1 Rural domestic sewage characteristics and treatment
Rural domestic water is generally used in combination with surface water (e.g., rivers, ditches, ponds, weirs, lakes and reservoirs, etc.), groundwater (wells, cellars), and tap water. China's rural sewage mainly comes from toilets and feces and their flushing water, bathing wastewater and kitchen catering water, etc., which can be divided into gray water and black water 2 categories. The former consists of kitchen drainage, sanitary shower water, laundry water; the latter water consists of feces and urine and their flushing water [3]. China's rural domestic wastewater is characterized by decentralization, large daily coefficient of variation (usually 3.0-5.0), intermittent discharge, and high ammonia and nitrogen content, high biochemical properties, containing heavy metals and other toxic and hazardous substances, etc [4].
Full understanding of the characteristics of rural sewage, in combination with the local economic level, natural conditions and environmental objectives, based on the development of rural sewage treatment technology suitable for China's national conditions, to alleviate the contradiction between the shortage of water resources, to improve the ecological environment in rural areas and improve the quality of life of the people are of great significance. According to the differences in sewage collection and treatment, sewage treatment mode can be divided into decentralized and centralized 2 categories.
Decentralized treatment is usually characterized by small investment, low operating costs, small amount of sludge, little influence from the outside world, simple and durable as well as easy to realize the recycling of water. This technology is suitable for the treatment of domestic sewage in rural areas with small scale, dispersed population, and sewage is not easy to collect centrally [5]. At present, China's rural areas apply more decentralized treatment technology has artificial wetlands, high-efficiency algae pond technology, earthworm ecological filter and other methods. However, as the composition of rural sewage is becoming more and more complex, the effluent of a single decentralized treatment process is difficult to meet the environmental needs of the receiving water body.
At the same time, different decentralized treatment of domestic sewage technology has its own advantages and disadvantages and scope of application, which also limits the application of decentralized treatment technology and effect. Based on this, the more common approach to rural sewage treatment is to combine a variety of processes to achieve the purpose of strengthening the purification capacity of the system. Currently, according to the mechanism of rural sewage treatment technology, can be roughly divided into three categories: biological combination of technology, ecological combination of technology, biological - ecological combination of technology.
2 biological combination technology for rural sewage treatment
Biological treatment technology refers to the technology of removing pollutants through microorganisms in aerobic and anaerobic conditions. This technology has a small footprint, low sludge production, good shock load resistance, and can treat sewage with large fluctuations in water quantity and quality. Biological treatment technology in the anaerobic unit (A) so that most of the organic matter in the sewage is degraded, reduce the sewage load, sedimentation of suspended solids; and aerobic unit (O) to further remove nutrients such as nitrogen and phosphorus and organic matter. Currently widely used in rural domestic wastewater biological combination of technologies, mainly by the combination of A and O into different processes.
2.1A/O process
A/O process is generally a combination of anaerobic treatment as the front unit, followed by aerobic treatment. This process has high pollutant removal rate and good system stability. Qingxue Li et al. added drop-water aeration and aerated biofilter treatment after an anaerobic folded plate reactor (ABR), respectively, and found that the removal rate of COD in rural domestic wastewater by these 2 combined processes increased by 9.5% and 24.9% compared with that of ABR treatment alone, which shows that the combined ABR-aerated biofilter process has a better effect on the removal of COD [6].
Aiming at the climatic characteristics of the northern region, which is characterized by more sunshine and less rain, He Gang et al. combined the anaerobic biofilter + solar aeration biofilter to provide oxygen through solar aeration system, which further reduced the energy consumption of the system [7].
Cao Dawei et al. developed a buried integrated biofilter, the energy-consuming equipment is only a small lifting pump, mainly composed of anoxic tank + biofilter, using the extraction duct ventilation and splash tray to strengthen the oxygenation [8].
The device has good resistance to shock load, and the removal of pollutants is also better, the average removal rate of COD, NH3-N, TN, TP were 63.1%, 92.2%, 68.6% and 47.5%, with no occupation of land resources, energy consumption and low operating costs, more suitable for promoting the use of the southern lagoon in rural areas where land resources are tight [ 9]. 9].
2.2A2/O process
The anaerobic-anoxic-aerobic (A2/O) process composed of adding one A unit in front of A/O is widely used in many domestic and foreign biological treatment technologies [10-11]. This combined process has a longer hydraulic retention time, lower organic load, so that the anoxic- aerobic unit can maintain a lower sludge content, greatly reducing the discharge of residual sludge, for the combined process to achieve sludge reduction.
Gao Dawen et al. used a combined ascending anaerobic sludge fixed bed (UAFB)-anoxic-aerobic membrane bioreaction (MRB) process to treat domestic wastewater, which not only achieved a removal rate of 93.3% and 90.6% of COD and NH3-N, but also was able to maintain a lower sludge content in the reactor for a long period of time, reducing the amount of residual sludge to be disposed of and mitigating the membrane contamination [12].
Zhou Jun et al. developed an improved combined johkasou by placing an anoxic tank at the front end of an anaerobic tank and adding a bed of microelectrolytic iron filings and composite biological materials. The johkasou adopts an anoxic-anaerobic- aerobic (A2/O) treatment process, which on the one hand effectively solves the problem of poor denitrification and nitrogen removal by denitrification due to the low content of organic matter in the wastewater and insufficient carbon source, and on the other hand, through the combination of the traditional activated sludge process, biological nitrification, denitrification and biological phosphorus removal process, it can be better synchronized with the nitrogen and phosphorus removal. The experimental results showed that the average removal rates of COD, TN and TP were 93.0%, 80.0% and 94.0% respectively in March when the HRT was 8h and the system reflux volume ratio was 75.0%; and 94.0%, 76.0% and 91.0% respectively in August [13].
Bai Xiaolong et al. also improved the small-scale johkasou by adopting a folded-flow anaerobic reactor-anaerobic biofilter-biological contact oxidation process, and domestic wastewater was fed by upflow effectively reducing equipment clogging and maintenance time[14].
2.3 Other Combination Processes
In addition to the common A/O, A2/O process, some other combination processes are also used in actual wastewater treatment.
Zhan Xu et al. 5-stage drop oxygenation biological contact oxidation method for rural sewage treatment, the raw water is pumped up through the 5-stage drop oxygenation, both to meet the required dissolved oxygen and exempted from the aeration equipment, reducing the investment costs and operating power consumption, so that management tends to be simple, the process of solving the relatively economically backward rural areas of the water pollution problem, has a more obvious benefit [15].
Shen Dongsheng et al. developed a kind of underground unpowered anaerobic standard treatment equipment (UndergroundUnpoweredAnaerobicReactor, UUAR), the device adopts the anaerobic sludge bed contact pool + anaerobic biofilter process, the process is simple, low energy consumption. Compared with aerobic biological treatment, the capital investment of UUAR technical equipment may be slightly higher than aerobic treatment, but there is no daily operation cost and no accumulation problem of residual anaerobic sludge, which is suitable for decentralized treatment of domestic wastewater in remote rural areas with land constraints, economic backwardness, and harsh natural climate [16].
Xu Gongdi et al. carried out aerobic pre-hanging membrane before the biological purification tank to form a combined O-A-O process, and this composite biological purification tank had a high removal of NH3-N and TN, which effectively reduced the loading of rural domestic wastewater with high uric acid content, and the removal rate of COD and phosphorus was 59.6% and 33.4% [17].
For the low carbon and nitrogen of domestic wastewater in rural areas of North China, Wu Di et al. used an autoflow anaerobic-3-stage aerobic-anoxic biofilm process to improve the nitrification capacity of the anaerobic section by using injected bioballs; at the same time, in the 3-stage aerobic-anoxic biofilm section, the nitrification and denitrification of phosphorus removal were realized in the same reactor through drop-water oxygenation, which effectively solved the problem of the supply capacity of the carbon source. The removal rates of COD, NH3-N, TN and TP in rural domestic wastewater by this process were 73.7%, 90.7%, 59.6% and
69.7% [18]. Subsequently, the 3-stage aerobic-anoxic biofilm technology was improved, and the new process added a return pump (return volume ratio of 2:1) and increased the loading rate of suspended filler in the anaerobic section, and the removal capacity of TN in the effluent after the improvement was improved to 63.9% [19].
3 Ecological combination technology to treat rural domestic wastewater
Ecological treatment technology is the use of soil-plant (animal)-microbial composite ecosystems, through the physical, chemical, and biological effects of the resources in the sewage to be utilized, and the pollutants in the sewage degradation and purification of the process [20]. Relative to biological treatment technology, ecological treatment technology is generally low construction and management costs, energy saving, has a certain landscape effect, and pays more attention to the value of ecological services. In China's vast rural areas, the current application of the implementation of ecological combination of treatment technologies include the combination of the same ecological technology and the combination between different ecological technologies.
3.1 Combination of the same ecological treatment technology
The composite vertical flow artificial wetland designed by Wu Zhenbin et al. connects the downward flow pond and the upward flow pond in series, and the bottom is connected, so that the wastewater enters into the wetland system in which the nitrification and denitrification are more adequate, and the system has a better effect on the removal of TN in the wastewater, and the removal rate is 43.6% [21].
For the low-content rural sewage in Dianchi area, Liu Chaoxiang et al. used two artificial composite ecological bed treatment processes, surface flow and submerged flow, and under the condition of high hydraulic loading (30cm/d), the removal rates of COD, TN, NH3-N and TP in submerged flow beds were 70.6%, 60.6%, 80.9%, and 66.0%, and in surface flow beds, they were. 63.1%, 61.2%, 90.2% and 60.2% [22]. Compared with the separate artificial wetland treatment technology, the combination of artificial wetlands improved the oxygen content and organic matter of the wetland, which improved the nitrification and the removal of pollutants from the wetland, and the effect of nitrogen removal was especially obvious.
The intake water of the tower-type composite artificial wetland (TICW) designed by Ye Fenxia et al. is divided into 2 segments, with a part of the effluent flowing through the lower intake to form a submerged flow artificial wetland, while the other part of the effluent flows down from the top of the tower to form a surface flow artificial wetland, which can provide sufficient carbon source for the denitrification of the wetland's latter segment [23].
Zhang Hongling et al. used a multistage soil percolation system to treat rural domestic wastewater in the Taihu Lake Basin, and the average removal rates of COD, NH3-N, TN, TP, and SS were 70.0%, 83.0%, 59.0%, 76.0%, and 94.0%, respectively [24]. Zheng Yanqiang et al. also achieved better results in the treatment of rural domestic wastewater by placing two underground percolation systems in parallel, with soil, ceramic granules, slag and two types of natural organic matter selected as the filling media [25].
Ji Zhumei et al. established an ecological pond by planting ecological plants on the water surface of a stabilization pond through floating bed technology, and this system has a high removal rate of COD, NH3-N, TN, and TP in high-content domestic wastewater, which can reach more than 55.0%, 70.0%, 80.0%, and 75.0%, respectively [26]. The tower earthworm eco-filter treatment system designed by Junshang Li et al. Each tower layer is a treatment unit with gradient tower layers and series stacked arrangement. The system has good effect on COD, ammonia nitrogen, TN and TP treatment, and low capital and operating costs, with a total operating cost of 0.671 yuan/m3. This technology has good application prospects in economically underdeveloped rural areas.
3.2 Combination of different ecological treatment technologies
Wang Xuehua et al. used ecological ponds as pretreatment and artificial wetland as subsequent treatment, and the removal rate of NH3-N, TN and TP in rural domestic wastewater in Sanshan Island, Taihu Lake, was as high as 95.0%-99.0%, 95.0%-98.0%, 92.0%-98.0%, and the SS content in influent was reduced, which effectively alleviate the clogging of the wetland system. The domestic wastewater through the tower earthworm eco-filter, the effluent load and pollutant concentration is reduced, and then into the horizontal submerged artificial wetland, further reducing the content of organic matter and nutrients, so that the effluent water quality is basically up to standard. This tower earthworm ecological filter + artificial wetland combination process, high degree of automation and easy to manage and operate, is more suitable for economically developed, densely populated rural areas to promote the use of.
Shi Jianwei et al. planted a combined system of plants in the form of biofloating beds on the water surface of high-efficiency algal ponds, which on the one hand significantly improved the stability of the system operation and the quality of the effluent water and saved land, and on the other hand, the roots of transplanted plants in the biofloating beds effectively inhibited the growth of algae by chemosensory effect, and at the same time, the plants themselves also had a certain effect of landscaping and beautification of the environment [30]. Gao Shengbing et al. used a combination of plant and soil percolation system to treat rural domestic wastewater, and the removal effect was good. The average removal rates of BOD5, COD, TN, NH3-N, and TP in domestic wastewater by this composite system were 73.5%, 76.0%, 85.0%, 89.0%, and 85.0%, respectively [31].
4 Biological + ecological combination technology to treat rural domestic wastewater
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, to give full play to their respective advantages, and to improve the effluent quality of the water and the stability of the system operation [32]. Compared with the biological combination technology and ecological combination technology, the biological + ecological combination technology needs to comprehensively consider the economic conditions of rural areas, the regional climate differences between the south and the north, as well as factors such as land use conditions, operation and management, sludge production and actual engineering cases. Since artificial wetland is one of the most commonly applied subsequent ecological treatment technologies, common biological + ecological combination technologies in rural areas of China mainly include biological + artificial wetland combination technologies and other biological + ecological combinations.
4.1 Biological + Artificial Wetland
The biological unit in the biological + artificial wetland combination system can effectively complete the degradation and nitrification of organic matter; at the same time, the artificial wetland system can further remove the pollutants such as nitrogen and phosphorus. The combined application of the 2 can improve the removal rate of various types of pollutants in sewage. Tang Jing et al. used a combined contact oxidation-artificial wetland process to treat rural domestic wastewater, and the removal rates of COD, NH3-N, TN and TP were 68.2%, 68.2%, 69.5% and 86.3%, respectively, with stable results. Among them, the falling water oxygenated contact oxidation tank contributed more to the removal of COD, while the artificial wetland contributed more to the removal of TN and TP [33].
In the rural area of Taihu Lake Basin, Bai Yonggang et al. used a combined process of drop-filter pond-artificial wetland to treat domestic wastewater, and the results showed that the removal rates of COD, NH3-N, TN, and TP in drop-filter ponds were 74.5%, 79.2%, 33.8%, and 47.5%, while those of the artificial wetland were 25.0%, 20.8%, 66.2%, and 52.5%, respectively [34 ].
Ren Shanshan et al. selected the septic tank-artificial submerged wetland process and Jiang Lalan et al. used the MBR-artificial wetland combination technology in the rural areas around Lake Taihu, which were also applied and achieved better removal results [35-36].
Aiming at the mild climate and high temperature in winter in Yunnan region, which is suitable for plant growth throughout the year, Li Wenqing used the modified A2/O a combination of artificial submerged wetland treatment system, and at the same time, a substrate with high adsorption of phosphorus was added to the artificial wetland. The average removal rate of COD, BOD5, TN, NH3-N and SS in the wastewater of this system reached 80.5%, 84.3%, 91.8%, 93.2% and 86.4% respectively, and the removal rate of TP was always kept above 75.7%.
Hetou Village, Xingan County, Ji'an City, Jiangxi Province, used solar-powered biofilter tower-artificial wetland combination process to treat domestic wastewater, which is powered by solar energy and oxygenated by jet injector, with good treatment effect and low operating cost, simple operation, unattended operation, which is suitable for popularizing and applying in the economically backward and remote rural areas [38].
Yu Hao et al. used "hydrolysis pool - drip filtration - artificial wetland" three process combinations to treat rural domestic wastewater, at the same time, perlite slag, ceramic and gypsum, etc., placed in layers of drip filtration pool, the hydraulic load can be up to 4.0-8.0m3/(m2˙d), the combination of processes for COD, TN and TP removal rate are more than 90.0% [40]. were more than 90.0% [39]. Zhong Qushuang et al. chose the anaerobic-contact oxidation canal-vertical submersible artificial wetland combination process to treat rural domestic wastewater, in which the contact oxidation canal serves to oxygenate the subsequent artificial wetland; whereas the anaerobic tank, the contact oxidation canal and the artificial wetland all have high removal rates of COD, NH3-N and TP, but the anaerobic tank removal rate is more stable, with the highest rates of 72.0%, 49.5% and 66.4% [40 ].
4.2 Other biological + ecological
The submerged biofilm-stabilized pond combination technology designed by Wu Zhaofu et al. eliminated the secondary sedimentation tank and sludge return, and the average removal rates of COD, NH3-N, TN, and TP in this system were 84.3%, 97.0%, 80.2%, and 77.3%, respectively, and the sludge content of effluent from biofilm system was basically 0, and the subsequent treatment by the The effluent water quality after stabilization pond treatment is good and meets the reuse standard [41]. Zhang Zengsheng et al. adopted the biological purification tank-enhanced ecological floating bed (BPT-EEFR) combination treatment process for rural domestic wastewater in Chongming Island, and the results showed that the combination technology not only had a better removal effect on COD, NH3-N, TN, TP and SS, but also had a small footprint, low cost and easy maintenance.
The ecological combined system consisting of biofloating bed, biological contact oxidation and river ecosystem also has good removal effect on NH3-N, TN, TP and COD in rural domestic wastewater. However, the removal of pollutants by this combined system was affected by the season, and the removal of pollutants was higher in summer compared to fall, and the removal of pollutants increased with the increase of HRT, while the change stabilized after 4 d [43].
5 Conclusion and Prospect
China is a vast country, and the selection of domestic wastewater treatment technology in rural areas should be adapted to local conditions, taking into account the local topography, hydrological and climatic conditions, and the level of economic development, as well as the characteristics of the various wastewater treatment technologies and the scope of application. Practical application through scientific design, optimization of combinations, to achieve technical complementarity, the development of a high hydraulic load and small-scale equipment features, easy to operate and manage a variety of treatment units of the composite innovative technology. Rural sewage treatment in the ascendant, a long way to go, the organic combination of scientific and social factors can greatly improve the effectiveness of rural water environment management, therefore, in addition to technological research and development, the following aspects should be strengthened:
(1) play the role of courtyard unit treatment. It is recommended that each household build septic tanks for preliminary treatment of toilet sewage, bathing sewage and catering sewage.
(2) Strengthen the research on the operation and management technology of sewage treatment processes or facilities, and further develop easy-to-operate, less-maintenance, low-cost operation modes and processes suitable for rural areas.
(3) Rural sewage treatment can be reused for resources. As rural domestic sewage is mainly N, P and other nutrients, after treatment can be used for agricultural irrigation in the vicinity.
(4) Raise farmers' awareness of ecological and environmental protection. Actively carry out the promotion of the use of biological bacterial fertilizer, organic fertilizer publicity and education activities, to guide farmers to reduce the use of chemical fertilizers.
(5) Gradually establish corresponding laws and regulations. According to China's rural reality, reference to relevant environmental protection standards, the development of relevant technical standards and regulations suitable for China, at the same time, the development of relevant policies and regulations, improve the reward and punishment policy, the governance of rural water pollution into the legalized track.
More about the engineering/service/procurement category of the tender writing production, enhance the winning rate, you can click on the bottom of the official website customer service free advice:/#/?source=bdzd