorus, is the sum of various phosphorus species in water. Human feces, food residues, and phosphorus-based detergents are primary sources of TP in domestic sewage. Like nitrogen, phosphorus is an essential element for biological growth and an indicator of nutrient pollution in water (Powers et al., 2016; Withers et al., 2009). Excess nitrogen and phosphorus in water may lead to the excessive multiplication of algae, giving rise to eutrophication. Phosphorus treatment is a ticklish problem for rural domestic sewage treatment since the biological phosphorus removal efficiency is not stable enough and the chemical phosphorus removal process is defective in difficult operation and maintenance and high price.

1.1.2 Main Characteristics

Rural domestic sewage features a large total amount of discharge, extensive discharge area, great fluctuation in water quality and quantity, remarkable regional characteristics of discharge, low treatment rate, and great potential for recovery.

  1. a.

    Large Amount of Discharge in Total

A majority of B&R countries are developing countries, which have a large rural population and a large total amount of domestic sewage discharge. There are 634,000 administrative villages and 22,000 towns in China, generating an annual discharge of domestic sewage exceeding 20 billion tons, accounting for more than half of the national total amount. 4.9962 million tons of COD, 245,000 tons of NH3–N, 446,500 tons of TN, and 36,900 tons of T are discharged from rural domestic sewage, accounting for 50.8, 35.0, 30.5, and 38.7% of the total discharge of domestic sewage pollutants (including the discharge of domestic source water pollutants in urban areas), according to China’s Second National Survey of Pollution Source shown in Fig. 1.1.

Fig. 1.1
A grouped column chart of pollutant emissions versus 4 pollutants plots values for urban and rural areas. The data are as follows. C O D, urban 483.82, rural 499.62. N H 4 N urban 45.41, rural 24.5. T N, urban 101.87, rural 44.65. T P, urban 5.85, rural 3.69.

Comparison of discharges of main pollutants in urban and rural sewage in China (second national survey of pollution source)

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  1. b.

    Extensive Discharge Area

The scattered living area of rural residents and a wide range of domestic sewage discharge areas are important sources of water source pollution due to defective sewage collection pipe networks and difficulty in concentrated treatment. Domestic sewage in rural areas is commonly treated with the following processes.

(i) leveraging treatment. Those villages near the urban sewage centralized collection system get access to the municipal and enterprise sewage pipe network for treatment; (ii) independent treatment. Rural areas with developed economies and management build their own domestic sewage treatment system (such as the integrated treatment equipment, and constructed wetlands) to discharge sewage upon treatment; (iii) simple treatment or direct discharge. In cases of centralized living, open ditches or underdrains, or simple facilities such as roadside ditches are adopted to collect and discharge sewage; in cases of scattered living, direct discharge is adopted (Jin, 2021; Li & Yu, 2021).

  1. c.

    Great Fluctuation in Water Quantity and Quality

The quantity and quality of rural domestic sewage fluctuate greatly under the impacts of factors such as daily routine, and periodic population flow. With the great mobility of the rural population, the discharge of domestic sewage is significantly increasing during holidays when a large number of people return home. Evident seasonal variation is presented in the discharge of rural domestic sewage. As residents have large demands for bathing in summer, leading to increased discharge of washing-up sewage, sewage discharge then is primarily characterized by large water volume and low concentration (COD, TN, TP). On the contrary, it is characterized by small water volume and high discharge concentration in winter. Similar discharge characteristics can be observed in spring and autumn, showing intermediate water quality and quantity. In addition, dilution by rainfall is also an important influencing factor. Lu et al. (2020) found that the average concentration of different pollutants in the rainy season is only 20–50% of that in the dry season upon studying villages in the Erhai Basin of Dali.

  1. d.

    Remarkable Regional Characteristics of Discharge

Sewage discharge in rural areas is closely bound up with climatic conditions and economic development levels, presenting remarkable regional characteristics. Different from residents in southeast China, rural residents in northwest China with less domestic water consumption under the impacts of the dry climate, particularly low frequency of washing up, discharge a smaller amount of sewage.

Liang et al. (2011) found that for a rural family of five, the water consumption for showering in South and North China is 70–140 L/(person d) and only 25 L/(person d) in summer, respectively. Also, for the water consumption in the kitchen, the water consumption of the farmer families in North China and Zhejiang is 50–70 L/d and 80–130 L/d, respectively.

Domestic sewage discharge in rural areas is positively correlated with the economic level of regional villages. With the high-speed economic growth, the output of rural domestic sewage is also increased together with the accelerated modernization of farmhouses and villages, the access of tap water to the farmhouse, and improved quality of rural life (such as the convenient use of modern kitchen appliances and bathroom products). Besides, the industrial structure also affects the discharge of domestic sewage in rural areas. The discharge of sewage from villages that are tourist destinations fluctuates remarkably in peak and low peak tourist seasons (Liang et al., 2011; Lu et al., 2020).

  1. e.

    Low Treatment Rate

The rate of domestic sewage treatment in rural areas of the B&R developing countries is low due to economic and technical constraints. The coastal developed areas in East and Southeast China with favorable economic growth took the initiative in rural sewage treatment and heavily invested in sewage treatment, achieving a complete rural environmental treatment infrastructure and full coverage of rural sewage treatment. Conversely, a low treatment rate of rural sewage can be observed in the central and western parts of China due to insufficient investment in rural environmental governance infrastructure as well as influence of living habits and natural conditions.

  1. f.

    Great Potential for Recovery

Rural domestic sewage is large in volume and extensive in distribution, which can be changed into large numbers of secondary water resources upon effective treatment for efficient onsite utilization. Besides, nutrient substances, such as N/P, abundant in rural sewage have a high value in recycling. Recycling N/P from rural domestic sewage (sludge) as fertilizer can realize the efficient utilization of nutrients, and avoid eutrophication by stopping nitrogen-containing compounds from entering the water. Moreover, carbon-rich domestic sewage has the potential for energy development. The sludge generated from the domestic sewage treatment has high chemical energy storage upon stabilized treatment, with the heat value reaching up to 32 MJ/kg (Hu et al., 2016; Li et al., 2020). It can be seen that the resource utilization of rural domestic sewage has great potential, which is especially significant for arid and semi-arid regions with scarce resources.

1.1.3 Environmental Risks

A range of environmental risks can be incurred after discharging rural domestic sewage without effective treatment/recycling, including (a) Contamination of drinking water sources. Discharging untreated domestic sewage or sewage below standard into surface water may contaminate drinking water sources, resulting in disease transmission. (b) Contamination of soil and groundwater. Rural domestic sewage discharged without effective treatment in areas with low groundwater levels may spark a high risk of groundwater contamination with indicators such as Escherichia coli exceeding the standard. (c) Disrupted balance of aquatic ecosystems and reduced stability and diversity of aquatic organisms affect fish survival and fishery production. (d) Black and odorous water. Continuous fermentation of organic matters in rural domestic sewage may generate odorous substances such as hydrogen sulfide, mercaptan, and ammonia, as well as black substances such as iron sulfide and manganese sulfide, resulting in black-odor water and loss of its function and affecting the landscape and human health. (e) Water eutrophication. The excessive multiplication of aquatic organisms would lead to a rapid drop in water transparency and dissolved oxygen, a sharp increase in pollutant indicators, and deteriorated water quality. (f) Breeding of mosquitoes and flies. Accumulation of domestic sewage provides an incubator for the reproduction of pests, such as mosquitoes and flies.

1.2 Management of Domestic Sewage in Rural Areas

Management policy is the driving force for addressing ecological and environmental problems. But the management of rural sewage is less developed than that of urban sewage. The United States and Japan are among the earliest countries to study rural sewage management. China, as a developing country and the second largest economy in the world, has also accumulated rich experience in rural sewage management.

1.2.1 United States

The U.S. began to address the problem of rural domestic sewage treatment as early as the middle of the 19th century and built domestic sewage treatment plants. Its rural decentralized sewage treatment technology evolved as outdoor toilets → sewage sumps → septic tanks → decentralized sewage treatment system. Complete systems concerning the regulatory system and financial subsidy system as well as technology and operation models have been established (Fan et al., 2009; Wen, 2016).

The same legal system was applied in rural and urban sewage treatment in the United States in the 1960s, highlighting the rural households’ independent management of domestic sewage; and rural domestic sewage treatment plants achieved full coverage in the 1980s. The treatment of non-point source pollution was included in the Water Quality Program in 1987 to request that programs and project funding should be established for decentralized sewage treatment in all states. But the investigation in 1997 by EPA found that the national decentralized sewage treatment system was not operated satisfactorily. Accordingly, a series of guidance documents on decentralized sewage treatment and management were released by EPA to strengthen the treatment of rural domestic sewage and determine jurisdiction and rights over the decentralized sewage systems in accordance with the management capacity and jurisdiction of local governments. Moreover, five operating modes (owner independent mode, agreement maintenance mode, licensed operation mode, centralized running mode, and centralized operation mode) with strengthened centralized management were proposed for decentralized sewage treatment in the “Guidelines for the Management of Decentralized Treatment System” issued in 2003. Besides, EPA has also established and governed various programs associated with the management of decentralized sewage treatment systems, such as the Water Quality Standard Project, the Total Maximum Daily Load Program, the Non-Point Source Management Program, the National Emission Reduction System Program, and the Water Conservation Program. Meanwhile, effective financial guarantee systems such as the state rolling fund programs provided by states for significant sewage treatment and environmental protection programs were also established for domestic sewage treatment in rural areas.

The complete decentralized sewage policy system, the multi-dimensional operation system, and the strong financial support system are essential for the successful treatment of rural domestic sewage in the U.S. This experience would be helpful for the rural domestic sewage treatment in B&R countries, which include:

  1. i.

    Domestic sewage treatment policies for rural areas should be flexibly formulated according to local conditions as well as the environmental needs, geographical conditions, economic development level, sewage discharge status, and sewage collection system.

  2. ii.

    A sound market mechanism and management mechanism of rural domestic sewage treatment should be established to give the roles of various forces such as the government, non-governmental organizations, and enterprises into full play.

  3. iii.

    An effective capital guarantee system should be built to expand the financing channels for domestic sewage treatment in rural areas.

1.2.2 Japan

With a small territory, large population and shortage of water resources, Japan has a significant demand for rural domestic sewage treatment. Its primary treatment processes are Johkasou treatment and centralized treatment, and has established a sound legal system, financial subsidy system, and operation mode for domestic sewage treatment. Overall, the comprehensive treatment level of rural domestic sewage in Japan is second to none among the developed countries.

Japanese enterprises began to apply Johkasou technology and facilities in rural manure treatment in the 1960s. The “Construction Benchmark Law” was promulgated by the Japanese government in 1969 for regulating and developing the market. In 1977, the rural domestic sewage treatment plan was launched, and the integrated water treatment equipment (Johkasou) with the biofilm process or activated sludge process was developed with reference to the urban sewage treatment technology. The Johkasou Law was officially enacted in 1983 to govern the decentralized treatment of rural sewage, and has become the principal legal basis for the treatment of rural domestic sewage in Japan. Further, a subsidy system was established for the “consolidated Johkasou setting and reconditioning business” in 1987, and the subsidy system for the “domestic drainage treatment business in specific areas” has been implemented since 1994 (Zhao et al., 2018).

The Japanese government, with the Johkasou Law as the core, has established a complete law and policy system for rural domestic sewage treatment, proposed requirements for the treatment and discharge of domestic sewage at different scales, established technical guidance and market supervision for the construction, operation, and maintenance of Johkasou facilities, strengthened the fund guarantee system for facility construction and operation and propelled the standard, professional and large-scale development of the Johkasou industry. On this basis, a sound policy environment has been built for the treatment of rural domestic sewage in Japan. Furthermore, the “Rules of Enforcing the Johkasou Law”, “Structural Standards and Explanations of Johkasous”, and “Design Guidelines for Drainage Facilities in Agricultural Villages” have been also introduced to support the implementation of the Johkasou Law. Meanwhile, local regulations and standards have been also unveiled by various prefectures to identify the responsibilities and obligations of relevant national departments, local governments, facility users as well as operation and maintenance agencies and personnel, laying a foundation for the orderly advancement and scientific management of rural domestic sewage treatment (Chen et al., 2019).

Rural domestic sewage treatment is fully covered in Japan, realizing the efficient utilization of resources. Specifically, the treated water in roughly 80% of the area is recycled as agricultural water, and approximately 71% of the sludge generated from the rural drainage facilities is recycled through farmland restoration. The collected biomass, for example, is used as fertilizers after being mixed and fermented with accessory materials (such as rice husks).

1.2.3 China

Compared with the United States and Japan, rural sewage treatment in China starts quite late in the early 21st century, and has experienced its infancy, development state, and rapid development stage.

Stage 1, infancy (2005–2008). China began to highly focus on rural environmental protection and attempted to guide industrial development via the formulation of policies. The State Council held the first National Rural Environmental Protection Work Conference in 2008, kicking off the comprehensive treatment of decentralized sewage in rural areas. Meanwhile, the Ministry of Finance and the Ministry of Ecology and Environment established a special fund for rural environmental protection to support rural domestic sewage treatment and refuse disposal.

Stage 2, development stage (2009–2015). Focusing on policy discussion, funding support, and demonstration base construction, the government issued the “Implementation Plan on the Accelerated Resolution of Outstanding Rural Environmental Problems with Awards to Promote Governance”, the “Rural Domestic Pollution Control Technology Policy”, the “Environmental Protection Act of the People’s Republic of China”, and the “Action Plan for Water Pollution Control”. The environment of 60,000 administrative villages has been comprehensively improved from decentralized pilots to centralized contiguous coverage through the implementation of promoting governance with awards. In this way, a number of demonstration cases were accumulated in environmentally-sensitive and economically-developed areas. Also, decentralized sewage treatment enterprises have been developed rapidly (HEXU Environment was founded in 2012), and they have developed a range of rural sewage treatment technologies and processes as per national conditions in China.

Stage 3, rapid development stage (2016–). It is a stage characterized by improved policies and mechanisms as well as the vigorous boost of regional integrated services. The Ministry of Housing and Urban-Rural Development issued the “Technical Standard for Domestic sewage Treatment Engineering in Rural Areas” in 2019 to provide guidelines for the technological development of the industry. Water pollutant discharge standards for local rural domestic sewage treatment plants have been established in 31 provinces, municipalities directly under the Central Government, and autonomous regions across the country as per actual conditions by 2020, meeting the requirements of decentralized sewage treatment in rural areas. The Ministry of Finance issued the “Management Measures for Rural Environmental Improvement Funds” in 2021 to provide fund support for rural sewage treatment nationwide, and the China Council for the Promotion of International Trade released the “Standards for Small Treatment Equipment for Domestic sewage”, “Evaluation Specifications for Small Treatment Equipment of Domestic sewage”, and “Technical Regulations for the Operation and Maintenance of Domestic sewage Treatment plants in Villages” successively. The environment of 130,000 administrative villages has been comprehensively improved under the implementation of the “Action Plan for Agricultural and Rural Pollution Control” during the 13th Five-Year Plan period, and the environment of 80,000 administrative villages will be comprehensively improved during the 14th Five-Year Plan period.

China has established complete legal systems, financial subsidy systems, and operation models, achieving significant improvement in the comprehensive treatment level of domestic sewage in rural areas. China’s experience accumulated in rural sewage treatment over the last two decades by drawing the advanced treatment concepts of the United States and Japan can be used as a reference for our B&R partners.