Diagnosis Of Problems In The Treatment Of Rural Domestic Sewage In Constructed Wetlands And Research On Enhanced Nitrogen And Phosphorus Removal Technology

Rural domestic sewage treatment has been rapidly developed in China in recent years,where constructed wetlands are one of the widely applied treatment units due to the low initial investment and simple operation&maintenance.However,investigations showed that there were many problems when constructed wetlands were applied to rural sewage treatment in scale,especially that the nitrogen and phosphorus removal was not as good as expected.This study extensively investigated the engineering application effect of constructed wetlands for treating rural domestic sewage in Haining City,and carried out diagnosis and analysis on the problems.On this basis,lab experiments were carried out for strengthening the nitrogen and phosphorus removal in a constructed wetland.The main conclusions are as follows:1.Seventy four A²O+constructed wetland facilities for rural domestic sewage treatment were randomly selected in Haining City,and their influent and effluent water quality was determined.It was found that the influent concentration was greatly affected by rainfall,and there was rainwater infiltration in the pipe network.The average influent COD concentration in the constructed wetlands was below 100 mg/L,with a very low carbon nitrogen ratio?COD/TN of only 1.4².4?,indicating the difficulty in biological nitrogen and phosphorus removal.2.29 of A²O+Horizontal flow constructed wetlands?HFCWs?and 50 A²O+Vertical flow constructed wetlands?VFCWs?were comparatively studied the pollutant removal performance,and the design and operation problems were diagnosed.The results showed that:?1?The principal unit for pollutant removal was of the VFCWs unit in the A²O+VFCWs process while the A²O unit in theA²O+HFCWs process.?2?The average removal rate of TN and TP in the A²O pretreatment unit of A²O+VFCWs was?16.0±18.5?%and?21.5±26.9?%in winter,and?10.9±14.3?%and?15.6±12.0?%in summer.The average removal rate of TN and TP in the A²O pretreatment unit of A²O+HFCWs was?31.3±24.1?%and?33.6±15.7?%in winter,and?23.8±14.9?%and?25±10.2?%in summer.The removal effect of A²O unit in both combined processes was not as good as expected,as might be attributed to the low carbon and nitrogen ratio in the influent as well as the low residual sludge discharge.The A²O+HFCWs removal rate of each pollutant was significantly lower than that of A²O+HFCWs,mainly attributed to the too small effective volume in the A²O and the insufficient dissolved oxygen in the wetlands.?3?The average removal rate of COD,NH₃-N,TN and TP in the VFCWs unit of A²O+VFCWs was?58.8±25.4?%,?61.4±24.4?%,?22.7±8.5?%and?27.4±21.2?%in winter,being 16.0%,36.9%,1.3%and 9.5%higher than that of HFCWs.The corresponding pollutant removal rate was?59.9±25.0?%,?71.6±26.5?%,?38.3±32.8?%and?39.2±32.9?%in summer,being 28.8%,52.6%,10.5%and 5.0%higher than HFCWs.The better pollutant removal performance in the VFCWs could be mainly attributed to its lower hydraulic load and higher DO concentration.?4?In addition,operation&maintenance affected the wetland performance.The uneven influent,the unreasonable design of the outlet and the long-time un-refurbishment of the filler caused blockage in the wetland as well as the decrease in the pollutant removal rate.Moreover,plant harvesting might cause temporary decrease in nitrogen and phosphorus removal,from the fact that the average removal rates of NH₃-N,TN and TP were 81.2%,45.3%,39.7%and 78.9%,respectively,before plant harvesting,while decreased to 72.2%,39.7%,32.3%and 63.5%,respectively,after plant harvesting.3.Experimental study was carried out for strengthening the denitrification and dephosphorization performance in constructed wetlands as influenced by low carbon/nitrogen ratio and insufficient internal DO.The findings are as follows:?1?Waste coconut shell as a slow-release carbon source was found to be effective for enhancing the denitrification in a constructed wetland.By replacing 25%of the shale in the wetland matrix in the control group with discarded coconut shells,the average TN effluent concentration in the test group was reduced by 6.5 mg/L,and the average TN removal rate was increased by 13.7%.In addition,the removal efficiency of ammonia nitrogen in the test group was found to be improved up to 30%when the outlet height was lowered?indirect namely DO increased?,the pollutant loading rate was decreased,or the influent was intermittently injected.?2?Both iron carbon filler and broken shell filler were found be effective for enhancing the phosphorus removal in a constructed wetland.The iron carbon filler group performed better.Under the same condition,the average TP removal was 70.1%in the iron carbon filler group and 61.8%in the broken shell filler group.The total phosphorus removal was further increased by 8.5%¹0.9%in both reactors as the DO concentration in the wetland was increased.Static adsorption experiments showed that the adsorption effects of crushed shell matrix and iron carbon filler on TP were in accordance with the quasi-first-order kinetic model,quasi-second-order kinetic model and?R²=0.97?.