With the advancement of new urbanization and beautiful rural construction, China has increased its focus on the treatment of rural domestic sewage. The United States is concerned about the early treatment of rural sewage. In order to solve the problem of groundwater pollution and meet the requirements of higher environmental water quality, a series of decentralized sewage treatment technologies have been introduced to replace traditional technologies. By reviewing the technical measures and management mechanisms of distributed domestic sewage treatment in the United States, we hope to find a way to treat rural sewage in China from the experience of decentralized sewage treatment in the United States.

1 Dispersed sewage treatment technology

The treatment technology of dispersed sewage in the United States can be basically summarized as a traditional land processing system and other alternative systems. Alternative systems include conventional aerobic treatment systems and anaerobic treatment systems, such as suspension growth and fixed membrane aerobic treatment systems, sequencing batch reactors, upflow anaerobic fluidized beds, and the like.

1.1 septic tank - soil absorption system

The septic tank-soil absorption system generally consists of two parts, the septic tank and the absorption site (see Figure 1). As a primary treatment unit for sewage, the septic tank functions as a sewage collection and primary treatment. The absorption site can be divided into two types, namely gravel type and gravel-free type, which can be divided into trench type and raft type according to the shape of the site.

The use of septic tank-soil absorption systems requires high soil and geological requirements. In order to improve the treatment effect, the structure of the septic tank has undergone a series of improvements, and a separator type and an upflow type septic tank have appeared. Mikhaeil et al. studied the performance of traditional, single-separator, double-separator, double-separator-filled septic tanks for domestic sewage 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 type> double partition type> single partition type> conventional type. Therefore, the double-separator type and the double-separated type septic tank are more suitable for treating high-concentration domestic sewage in rural areas. Sabry uses a modified septic tank, the upflow septic tank/separator reactor (USBR), to treat rural domestic sewage. During the one-year operation period, the removal rates of COD, BOD and TSS reached 84%, 81% and 89%. The USBR system is not affected by the impact load and is less affected by low temperatures.

1.2 mound system

The mound system is a method of disposal of sewage land that is higher than natural soil and loaded with packing. Fillers are an important processing unit in mound systems. Different structure fillers are applied to the mound system and have different treatment effects. Some fillers can enhance the phosphorus removal effect of the system due to the inclusion of aluminum, iron and calcium. At the same time, it is also a major trend to recycle industrial waste as a filler, such as blast furnace slag, fly ash, and the like. Charles et al. filled the sandstone mound system with industrial by-products to improve the phosphorus removal effect, and found that the average removal rate of phosphorus was as high as 98%, while the average removal rate of nitrogen was low, only 19%.

1.3 sand filter system (Sandfilters)

The sand filter system refers to a treatment device that uses a medium to intercept and biodegrade pollutants, and has various structures, flexibility, and wide application range. According to the structure, the sand filter system can be divided into three types: buried type, intermittent type and circulating type. For sand filtration systems, clogging is the main cause of failure, which reduces the system's permeability coefficient and, in severe cases, leads to complete system failure. The discussion of clogging problems is mainly due to genetic analysis, including the retention of suspended solids, biofilm growth, vegetation growth, and chemical effects. However, the degree of clogging of the sand filter system is not easy to be evaluated, especially the underground sand filter system. In the past, the filter head loss was usually measured, and the early warning parameters were also studied. Petitjean et al. studied the early warning parameters of the blocking condition of the buried vertical flow sand filtration system. It is recommended to use the NH3-N/NO-3-N effluent concentration ratio and the oxygen concentration in the porous medium to detect early occlusion.

1.4 sewage evapotranspiration system

The sewage evapotranspiration system is a sewage treatment facility that discharges sewage into the atmosphere through physical forces such as soil surface evaporation and plant transpiration. It is mainly composed of anti-seepage liner, cloth pipe and sand bed, and vegetation can be planted on the surface of the sand bed. The evapotranspiration rate of the evapotranspiration system is related to the growth of the plants and the accessibility of the nutrients, and the treatment effects of different tree species are also different. In temperate climate regions, willow and poplar are more popular because of their high transpiration rate, water flooding and high tolerance to contaminant concentrations during the growing season.

1.5 integrated processing device (PackagePlants)

The integrated treatment unit is a pre-fabricated sewage treatment facility for use in areas with a small population and low sewage flow. Conventional processing equipment requires in-situ installation, while the processing components of the integrated processing unit are assembled directly at the factory and then transported to a designated location. Its compact structure and small amount of construction work. Integrated wastewater treatment facilities typically have basic physical processing units including coagulation, flocculation, sedimentation, filtration, and the like. The most common integrated treatment devices are time delay aeration reactors, sequencing batch reactors, oxidation ditch, contact stabilization tanks and rotating biological contactors.

2 Enlightenment from the choice of dispersed sewage treatment technology

2.1 Meet geographical conditions and meet the site selection requirements

Different technologies have their own geographical conditions. The treatment unit of the septic tank-soil absorption system is soil, so there are certain requirements for soil and geological conditions. The proper soil needs to have good permeability, and the soil moisture remains unsaturated at a certain depth, thus ensuring the soil treatment capacity. The location of the treatment system should be much higher than the groundwater level and bedrock and at a distance from the location of the drinking water source to reduce the threat to natural water such as groundwater. For the geographic requirements of site selection, the United States has corresponding rules and regulations. When some land treatment systems are not suitable, the treatment can be improved by adding auxiliary facilities, or some other alternative technologies such as aerobic, anaerobic and other engineered biological treatment facilities can be directly used.

2.2 Processing capacity meets the corresponding environmental water quality requirements

In the early stage of technology selection, detailed surveys should be conducted on the area, including the water quality, quantity of water, location of environmentally sensitive areas, and population density. Secondly, determine the surface water and groundwater quality, assess the degree of pollution of the receiving water body and the self-purification capacity of the water body, and determine the treatment standard according to the environmental and health risks. Different local areas should have different treatment requirements for in-situ treatment systems. For water-sensitive areas such as drinking water sources and nature reserves, the effluent quality requirements of the in-situ treatment system are high.

2.3 The price is suitable

The United States attaches great importance to the economic accounting of existing technologies because it helps guide subsequent technological choices. As early as 1979, the EPA released a more comprehensive summary of traditional wastewater treatment technology capital and operating costs. The choice of rural sewage treatment technology needs to combine the local economic level and environmental needs, and comprehensively consider the relationship between cost and treatment effect, so as to choose a cost-effective technology. The cost estimate should include not only the construction costs, but also the operating and maintenance costs of the service period, as well as the long-term necessary repair and replacement costs. Only when the village correctly evaluates the expenses that it can bear, can it prevent money from building, run without money, and “bath the sun” after the sewage treatment facilities are built.

3 Management mechanism for decentralized sewage treatment

3.1 Improve management regulations

In order to maintain the good operation of the distributed sewage treatment system, the United States' dispersed domestic sewage treatment is guaranteed by the relevant laws and regulations. At the state level, the management requirements for decentralized domestic sewage treatment usually appear in the form of laws and regulations related to related fields, and there are many types of laws and regulations, involving public safety law, nuisance behavior law, environmental protection law or building regulations. However, in most states, the jurisdiction of the local processing system is generally delegated to the county or other local administration. Local jurisdictions can directly enforce state-enforced regulations or enforce more stringent regulations that are self-developed during law enforcement. In addition to the mandatory legal requirements, EPA also focused on the publication of the Guidebook to guide the management of decentralized processing systems. In 2003 and 2005, the Dispersed Processing System Management Guide and the Dispersed Sewage Treatment System Management Manual were issued. Related management documents help community managers and household owners to handle the operation and maintenance of the system.

3.2 outstanding operation and maintenance

The success or failure of a processing system depends largely on the operation and maintenance phase. The management of decentralized sewage treatment facilities in the United States is divided into five modes, namely, household ownership mode, maintenance contract mode, operation permit mode, centralized operation mode and centralized operation mode. A simpler system can be maintained by the head of the household, relying on the owner to maintain it; a more complex system requires the supplier to provide maintenance services; the system operated by the management responsible entity is maintained by the management entity. Hierarchical management and maintenance can guarantee the performance of the processing system most efficiently. There are differences in the maintenance content of different processing systems. In order to ensure that the repair service is easy to trace, it is necessary to register after each service and use a networked monitoring facility if necessary. The town of Jamestown, Rhode Island has a routine maintenance monitoring program for the in-situ processing system, and inspectors record monitoring information in a network database. Check the system every 3 or 5 years depending on the size, type and water usage of the system.

3.3 Focus on public education

Looking at the reasons for the failure of the processing system, most of the cases are due to the lack of experience of the head of the household, the failure to repair and update the faulty processing system in a timely and effective manner, and ultimately the system collapse. Public education is conducive to improving the awareness and ability of the handling system and the maintenance system of the management personnel. The main contents include reminding users to maintain regularly and distributing multimedia materials. For example, the head of the household is encouraged to inspect every 3 years, and the septic tank system is cleaned once every 3 to 5 years; the water resources are rationally used, and the waste is not discarded at will; avoiding parking, planting, and resetting the rainwater discharge system around the land treatment system. At the same time, the consultation object of the treatment system is provided. When the owner has doubts about the maintenance of the septic system, he can contact the local health department or the regulatory agency, or go to the experts in the area through the database of the septic system service provider. .

3.4 Strengthening departmental cooperation

3.4.1 Federal, State, and Local Governments

The Clean Water State Revolving Fund (CWSRF) established by the US Environmental Protection Agency is a source of funding for many municipal infrastructure projects and an important source of funding for decentralized wastewater treatment projects. The good functioning of funds is the result of good collaboration between federal, state and local governments. The Clean Water State Working Capital Fund relies mainly on federal-state funding relationships to provide low-interest or interest-free loans to communities and individuals. Loans are returned to the Working Capital Fund and then allocated to subsequent projects. The federal government is primarily responsible for providing grants to states, and states are responsible for the operation of the state's CWSRF program, providing various types of financial assistance, including loans, refinancing, purchasing or guaranteeing local debt and buying bond insurance. When the fund cannot be directly delegated to a private entity, the local government entity can be funded first by revolving the loan, and then the owner or other entity can provide loans or grants.

3.4.2 Federal and Cooperative Organizations

In order to strengthen the cooperation between EPA and the relevant practitioners of the dispersion system, the EPA and the relevant cooperative organizations for decentralized wastewater treatment established a memorandum of understanding in 2005, including the National Township Association, the National Association of Wastewater Technicians, and the National Environmental Health Association. The responsibilities of the EPA and contracting organizations include providing information on projects, regulations, and plans for personnel engaged in the dispersed wastewater treatment system industry, preparing training, awarding certificates, raising public awareness, and preparing materials for organizations that implement decentralized systems. In 2014, MOU expanded to 18 partners, updated the commitment of EPA and partner organizations, further strengthened the management of decentralized systems, and increased EPA cooperation with state, local government, decentralized system-related practitioners and suppliers.

3.4.3 State and State

The problem of basin-based sewage can only be solved by the state alone. For decentralized wastewater treatment, regional cooperation and information sharing are emphasized among states in the United States. In 2015, in order to better achieve the maximum daily load target for pollutants in the Chesapeake Bay, Delaware, Maryland, Pennsylvania, Virginia and West Virginia signed a memorandum of cooperation, promised to use The data is shared by the pre-processing unit of the in-situ processing system that reduces the nitrogen load in the Chesapeake Bay for a period of five years. This inter-state collaborative development tool for sharing data is the first in the in-situ wastewater treatment industry. The approval of the previous processing system was carried out separately in each state, which greatly simplified the approval process of processing technologies in each state and reduced the cost of users and manufacturers.

4 summary

The treatment of decentralized sewage in the United States has experienced twists and turns. The early misuse and mismanagement of land treatment systems led to serious problems of groundwater pollution, and gradually established a good technology selection model and management mechanism. There have been many demonstration projects in China's decentralized wastewater treatment, but there are still many problems. Rural domestic sewage treatment should establish guidelines for technology selection, select technologies according to local geological conditions, environmental water quality requirements, and economic conditions, and give priority to technologies with low cost and convenient management. Accelerate the establishment of laws, regulations and management guidelines for decentralized wastewater treatment, establish standards for decentralized sewage facilities, and standardize the design, installation and operation of treatment facilities. Provide farmers with knowledge of sewage treatment, provide channels for handling system maintenance, increase farmers' enthusiasm for participating in wastewater management, and ensure that the treatment system is well maintained after it is built. Finally, strengthen cooperation between provinces and cities, departments and governments and cooperative organizations to jointly solve regional decentralized wastewater treatment problems.

Original title: "Life decentralized sewage treatment in the United States experience and enlightenment", Author: Tianze Yuan, Wu Deli, Alec Zhang, published in "Water & Wastewater" 2017 05.

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Editor in charge: Yin Yingying