In rural areas, people's awareness of environmental protection is weak, the economy is relatively underdeveloped, and domestic sewage drainage and collection pipe network system and centralized treatment facilities are also lacking; Or the residence is relatively scattered, which makes it difficult to collect domestic sewage centrally, resulting in more than 91% of domestic sewage being directly discharged into rivers and lakes without treatment. At present, the environmental pollution caused by domestic sewage in rural areas seriously threatens the safety of water resources in rural water sources, and also intensifies the crisis of fresh water resources, which makes the irrigation of cultivated land unable to be effectively guaranteed and ultimately harms the survival and development of people and animals. Today, when the country emphasizes the construction of ecological civilization, it is particularly urgent and important to strengthen the control and treatment of rural domestic sewage pollution.
1 characteristics and treatment of rural domestic sewage
rural domestic water is generally used in combination with surface water (for example, rivers, ditches, ponds, weirs, lakes and reservoirs), groundwater (wells and pits) and tap water. Rural domestic sewage in China mainly comes from toilet manure and its washing water, bath wastewater and kitchen catering water, which can be divided into gray water and black water. The former consists of kitchen drainage, sanitary shower water and laundry water; The latter water consists of feces, urine and its washing water [3]. Rural domestic sewage in China has the characteristics of dispersion, large daily variation coefficient (usually 3.1 ~ 5.1), intermittent discharge, high ammonia nitrogen content, strong biodegradability and less toxic and harmful substances such as heavy metals [4].
it is of great significance to fully understand the characteristics of rural domestic sewage, develop rural domestic sewage treatment technology suitable for China's national conditions, alleviate the contradiction of water shortage, improve the ecological environment in rural areas and improve people's quality of life on the basis of local economic level, natural conditions and environmental objectives. According to the differences of sewage collection and treatment methods, sewage treatment modes can be divided into decentralized and centralized types.
Decentralized treatment usually has the characteristics of small investment, low operating cost, small sludge production, little external influence, simplicity and durability, and easy realization of water recycling. This technology is suitable for the treatment of domestic sewage in rural areas with small scale, scattered population and difficult centralized collection of sewage [5]. At present, there are many decentralized treatment technologies widely used in rural areas of China, such as constructed wetland, high-efficiency algae pond technology and earthworm ecological filter. However, with the composition of rural domestic sewage becoming more and more complex, the effluent from a single decentralized treatment process is difficult to meet the environmental protection needs of the receiving water body.
At the same time, different domestic sewage decentralized treatment technologies have their own advantages and disadvantages and application scope, which also limits the application scope and effect of decentralized treatment technology. Based on this, the common method to treat rural sewage at present is to combine various processes to achieve the purpose of strengthening the purification capacity of the system. At present, according to the mechanism of combined treatment technology of rural domestic sewage, they can be roughly divided into three categories: biological combination technology, ecological combination technology and biological-ecological combination technology.
2 Treatment of rural domestic sewage by biological combination technology
Biological treatment technology refers to the technology of removing pollutants by microorganisms under aerobic and anaerobic conditions. This technology has the advantages of small floor space, low sludge output, good impact load resistance, and can treat sewage with large fluctuation in water quantity and quality. Anaerobic unit (a) in biological treatment technology can degrade most organic matter in sewage, reduce sewage load and settle suspended matter; The aerobic unit (O) further removes nutrients and organic matter such as nitrogen and phosphorus. At present, the biological combination technology widely used in rural domestic sewage is mainly a different process composed of A and O.
2.1A/O process
A/O process is generally a combined process with anaerobic treatment as the pre-unit, followed by aerobic treatment. The process has higher pollutant removal rate and better system stability. After anaerobic baffled reactor (ABR), Li Qingxue and others added drop aeration and biological aerated filter respectively, and found that the removal rate of COD in rural domestic sewage by these two combined processes was 9.5% and 24.9% higher than that by ABR alone, so it can be seen that the combined process of ABR and biological aerated filter has a better removal effect on COD [6].
in view of the climate characteristics of sunny and sunny in the northern region, He Gang and others combined anaerobic biofilter with solar aerated biofilter to provide oxygen through solar aerated system, which further reduced the system energy consumption [7].
CaoDaWei and others have researched and developed the buried integrated biofilter, and the energy consumption equipment is only 1 small lift pumps, which are mainly composed of anoxic pool and biofilter, and the aeration is enhanced by ventilation with exhaust pipe and splash tray [8].
the device has good impact load resistance, and the removal effect of pollutants is also good. The average removal rates of COD, NH3-N, TN and TP are 63.1%, 92.2%, 68.6% and 47.5%, respectively. It has the characteristics of not occupying land resources, low energy consumption and low operating cost, and is suitable for popularization and use in rural areas around lakes in the south where land resources are tight [9].
2.2A2/O process
The anaerobic-anoxic-aerobic (A2/O) process consisting of adding 1 A units before A/O is widely used in many biological treatment technologies at home and abroad [11-11]. This combined process has a long hydraulic retention time and a low organic load, so that the anoxic-aerobic unit can maintain a low sludge content, greatly reduce the discharge of excess sludge and realize sludge reduction for the combined process.
Gao Dawen and others used the combined process of upflow anaerobic sludge fixed bed (UAFB)- anoxic-aerobic membrane biological reaction (MRB) to treat domestic sewage, which not only removed 93.3% and 91.6% of COD and NH3-N, but also maintained the low sludge content in the reactor for a long time, reduced the residual sludge treatment capacity and alleviated membrane pollution [12].
Zhou Jun et al. put the anoxic tank at the front end of the anaerobic tank, added a micro-electrolysis iron filings bed and composite biomaterials, and developed an improved combined purification tank. The purification tank adopts anoxic-anaerobic-aerobic (A2/O) treatment process. On the one hand, this process effectively solves the problems of low organic matter content, insufficient carbon source and poor denitrification effect in sewage; on the other hand, it can achieve better simultaneous nitrogen and phosphorus removal through the combination of traditional activated sludge process, biological nitrification, denitrification process and biological phosphorus removal process. The experimental results show that the average removal rates of COD, TN and TP in March are 93.1%, 81.1% and 94.1% respectively when the HRT is 8h and the reflux volume ratio of the system is 75.1%. In August, it was 94.1%, 76.1% and 91.1% respectively [13].
Bai Xiaolong and others also improved the small purification tank, adopting the baffled anaerobic reactor-anaerobic biofilter-biological contact oxidation process, and adopting the upflow inflow of domestic sewage to effectively reduce the equipment blockage and maintenance time [14].
2.3 other combined processes
besides the common A/O and A2/O processes, some other combined processes are also used in the actual sewage treatment.
Zhan Xu et al. used the five-stage biological contact oxidation method to treat rural sewage. The raw water was pumped up by a water pump, and the five-stage aerobic treatment not only met the required dissolved oxygen, but also eliminated the aeration equipment, reduced the investment cost and operation power consumption, and made the management work simple. This process has obvious benefits in solving the water environmental pollution problem in rural areas with relatively backward economy [15].
Shen Dongsheng and others have developed 1 underground unpowered anaerobic reactor (UUAR), which adopts anaerobic sludge bed contact tank+anaerobic biofilter process, with simple process and low energy consumption. Compared with aerobic biological treatment, the capital investment of UUAR technology and equipment may be slightly higher than that of aerobic treatment, but there is no daily operating cost, and there is no accumulation of surplus anaerobic sludge, which is suitable for decentralized treatment of domestic sewage in remote rural areas with tight land, backward economy and bad natural climate [16].
Xu Gongdi and others carried out aerobic pre-hanging membrane before the biological purification tank, forming an O-A-O combined process. The compound biological purification tank has a high removal capacity of NH3-N and TN, effectively reducing the load of rural domestic sewage with high urine content, and the removal rates of COD and phosphorus are 59.6% and 33.4%[17].
In view of the low carbon and nitrogen of domestic sewage in rural areas of North China, Wu Di and others adopted the self-flow anaerobic-three-stage aerobic-anoxic biofilm process, and used the added water beads to improve the nitrification capacity of anaerobic section; At the same time, in the three-stage aerobic and anoxic biofilm section, nitrification and denitrification dephosphorization are carried out in the same reactor by falling water and oxygenation, thus effectively solving the problem of carbon source supply capacity. The removal rates of COD, NH3-N, TN and TP in rural domestic sewage are 73.7%, 91.7%, 59.6% and
69.7%[18]. Subsequently, the three-stage aerobic-anoxic biofilm technology was improved, and a reflux pump (reflux volume ratio of 2:1) was added to the new process, and the filling rate of suspended filler in anaerobic stage was increased. After the improvement, the TN removal capacity of effluent was greatly improved, reaching 63.9%[19].
3 ecological combination technology to treat rural domestic sewage
ecological treatment technology is a process that utilizes soil-plant (animal)-microorganism composite ecosystem, utilizes the resources in sewage through physical, chemical and biological actions, and degrades and purifies pollutants in sewage [21]. Compared with biological treatment technology, ecological treatment technology generally has low construction management cost, energy saving and consumption, has certain landscape effect, and pays more attention to ecological service value. In the vast rural areas of China, the ecological combination treatment technologies currently applied include the combination of the same ecological technology and the combination of different ecological technologies.
3.1 Combination of the same ecological treatment technology
The composite vertical-flow constructed wetland designed by Wu Zhenbin and others connects the down-flow pool and the up-flow pool in series, and the bottom is connected, which makes the nitrification and denitrification of sewage enter the wetland system more fully. The system has a good removal effect on TN in sewage, with a removal rate of 43.6%[21].
for low-content rural sewage in Dianchi lake area, Liu chaoxiang and others adopted two artificial composite ecological bed treatment processes, namely surface flow and subsurface flow. under the condition of high hydraulic load (31cm/d), the removal rates of COD, TN, NH3-N and TP in subsurface flow bed were 71.6%, 61.6%, 81.9% and 66.1% respectively, while those in surface flow bed were respectively. Compared with the single constructed wetland treatment technology, the combined constructed wetland technology improves the oxygen content and organic matter in the wetland, thus improving the nitrification and the pollutant removal ability of the wetland, and the denitrification effect is particularly obvious.
The influent of tower-type composite constructed wetland (TICW) designed by Ye Fenxia et al. is divided into two sections. One part of sewage flows into the lower section to form subsurface constructed wetland, while the other part flows down from the top of the tower to form surface constructed wetland, which can provide sufficient carbon source for nitrogen removal in the latter section of wetland [23].
Zhang Hongling and others used multistage soil infiltration system to treat rural domestic sewage in Taihu Lake Basin, and the average removal rates of COD, NH3-N, TN, TP and SS were 71.1%, 83.1%, 59.1%, 76.1% and 94.1% respectively [24]. Zheng Yanqiang and others put two sets of underground infiltration systems in parallel, and the filling media were soil, ceramsite, slag and two kinds of natural organic matter, which also achieved good results in the treatment of rural domestic sewage [25].
Ji zhumei and others established an ecological pond by planting ecological plants on the surface of the stable pond through floating bed technology. The system has high removal rates of COD, NH3-N, TN and TP in high-content domestic sewage, which can reach above 55.1%, 71.1%, 81.1% and 75.1% respectively [26]. In the tower-type earthworm ecological filter treatment system designed by Li Junxiang, each tower is a treatment unit, and the gradient tower layers are arranged in series and stacked. The system has a good treatment effect on COD, ammonia nitrogen, TN and TP, and the capital construction and operation costs are low, and the total operation cost is 1.671 yuan /m3. This technology has a good application prospect in economically underdeveloped rural areas.
3.2 Combination of different ecological treatment technologies
Wang Xuehua and others used ecological ponds as pretreatment and constructed wetlands as follow-up treatment, and the removal rates of NH3-N, TN and TP in rural domestic sewage of Sanshan Island in Taihu Lake were as high as 95.1% ~ 99.1%, 95.1% ~ 98.1% and 92.1% ~ 98.1%, and the SS content in the influent was reduced. After the domestic sewage passes through the tower earthworm ecological filter, the effluent load and pollutant concentration are reduced, and then it enters the horizontal subsurface constructed wetland, further reducing the contents of organic matter and nutrients, and making the effluent quality basically meet the standard. This combined process of tower earthworm ecological filter and constructed wetland has high degree of automation and convenient management and operation, and is suitable for popularization and use in rural areas with developed economy and dense population.
Shi Jianwei and others planted a combined system of plants in the form of a biological floating bed on the water surface of an efficient algae pond. On the one hand, it significantly improved the stability of the system operation and the water quality of the effluent, and saved the land. On the other hand, the roots of transplanted plants on the biological floating bed effectively inhibited the growth of algae through allelopathy, and the plants themselves also had a certain landscape effect and beautified the environment [31]. Gao Shengbing et al. used plant and soil infiltration system to treat rural domestic sewage, and the removal effect was good. The average removal rates of BOD5, COD, TN, NH3-N and TP in domestic sewage by this composite system are 73.5%, 76.1%, 85.1%, 89.1% and 85.1% respectively [31].
4 biological+ecological combination technology to treat rural domestic sewage
biological+ecological combination technology is a combination of biological and ecological treatment processes. The former biological treatment mainly removes organic matter and some nutrients, and the subsequent ecological treatment further removes nitrogen and phosphorus, giving full play to their respective advantages and improving the effluent quality and the stability of system operation [32]. Compared with biological combination technology and ecological combination technology, biological+ecological combination technology needs to comprehensively consider the economic conditions in rural areas, the climate difference between the north and the south, as well as land use conditions, operation management, sludge production and practical engineering cases. Because constructed wetland is the most widely used follow-up ecological treatment technology, the common biological+ecological combination technology in rural areas of China mainly includes biological+constructed wetland combination technology and other biological+ecological combinations.
4.1 Biological+Constructed Wetland
The biological unit in the combined system of biological+Constructed Wetland can effectively complete the degradation and nitrification of organic matter; At the same time, the constructed wetland system can further remove pollutants such as nitrogen and phosphorus. The combination of the two can improve the removal rate of various pollutants in sewage. Tang Jing and others used the combined process of contact oxidation and constructed wetland to treat rural domestic sewage, and the removal rates of COD, NH3-N, TN and TP were 68.2%, 68.2%, 69.5% and 86.3% respectively, and the effect was stable. Among them, the contact oxidation pool with falling water and oxygenation contributes greatly to the removal of COD, while the constructed wetland contributes greatly to the removal of TN and TP [33].
In the rural area of Taihu Lake Basin, Bai Yonggang et al. used the trickling filter-constructed wetland combined process to treat domestic sewage. The results showed that trickling filter could treat COD, NH3-N, TN and