Study On The Purification Of Micro-polluted River Water By Gravel Contact Oxidation-horizontal Subsurface Flow Constructed Wetland

In recent years,China’s water environment treatment has achieved initial results,but it has not achieved the goal of sustainable and ecological treatment.Some rivers are still polluted,and the water quality can not meet the required surface water quality standards,and become a typical micro polluted river.Aiming at the problem of micro-polluted river course,this paper proposed the compound technology of gravel contact oxidation-horizontal subsurface flow constructed wetland,and studied the water purification effect of the compound technology.After stable operation,the effects of HRT,the inflow water distribution ratio in the gravel contact oxidation aeration area and non aeration area（gravel O/A area）and the aeration intensity in gravel O area on the purification effect of gravel contact oxidation and post horizontal subsurface flow constructed wetland were discussed,and the purification effect of composite process under the condition of low pollution and low carbon nitrogen ratio was studied.During the start-up period,it can be seen that there was obvious microbial film on the surface of gravel packing,and the removal rates of COD and NH₄⁺-N in the gravel contact oxidation（referred to as gravel area）were all stable at about 75%,and the removal rates of TN were about 50%.After the success of film hanging,it can be seen that the removal efficiency of COD and NH₄⁺-N was the highest in gravel O area,while TN was the highest in gravel a area.The removal efficiency of COD and NH₄⁺-N in horizontal subsurface flow constructed wetland was better than that of TN.The operation parameters optimization experiment showed that when the HRT of gravel area was5h,the removal efficiency of pollutants in gravel area was better,and the average removal efficiency of COD,NH₄⁺-N and TN can reach 71.8%,75.3%and 67.8%respectively;when the inflow water distribution ratio in gravel O/A area was 1:1,the average removal efficiency of COD and NH₄⁺-N was higher.At 1:2,the TN removal rate of composite process was the highest,reached 78.9%,and the horizontal subsurface flow constructed wetland with post series connection can make up for the problem of low treatment effect of NH₄⁺-N caused by subsection inflow,and the removal rate of NH₄⁺-N of composite process can still reach more than 80%;When the DO concentration of gravel O area was 5-6mg/L,the composite process had good treatment effect on all pollutants.When the influent concentration was slightly higher than the class V water in the environmental quality standard for surface water（GB 3838-2002）,most of the treated effluent can reach the class II standard.The better parameter combination was:HRT in gravel area was 5 h,the proportion of subsection water inflow in gravel O/A area was 1:1,and do concentration in gravel O area was 5-6 mg/L.In the micro-polluted river with low pollution and carbon nitrogen ratio,the composite process can still achieve better treatment effect,and the removal rates of COD,NH₄⁺-N and TN can reach 70%,80%and 50%respectively.Based on the above experimental results,the water purification engineering design of the small watershed outside the Xikou of Miyun reservoir was carried out,which mainly includes the process design,the calculation of the size of each structure,and the estimation of the project investment,in order to provide the basis for different micro-polluted river treatment projects.