Optimization Of Water Purification Process In Aerobic/anaerobic Subsurface Flow Constructed Wetlands

With the rapid urbanization in China, more and more domestic sewage are discharged directly into watercourses, lakes, rivers or sea in rural areas. Water pollution caused by high COD and N is largely evident. Rural sewage with high dispersion, high pollution, big changes, away from the municipal network and large-scale wastewater treatment outfall, generally discharged without any handle into the surrounding rivers and lakes, causing serious environmental pollutions. Rural sewage pollution has become an important factor in natural water. Small water purification process for rural domestic sewage characteristics is an urgent demand. Constructed wetlands purification systems is simple, cheap and has other advantages, in particular, suiting to China's situation of towns'sewage treatment. It is the most appropriate rural sewage treatment.The removing of nitrogen in constructed wetlands mainly depends on ammonification, nitrification and denitrification. Ammonification and nitrification are occured in aerobic environments, while denitrification is in anaerobic conditions. Duing to its structural limitations, conventional subsurface flow constructed wetlands (SFCWs) can not provide aerobic and anaerobic environment at the same time. TN removal of a single type of constructed wetlands is often not high, generally between 40% and 55%. To fundamentally solve the problem, the design must ensure adequate dissolved oxygen in water to complete the nitrification of NH4+-N, also a optimum anaerobic environment and adequate organic sources. The purpose of this study is through the optimization of dissolved oxygen distribution in conventional SFCWs, improving the quantity and activity of aerobic and anaerobic microorganisms, improving its sewage water purification efficiency.In this study, the traditional SFCWs were divided into different sections to optimize the structure and enhance its function. Different kinds of aerobic/anaerobic SFCWs were designed. The influences of ratio and location of aerobic/anaerobic, artificial aeration and other factors on removal efficiency were studied. The purification efficiency of the water in this study was compared with that in traditional SFCWs. The results showed that when HRT was 5d, influent average concentration of COD, NH4+-N and TN were 184.55,21.72 and 22 mg·L-1, the removal rates of the conventional SFCWs were 70.3%,18.4% and 40.6%, while aerobic-anaerobic-aerobic SFCWs with aeration (O-A-0 SFCWs with aeration) were 90.1%,99.7% and 50.7%. O-B-A-A-O with aeration and added water treatment were 91.6%,100% and 87.7%. It showed that in the experimental conditions, O-B-A-A-O with aeration and added water treatment had the highest removal efficiency, which could purify the demand for optimization.The water quality of the same structure SFCWs had significant improvement after aeration. Aeration could improve the COD and NH4+-N removal efficiency. In addition, the structure of SFCWs and location of the aeration section could significantly affect COD and NH4+-N removal efficiencies. The dissolved oxygen and carbon sources are important factors affected denitrification. Adding water could increase removal rate of TN remarkablely.