| The water quality requirements become more and more rigorous after the construction of the Three-Gorges Project. With the investment of environmental protection measures by nation, point source pollution is controlled gradually in the Three Gorges reservoir area, the key is transferred to disperse and wide rural water pollution. The rural domestic sewage, especially dispersion-type wastewater, is important part of non-point source pollution, it must be treat in stages.Because of complicated topography, rolling terrain and economic restriction in the Three Gorges reservoir area, we must follow the principles of low cost, stable effect and simple operation to choose sewage treatment technology which is suitable to typical village in this area. This paper have recommended the anaerobic biological contact pool-water drop reaeration and the anaerobic biological contact pool-water drop reaeration-surface/subsurface flow constructed wetland processes (hereinafter short for process A, process B). Firstly, this paper discussed the factors and optimum conditions of water drop reaeration. On these bases, the pilot scale test device was designed to study wastewater treatment efficiencies of two processes. Finally, we chose typical village in the Three Gorges reservoir area to build a project of process A and process B to correct parameter further. We monitored the operation effect continuously in next8months to study operation effects of two processes.Firstly, water drop height, water drop depth, water drop width, flow were chose to be the test factors, on these bases, we carried on orthogonal test to investigate the effects of water drop reaeration for test condition optimization. We could find that the optimum conditions are0.6m in water drop height,0.35m in water drop depth,0.15m in water drop width,0.7L/min in flow. The higher of water drop height, the better of effect of reaeration in some Fields. It is complex that the effects of water drop depth and flow to reaeration. The relationship between water drop width and reaeration is a second order relationship. The ranking of importance of this four factors that affect water drop reaeration is flow, water drop depth, water drop height, water drop width. It is obvious that we must fully investigate discharge condition of domestic sewage before designing so that we can improve removal rate of pollution by designing proper flow.The research of pilot scale test found that CODcr removal rate was comparatively stable in different inflows. This system had a good resistance to shock-loading for CODcr, CODcr removal rate of anaerobic phase was43%-55%, and average removal rate of water drop reaeration water outlet was52.33%and constructed wetland water outlet was82.56%. Anaerobic phase contributed most to CODcr removal and constructed wetland had an invigorating effect. Nitrification is the main way to remove NH3-N. As the increase of flow rate, the NH3-N removal rate decreased. The removal rates were29.06%,61.81%,69.70%in anaerobic phase process, water drop reaeration water outlet and constructed wetland water outlet. Water drop reaeration contributed most to NH3-N remove. TP removal rate of water drop reaeration water outlet was only34.35%at most, and DO had a decisive influence on it. Constructed wetland contributed most to TP removal, but its rate was only52.92%at most, which means the removal efficiency of TP was not very good. As the increase of flow rate, the TN removal rate decreased, and the average removal rate was66.87%, so that the effect was good. We could find that all the NH3-N, TP, TN removal efficiencies of process A and process B were good. Above800L/d in flow, the average effluent concentration of CODcr of process A could not reach the secondary standard of discharge standard of pollutants for municipal wastewater treatment plant(hereinafter referred to as standard).So we must use process B in the areas where domestic sewage is abundant for reaching the secondary standard.Through sampling analysis of verified wastewater treatment project, we could find that influent and effluent pH of domestic sewage fluctuated in normal range between6and9, but effluent pH was slightly higher than influent in process A and process B. Effluent concentrations of CODcr of process A and process B were comparatively stable. The two kinds of processes had good resistance to shock-loading for CODcr, and whose CODcr removal effects were good too. After the systems became stable, SS removal effects were good in both processes, but they may had a correlation with external environment and air temperature. Influent concentration of NH3-N of process A was also reached the secondary standard, and all its effluent concentration was lower than the B standard of primary standard or even the A standard, indicating that the removal efficiency of NH3-N in process B was stable and good, and its NH3-N removal rate was little affected by the air temperature. Both process A and B had a good effect on TP removal and had a good resistance to shock-loading. Effluent concentrations of TP of process A and process B could reach the secondary standard. Their TN removal effect was good while the air temperature had little effect on it. |
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