What are the principles of rural sewage treatment?

The "Eleventh Five-Year Plan" puts forward a major historical task of building a new socialist countryside, and defines the construction goals of "production development, affluent life, civilized rural customs, clean village appearance and democratic management". Strengthening the treatment of rural domestic sewage is an organic part of the transformation of village appearance and an important part of the construction of new socialist countryside. The environmental pollution caused by rural domestic sewage is not only a potential safety hazard of rural water sources, but also aggravates the crisis of fresh water resources, which makes the irrigation of cultivated land not effectively guaranteed and harms the survival and development of farmers. Therefore, strengthening the construction of rural domestic sewage collection, treatment and recycling facilities, avoiding the pollution of rural water, soil and agricultural products caused by direct discharge of domestic sewage, and ensuring the safety of rural water sources and the physical and mental health of farmers are important contents of strengthening infrastructure construction and promoting village renovation in the construction of new countryside, and also an urgent problem to be solved in improving rural living environment. 1. Environmental Problems Caused by Rural Domestic Sewage Rural areas produce about 8 billion tons of domestic sewage every year, but 96% of villages have no drainage channels and sewage treatment systems, and domestic sewage is discharged at will. Some people describe the current situation of rural human settlements in China as follows: if the villages in the whole country are merged into 10 villages, 4 villages have no running water; Three villages dug a well beside the pigsty or toilet for people to drink; 10 village has dirty water spilled out; Nine villages still use traditional dry toilets; Nine villages still randomly find places to bury garbage; Four villages can't get out when it rains; Five villages are not allowed at night. Domestic sewage in rural areas of China has the following characteristics: (1) is widespread and scattered. The geographical distribution characteristics of scattered villages make the sewage scattered and difficult to collect. (2) There are many sources. In addition to human feces and kitchen sewage, there are also sewage generated by household cleaning and domestic garbage accumulation and infiltration. For example, the laundry wastewater in Taihu Lake accounts for 2 1.6% of domestic sewage, and Chaohu Lake and Dianchi Lake account for 17.9%. (3) rapid growth. With the improvement of farmers' living standards and the change of rural lifestyle, the amount of domestic sewage also increases. (4) The treatment rate is low. Taking rural pollution in Lishui City, Zhejiang Province as an example [3], the total amount of rural human excrement produced in this city is about 6.5438+0.8 million tons each year, and the septic tank treatment capacity is about 23.03 million tons, with a treatment rate of only 654.38+0.2%. Untreated domestic sewage is discharged at will, which seriously pollutes the rural ecological environment and directly threatens the health of farmers and rural economic development. On the one hand, untreated domestic sewage flows into low-lying rivers, lakes and ponds, seriously polluting all kinds of water sources; On the other hand, domestic sewage is also the source of disease transmission, which is easy to cause the occurrence and prevalence of infectious diseases, endemic diseases and zoonosis in some areas. At present, the penetration rate of tap water in rural areas in China is only about 34%, and there are still more than 300 million farmers with drinking water safety problems. In Lishui city, Zhejiang province, the sampling test results of household water quality of farmers showed that 72% of 63 water samples exceeded the standard, such as Escherichia coli and turbidity. There is a direct causal relationship between the low water quality of the water source and the direct discharge of rural domestic sewage without treatment. Second, the technical route of rural domestic sewage treatment abroad. The highest goal of sewage treatment is to realize resource consumption reduction, product value Reuse and waste Recycle. The utilization of water resources should be transformed from a one-way linear water metabolism system of "water supply-water use-drainage" to a closed-loop water circulation system of "water supply-water use-drainage-sewage reuse". For decentralized domestic sewage in rural areas, the decentralized sewage treatment and reuse (DESAR) system with simple process, guaranteed treatment effect and simple operation and maintenance is the best choice for comprehensive benefits, which contains the dual significance of sewage treatment and resource utilization, emphasizes on-site quality treatment and recovers nutrients as much as possible. Some foreign countries have accumulated a lot of experience in the research and application of decentralized domestic sewage treatment technology in rural areas, which is worth learning and learning from. (1) Australia's "filter" sewage treatment and recycling system This system uses sewage irrigation to achieve the purpose of sewage treatment, which can effectively achieve the dual goals of pollutant removal and sewage reduction, which can not only meet the needs of crops for moisture and nutrients, but also reduce the nitrogen, phosphorus and potassium contents in sewage and avoid eutrophication of water after sewage is directly discharged into water. The removal rates of total phosphorus (TP), total nitrogen (TN), biological oxygen consumption (BOD5) and chemical oxygen consumption (CODcr) can reach 97%~99%, 82%~86%, 93% and 75% ~ 86% respectively [6] (p79-81). (II) Soil Capillary Infiltration System This system puts sewage into a infiltration ditch with a certain structure on the soil surface, and treats and purifies pollutants through physical, chemical and microbial degradation and plant absorption and utilization. The United States, Japan, Australia, Israel, Russia and Western Europe have always attached great importance to the research and application of the system, standardized and serialized the process flow, purification methods and construction facilities, and compiled corresponding technical specifications. The removal rate of suspended solids, organic matter, ammonia nitrogen, total phosphorus and Escherichia coli by this technology is relatively high, generally reaching 70%~90%[7](P953-964). Moreover, the technology has the advantages of low capital investment, low operating cost and simple maintenance. The whole system is buried underground, which will not emit odor, ensure stable operation in winter and facilitate on-site treatment and reuse of sewage. Therefore, this technology has strong technical and economic advantages for the vast rural areas where the contradiction between supply and demand of water resources is becoming increasingly tense and domestic sewage pollution is becoming increasingly serious. Units that need to treat wastewater can also consult enterprises with similar sewage treatment experience on the sewage treasure project service platform. (3) Constructed wetland treatment system This system is generally composed of artificial substrates (mostly gravel) and wetland plants (reeds, cattails, rushes and hemp, etc.). ) grow on them. It is a unique "soil-plant-microorganism" ecosystem, which uses the interaction of various plants, animals, microorganisms and soil to filter and absorb pollutants in sewage step by step, thus purifying sewage. This technology has been widely used in Europe, North America, Australia, New Zealand and other countries. Its disadvantage is that it needs a lot of land, and it has to solve the problem of sufficient oxygen supply for soil and water, as well as the influence of climate temperature and plant growing season. (IV) Biofilm technology Biofilm method is an artificial treatment technology mainly used for decentralized domestic sewage treatment, including anaerobic and aerobic biofilm. Anaerobic or aerobic microorganisms adhere to the surface of the carrier to form a biofilm to adsorb and degrade pollutants in sewage, thus achieving the purpose of purification. The method has the advantages of simple equipment, low operation cost and high treatment efficiency. The reactor generally consists of three parts: filler (carrier), water distribution device and drainage system. The fillers used are inorganic substances (ceramsite, slag, activated carbon, etc.). ) and organic (PVC, PP, plastic, fiber, etc. ). At present, new biofilm reactor and immobilized microorganism technology have also been widely studied. (5) Stabilizing pond This technology mainly uses the interaction between bacteria and algae to remove pollutants in sewage, which has the advantages of less capital investment, low operating cost, simple maintenance, easy operation, effective removal of organic substances and pathogens in sewage, and no need for sludge treatment. Germany and France have 3,000 and 2,000 stabilization ponds respectively, while the United States has tens of thousands of stabilization ponds. Oswald of the United States proposed and developed an efficient algae pond to make full use of oxygen produced by algae and the relationship between bacteria and algae to treat pollutants efficiently. (VI) Integrated integrated device treatment technology The development of small and medium-sized integrated sewage treatment devices integrating pretreatment, secondary treatment and advanced treatment is a trend of decentralized sewage treatment at home and abroad. The integrated device studied in Japan mainly adopts the combined process of anaerobic-aerobic-secondary sedimentation tank, which has the functions of degrading organic matter and denitrification, and its effluent BOD? 5? & lt20/L, total nitrogen