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Development and application of biogas project for domestic sewage treatment in rural China: opportunities and challenges

The biogas project for domestic sewage treatment (BPDST) is considered a promising facility for wastewater management in rural areas of China. This paper explores previous experimental works, cost analysis, and BPDST structure and design based on Chinese literature. Opportunities for developing decentralized or neighborhood-based BPDSTs include fulfilling Millennium Development Goals (MDGs) and Sustainable Development Goals (SDGs), the water pollution situation and deficiency of wastewater treatment facilities, the advantages of BPDSTs compared with centralized sewage plant, government support and policy drive for rural wastewater treatment, and reuse demand for resources. Meanwhile, challenges faced are emphasized as follows: uncertain responsibility for BPDSTs under different governmental departments restricts BPDST development and should be specified; uncertain effluent quality due to low efficiency of nutrient removal requires aerobic post-treatment to some extent; rural environmental awareness is still low and should be heightened; more funds should be invested in R&D for improvement of technology innovation; more reuse and resource recovery elements should be considered during implementation; follow-up services are lacking and should be improved; and BPDST maintenance should be trained. This paper could provide valuable reference for other developing countries.


INTRODUCTION
. This technique has been evaluated as one of the most energy-efficient and environmentally beneficial technologies for bioenergy production (Bond & Templeton ).
The dominant biogas digester in rural areas mainly aims to treat animal waste and is integrated in livestock and poultry breeding and agriculture production systems. However, another mainstream type of biogas digester, the domestic sewage digester (DSD), is applied to treat domestic sewage (Cheng et al. ). DSD was developed in the 1980s; in view of sewage characteristics, the household-scale biogas digester replaces a septic tank to treat domestic sewage and fecal sludge. Rural energy departments at all levels began to promote DSD in rural China (Zheng et al. ).
DSD is the core of any biogas project for domestic sewage treatment (BPDST), which is a type of decentralized wastewater technology. BPDST is suitable for places where sewer pipeline systems are unavailable and serves residential buildings, office buildings, hotels, schools, public toilets, and hospitals. Table 1 presents the number of BPDSTs in recent years. The number of decentralized BPDSTs reached its peak in 2013. An increasing number of villages and townships started to construct centralized WWTPs due to the expansion of urbanization; thus, more wastewater is collected in WWTPs via sewer pipeline networks. As a result, the number of BPDSTs is decreasing, and this trend will continue. However, the rate of decrease is not high, and BPDSTs will still function in rural areas in the long run. In China, BPDST is more popular in Sichuan, Jiangsu, and Zhejiang Provinces due to the high disposal rate of rural domestic sewage in these provinces. These regions are located in the south of China, where the climate is considerably suitable for anaerobic digestion. In addition, local governments emphasized rural wastewater management and provided preferential policies and guidelines for such projects. Internationally, similar systems are also developed and introduced as decentralized wastewater treatment systems (DEWATS) or decentralized sanitation and reuse (DESAR).
BPDST has numerous advantages, such as easy onsite construction, minimal land occupation, easy maintenance, low energy input, and good environmental benefits.
BPDST has been promoted in rural China in recent decades. However, the situation of BPDST is not at the optimum level. This paper mainly reviews BPDST technology by presenting the opportunities for its development in rural China and discussing its many impediments and challenges.

CHINESE LITERATURE REVIEW
Previous studies and evaluation of BPDST have mainly focused on the hygiene effect of pathogen removal by DSD (Wu & Xu ). A study by Sichuan Province Institute of Parasitic Disease Prevention and Control tested the BPDST treatment effect in six projects. Generally, the quality of treated sewage improved considerably after 1-4 years of operation. Thermotolerant coliforms are >10 À4 . The number of parasitic ova ranges from 0.565/L to 1.074/L; biochemical oxygen demand (BOD) is <50 mg/L; suspended solids (SS) is <60 mg/L; color is <100. These indicators could meet the requirement of Integrated  Table 2). BPDST hygiene effect and qualified effluent in dormitory buildings and office buildings showed no obvious difference (see Table 3). However, the treatment effect during spring and summer was better than in winter and autumn (see Table 4). Although  BPDSTs fail without correct long-term operation, which will be discussed in the subsequent sections of this paper.

STRUCTURE AND DESIGN OF BPDST
In 1991, the Sichuan Rural Energy Office compiled the first drawing collection of BPDST, in which 10 types of BPDST were presented and some types were improved (Mao ). The digester can be strip-type, rectangular, or round  Figure 1 and Table 6.  At present, BPDST is composed of a sediment tank or sediment zone, anaerobic zones I and II, and post-treatment zone. The sediment tank or sediment zone is used for removing non-biodegradable and large solid bodies, whereas anaerobic zone I digests organic pollutants. Soft packing is filled into anaerobic zone II as a microbe carrier for further degradation of organics. The post-treatment zone is installed with packing and filler, which also act as a filter. The two types of inflow system are separated inflow and combined inflow. The combined inflow type is better than the separated inflow type in terms of investment. However, the concept of source separation has become increasingly known and is encouraged by the government (Hu et al. ). The separated inflow type meets the requirements for future development. Type A adopts separated inflow system and tunnel-type tanks, and the soft packing is filled into anaerobic zone II (see Figure 1). Type B fits the separated and combined inflow systems. A 10% gradient exists at the bottom of a sand sediment tank. An extra inflow hole is set in anaerobic zone II for other types of wastewater.
A ventilation pipe is installed in post-aerobic treatment.
Type C adopts two cylinder-shaped digesters, and a baffled wall is placed inside digester II. Post-treatment adopts a facultative biofilter, with the filler at the particle size of Such a pipe is similar to the ventilation pipe in the national standard septic tank (CIBSDR ).  Sewage digesters are ideal for treating the domestic wastewater of small cities, townships, and villages, as well as communities in peri-urban areas, which have no sewage pipeline network.

Reuse demand
China is a water-scarce country. The annual water shortage gap reaches more than 30 billion m 3 in the agriculture sector, and 60% of cultivated land lacks irrigation water.
Consequently, the output of food produce is reduced by  • The drawings aim at brick-concrete structure of BPDSTs.
• Three series (Types A, B, and C) should meet different economic and environmental conditions, as well as effluent requirements.
The effective volume varies between 20 m 3 and 200 m 3 .
• Type A adopts combined drainage system; effluent should meet the hygienic requirement for harmless disposal of human waste, which is roundworm mortality !95%; living eggs of Schistosoma and hookworm are undetectable; salmonella is undetectable; and thermotolerant coliforms !10 À4 .
• Type B adopts separated drainage system; effluent should meet the grade 3 discharge standard of pollutants for municipal WWTPs.
• Type C adopts separated drainage system; effluent should meet the grade 2 discharge standard of pollutants for municipal WWTPs.

NY/T 2601-2014
Construction regulations of BPDST It sets out the construction procedure and technical requirement for BPDST and suits the newly built, expanded, and rebuilt BPDST but excludes household biogas digesters in rural areas.
• Corresponding approval documents should be obtained before construction.
• Main materials should be qualified and certificated.
• Pipeline work should follow related standards.
• Installation of filler should follow related requirements.
• Water tightness and air-tight test should be done once construction is finished.
• Local rural energy office is responsible for accepting completed projects, including midterm and final acceptance.

NY/T 2602-2014
O&M specifications of BPDST It sets out the requirement and methods for O&M of BPDST.
• The O&M staff should be trained and certificated.
• Gas tightness should be checked annually.
Overhaul should be conducted every 2-4 years.
• The effluent should be monitored regularly.
• No need to collect biogas for less than 10 m 3 anaerobic digester.
• Measures should be considered for safety control.
• Key data, such as drawings, should be  Table 8. In addition, the incoming water flow for decentralized systems fluctuates more than in centralized systems, which may result in an unsteady and uncertain effluent quality.

Low environmental awareness
A survey reveals the public opinion that environmental pollution has become a serious problem for China (Liu et al. ). Public awareness toward the problem of wastewater pollution has grown tremendously in recent years. However, the education gap between urban and rural areas is still large. When residents or farmers were interviewed about the BPDST in their area, they had no idea about these structures; to a certain extent, this reflected the attitude of the local people toward the technology.
Another issue is public acceptance. Farmers prefer chemical fertilizer instead of organic fertilizer. They expect a payment if they use the 'waste sludge' from biogas digesters. This situation shows a great need for dissemination of knowledge about the reuseoriented wastewater treatment systems.

Lack of funds and technological innovation
Money is not often the most serious problem for pilot or demonstration projects. However, a general helplessness exists