The water quality in China’s rural areas is continually deteriorating, with the social and economic development and the transformation of life and production mode. It is showed in survey that rural sewage is one of the main sources of water environment, a potential threat to water resources security, restricting the sustainable development of the rural society and economy. Thus, the sustainable rural sewage treatment technology research with high efficiency, low consumption has become one of the important research directions in the field of water pollution control.
According to the rural water pollution characteristics and the current situation of the rural areas, in the aim of energy conservation, emissions reduction and resource utilization, a new process of rural sewage treatment was developed combined with large depth-diameter ratio anaerobic reactors, anoxic reactors, dripping aeration reactors and artificial wetland, which is called biological and ecological wastewater treatment system. The combined system is highly efficient to remove pollutions as anaerobic-anoxic-aerobic biological treatment technology, and also easy to manage and maintain as artificial wetland ecological process. In this thesis, the pollution removal efficiency of the combined system was studied, in the aspects as followed: 1.The process optimization and operation characteristics analysis on different temperature of large depth-diameter ratio anaerobic reactors; 2.The process optimization of denitrification deodorization in anoxic reactors; 3. The pollution removal efficiency and biological membrane characteristics analysis in dripping aeration reactors; 4.The research on nitrogen and phosphorus resource utilization and pollution removal mechanism and the operation mode optimization in the aquatic vegetables type artificial wetland. Based on the research above, a combined system process model is also established, and verified in practical treatment. The main research results are briefly as follows:
1. The effects of temperature and hydraulic retention time (HRT) on organic pollution removal efficiency, gas production and effluent odor threshold were researched in the large depth-diameter ratio anaerobic reactor. The gas production rate is proportional to the temperature; HRT and organic matter removal rate have significantly positive correlation, and with the extension of HRT, the gas production rate decreased, odor threshold also lowered. When the water temperature was 15℃~25℃, the economic HRT was 72 h, organics removal efficiency reached 79.7%; When the temperature was 8℃~15℃, the economic HRT was 144 h, organics removal efficiency reached 66.5%. As for the sewage with high carbon concentration, organic matter can be converted to biogas by anaerobic fermentation, simultaneously realizing the recycling use of organic matter; as for sewage with low carbon concentration, the removal of organic matter should be first priority.
2. The effects of temperature, reflux ratio and HRT on denitrification deodorization were researched in the anoxic regulation reactor hung with elastic filler. With the increase of reflux ratio, the value of total nitrogen, hydrogen sulfide, Volatile fatty acid (Volatile fatty acid, VFA) and odor threshold which was in the anoxic period has dropped significantly. When the water temperature was 15℃~25℃, the reflux ratio was regulated to 200%, the removal rate of total nitrogen, hydrogen sulfide, VFA and threshold value can be obtained as 52.3%, 82.6%, 83.6% and 82.6% respectively; When temperature was 8℃~15℃, the reflux ratio was required to increase to 400% in order to effectively remove odor, the removal rate of total nitrogen, hydrogen sulfide, VFA and odor threshold can be obtained as 30.1%, 64.1%, 72.2% and 64.1% respectively.
3. The design of the dripping aeration devices was optimized: an aerobic reactor with five steps plunge pool which each step was 0.4m depth were set up, filled with non-woven fabric; a dripping baffle with horizontal spacing of 5cm was used, and the first and secondary reactor was devised with two baffles each, with the vertical interval of 25cm, and other reactors with one baffle. With the improved baffles, when the HRT was 2h, the removal rate of COD, NH4+-N reached 51.4% and 54.1% respectively. Under the test condition, when COD and NH4+-N volume load was less than 1.5 kgCOD/(m3?d), 0.2 kgNH4+-N/(m3?d) respectively, the reactor have reached to a better treatment effect in system.
4. The most suitable hydraulic load for the aquatic vegetables type artificial wetland is 0.2 m3/(m2?d), with the planting depth of 10 cm, and the TN removal load can reach maximum of 1.87 g/(m2?d); Using the harvest method of 1/4 vegetable, the effective nutrient removal and higher vegetable production can be ensured. The nitrogen phosphorus removal pathways in aquatic vegetables type constructed wetland were analyzed. The absorption of water spinach and microbial degradation are the main ways of TN removal, the rates of which were between 47.68~59.57% and 39.43~51.34%; The absorption of water spinach is the main approach of TP removal, the rates of which were between 58.4~77.5%. The contribution of plants in the aquatic vegetables type artificial wetland to remove phosphorus is much higher than those in normal artificial wetland.
5. In the Subsurface flow constructed wetland(SFCW), the wetland with cinder as substrate had although a low removal rate under a low hydraulic load (i.e. 0.1 m3/(m2?d)), but as the hydraulic load increased to 0.2 m3/(m2?d), the removal rate remained stable around 50%, which showed its stronger ability to resist shock; However, the wetland with rubble, gravel, fine sand as substrate had obvious removal of the pollutants only at low hydraulic load, and when hydraulic load increased from 0.1 m3/(m2?d) to 0.2 m3/(m2?d), the removal rate was gradually reduced from 70.2% to 50.9%.
6. The aquatic vegetables type artificial wetland and SFCW were combined to examine the influence of the tandem sequence and the hydraulic load to tail water pollutants removal. When the aquatic vegetable type artificial wetland was in the former, and the hydraulic load was 0.3 m3/(m2?d), the pollutant removal load is higher, e.g. COD, TN, TP, respectively was 8.41 g/(m2?d) and 2.6 g/(m2?d) and 0.18 g/(m2?d), having reached Standard A of the first class; When the SFCW was in the former, the optimum hydraulic load is 0.3 m3/(m2?d), the COD, TN and TP removal load respectively was 7.37m3/(m2?d), 1.96m3/(m2?d) and 0.14m3/(m2?d), having barely reached Standard B of the first class. It is recommended that SFCW is placed at the rear, in order to guarantee the stable effluent standard.
7. The food security, agricultural vegetable pesticide residues and heavy metals of vegetables (water spinach and water celery) in the different combination wetland system were analyzed, according to "Monitoring directory of 55 Pollution-free agricultural products" (The Ministry of Agriculture 17th file in 2013) and "the Contamination Limitations in Food" (GB 2762-2012), which were not beyond the regulations. Therefore, the utilization of aquatic vegetable type artificial wetland after biological treatment can realize nitrogen and phosphorus resource recycle, build pollution purification type agriculture, combine sewage disposal with agricultural production.
8. Rural domestic sewage pollution removal models with biological-ecological treatment system were built based on material conservation, and the models were also coupled. The results of process model on pollutants removal of simulation were similar with the practical measured values, with the deviation maintained at 5%~25%. Each element model and coupling model can be used to predict combination system of whole flow operation effect and operation condition adjustment.
In this thesis, the research results showed that “anaerobic-anoxic-dripping aerobic-artificial wetland” combination system can be used to effectively treat the rural domestic sewage, and realize the organic matter, nitrogen and phosphorus recycling use in sewage, in order to construct a sustainable rural sewage treatment system with high efficiency and low consumption, improving the rural water environment quality.