In the process of beautiful rural construction, rural sewage treatment is a key element. The United States experienced in rural sewage treatment, treatment technology is more mature, similar to the soil absorption system, sand filtration system and the choice of technology, management mechanism for the establishment of cooperation between all levels of experience are worth learning.
With the advent of new urbanization and beautiful rural construction, China has increased its attention to the treatment of rural domestic sewage. The United States concerned about rural sewage treatment earlier, in order to solve the problem of groundwater pollution to meet the higher environmental quality requirements, there have been a series of alternative to the traditional decentralized sewage treatment technology. By summarizing the technical measures and management mechanism of decentralized domestic wastewater treatment in the United States, it is hoped to find a suitable way for rural sewage treatment in our country from the experience of decentralized sewage treatment in the United States.
1 decentralized sewage treatment technology
Dispersal wastewater treatment technology in the United States can be basically summarized as the traditional land treatment system and other alternative systems. Alternative systems include conventional aerobic treatment systems and anaerobic treatment systems, such as aerobic treatment systems for suspended growth and fixed membranes, sequencing batch reactors, upflow anaerobic fluidized beds, and the like.
Septic tank - soil absorption system
Septic tank - soil absorption system generally includes septic tanks and absorption of two parts (see Figure 1). Septic tank as a primary treatment unit for sewage, sewage collection and primary treatment functions. Absorption site can be divided into two types, namely gravel and gravel-free, according to the site shape can be divided into furrows and furrows.
Septic tank - the use of soil absorption system on the soil, geology demanding. In order to improve the treatment effect, the structure of the septic tank has undergone a series of improvements, such as the separator type and the upflow type septic tank. Mikhaeil et al. Studied the performance of conventional, single-separator, double-separator, double-separator packed septic tanks for domestic wastewater treatment under three hydraulic retention times. For different types of septic tanks, COD, BOD and TSS removal rates are positively correlated with HRT, and the removal rate is double-separator filled> double-separator> single-separator> conventional. Therefore, double-septum and double septum-filled septic tank is more suitable for the treatment of high concentrations of domestic sewage in rural areas. Sabry uses a modified septic tank-upflow septic tank / septum reactor (USBR) to treat rural domestic wastewater. Within one year operation period, COD, BOD, TSS removal rate reached 84%, 81%, 89%. The USBR system is unaffected by the impact load and is less affected by low temperatures.
1.2 mound system
Mound system is higher than the natural soil surface, filled with sewage treatment of land disposal. Fillers are an important processing unit in mound systems. Different structure of the filler applied to the mound system, with a different treatment effect. Some fillers contain aluminum, iron, calcium, phosphorus can enhance the system effect. At the same time, recycling of industrial waste as a filler, thereby reducing costs is also a major trend, such as blast furnace slag, fly ash and so on. Charles et al. Filled industrial gravel mound systems with industrial by-products to improve phosphorus removal, finding an average phosphorus removal of up to 98% and a mean nitrogen removal of only 19%.
1.3 Sand filter system (Sand filters)
Sand filtration system refers to the use of media for pollutant interception and biodegradation of the processing device, its structure, flexibility, wide range of applications. Sand filtration system in accordance with the structure can be divided into buried, intermittent and circulating 3 kinds. For the sand filtration system, the blockage is the main reason for its failure, it will reduce the system's permeability coefficient, serious cases will lead to complete failure of the system. The main problems of plugging are the genetic analysis, including the suspension of suspended matter, biofilm growth, vegetation growth and chemical effects. However, the degree of clogging of the sand filtration system can not be easily assessed, especially the underground sand filtration system. In the past, the loss of the filtration head was usually used for the measurement. At present, the research on the warning parameters also appeared. Petitjean et al. Studied the early-warning parameters for the clogging of a buried vertical flow sand filtration system. It is recommended that the early blockage be detected using the NH3-N / NO3-N effluent concentration ratio and the oxygen concentration in the porous media.
1.4 sewage evapotranspiration system
Evapotranspiration system is through the soil surface evaporation and transpiration of plants and other physical forces to sewage into the atmosphere of sewage treatment facilities. Mainly by impervious liner, cloth pipes and sand bed composition, sand bed surface can be planted with vegetation. The evapotranspiration rate of the evapotranspiration system is related to the growth of the plant and the availability of nutrients, and the treatment effects of different tree species are also different. Willow and poplar are prevalent in temperate climates because of their high transpiration rate during the growing season, water flooding and high pollutant concentration tolerance.
1.5 Integrated Plants (Package Plants)
Integrated treatment plant is pre-fabricated sewage treatment facilities on-site, for less population, sewage flow smaller areas. Traditional processing units need to be installed on site, while the processing units of the all-in-one processing unit are assembled directly at the factory and shipped to designated locations. Its compact structure, the construction project is small. Integrated wastewater treatment facilities generally have basic physical treatment units, including coagulation, flocculation, sedimentation, filtration and the like. The most common integrated processing devices are delayed aeration reactors, sequencing batch reactors, oxidation ditches, contact stabilization tanks and rotating biological contactors.
Inspiration of Choice of Dispersal Sewage Treatment Technology
2.1 meet the geographical conditions to meet the site requirements
Different technologies have their own geographical conditions. Septic tank - soil absorption system processing unit is the soil, so the soil, geological conditions have certain requirements. The right soil needs to have good permeability, and the soil moisture to maintain a certain depth of unsaturated state, so as to ensure the soil processing capacity. The location of the treatment system should be well above the groundwater level and bedrock and be at a distance from the source of the drinking water source in order to reduce the threat to natural water bodies such as groundwater. For the geographical requirements of the site, the United States has the corresponding rules and standards. When some land treatment systems are not available, treatment can be improved by adding ancillary facilities, or some other alternative technologies can be used directly, such as aerobic, anaerobic and other engineered biological treatment facilities.
2.2 Processing capacity in line with the corresponding environmental water quality requirements
Technical options should be conducted early in the region for a detailed survey, including sewage water quality, water, environmentally sensitive areas, population density and so on. Second, determine the quality of surface water and groundwater, assess the pollution degree of the receiving water bodies and the self-purification capacity of water bodies, and determine the treatment standards according to the environmental and health risks. The handling requirements set by the local treatment system should be different in different regions. For drinking water sources, natural protection areas and other water quality sensitive areas, the effluent quality of the in-situ treatment system is required to be higher.
2.3 affordable
The United States places great emphasis on the economic accounting of the prior art as it helps to guide subsequent technological choices. As early as 1979, the EPA released a more comprehensive summary of the capital and operating costs of conventional wastewater treatment technologies. The choice of rural sewage treatment technology needs to combine the local economic level and environmental needs, considering the relationship between cost and treatment effect, so as to choose a higher cost of technology. Cost estimates should include not only construction costs, but also operational life, maintenance costs, and long-term maintenance and replacement costs. Only when villages correctly assess the costs they can afford can they prevent money-rich construction and money-free operation and "basking in the sun" after the sewage treatment facilities are completed.
3 decentralized sewage treatment management mechanism
3.1 improve the management regulations
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