rural areas with weak economic conditions (Zheng et al., 2020), making these facilities prone to failure in operation at a later stage due to the lack of funds. iii Select a process with simple and less operation and maintenance. There are various rural sewage stations widely distributed in rural areas, lacking sewage treatment professionals for operation and maintenance. Therefore, facilities that are simple in operation and maintenance without frequent process testing or adjustment are essential in rural sewage treatment (Wang, 2021a, 2021b). iv. Select a process that is conducive to resource utilization. Sewage resource utiliza-tion means the sewage meets specific water quality standards upon harmless treatment and can be used as reclaimed water for residents’ living, ecological water supply, and agricultural irrigation in replacement of conventional water resources, or other resources and energy can be extracted from sewage. It is of great significance to increase the supply of water resources, alleviate the shortage of irrigation water, and ensure the safety of water ecology. v. Adjustment should be made as per local conditions. Field investigation should be conducted before the selection of the rural sewage treatment process. Based on the functional requirements of the receiving water, the treatment process suit-able for the local area can be determined with consideration of the economic conditions of rural areas, the complete status of infrastructure, natural environ-ment, rural water-using habits, water consumption, the permanent population, climatic conditions as well as sewage situation and the final drainage destination of surrounding factories and aquaculture farms

Natural biological treatment is a process technology that purifies sewage using natural ecological or artificial ecological functions. Sewage is purified through the physical and chemical effects of soil or artificial carriers, the purifying function of natural organisms, and the interception and absorption of aquatic plants. It has been widely applied with its features of high purification efficiency, low operation and maintenance costs, and low management level required. Constructed wetlands and stabilization ponds are two commonly used processes in rural sewage treatment

The quality and quantity of domestic sewage differ due to great differences in economic and social conditions, population factors, and customs under a diversity of geographical, climatic, and ecological conditions in rural areas worldwide. More-over, different treatment processes selected as the sewage treatment terminal may lead to a big difference in infrastructure cost, treatment effect, and the complexity of operation and maintenance management. Evidently, a reliable and safe sewage treatment process with favorable treatment effect, low energy consumption, simp

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

sewage management. China, as a developing country and the second largest economy in the world, has also accumulated rich experience in rural sewage management

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 ecosys-tems 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 multipli-cation 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

The rate of domestic sewage treatment in rural areas of the B&R developing coun-tries 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 treat-ment. 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 condition

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.

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

(i) leveraging treatment. Those villages near the urban sewage centralized collec-tion 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 inte-grated treatment equipment, and constructed wetlands) to discharge sewage upon treatment; (iii) simple treatment or direct discharge.

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.

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.

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

Human feces, food residues, and phosphorus-based detergents are primary sources of TP in domestic sewage. Like nitrogen, phos-phorus 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.

Chapter 1 Characteristic and Management of Rural Domestic Sewage 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 poten-tial resource.

Chinese enterprises, for instance, have carried on many environmental protection supporting projects in B&R countries, and water treatment plants made in China have been exported to Southeast Asia, Central Asia, the Middle East, Africa, and other regions in large quantities. Combining sewage pretreatment, biological treatment, sedimentation, and disin-fection, the integrated sewage treatment equipment, featuring compact structure, small footprint, short construction period, high treatment efficiency, and econom-ical rationality, is particularly suitable for decentralized domestic sewage treatment in rural areas without pipe networks.