Study On Combination Of Ecological Ditch And Bioretention Pool And Its Application

As an ecological measure,ecological ditch is widely used in the control of rural non-point source pollution.However,under the strong rainfall intensity,its low pollutants removal leads to the bad impact of the effluent on the water environment.To solve this problem,a two-stage treatment system,namely ecological ditch-bioretention pond combined system,was established to enhance the removal of non-point source pollution.Carbon deficiency is often an important factor limiting microbial nitrogen removal,so alkali modified straw and sawdust were added to the bioretion ponds as an external carbon source.The effects of external carbon sources,HRT and drought days on the removal of organic compounds,nitrogen and phosphorus in the combined system were discussed.Based on the results of laboratory research,the demonstration project of ecological ditch and bioretention pond combined system was built in Jian Dong village,Lishui,Nanjing.The main results are as follows.?1?After alkali treatment,the rate of carbon release and specific surface area of rice straw and sawdust were both increased.The average carbon release rates of the first 5 days increased from 4.77,1.29mg/?g·d?to 10.22 and 1.65 mg/?g·d?,and the specific surface area increased from 1.08,0.52m²/g to 1.85and 0.85m²/g,respectively.The removal efficiency of TN with carbon source addition were increased by18.55%and 21.69%compared with the control group,while the removal rates of COD and NH₄⁺-N were lower than that of the control group.The removal rate of TP in different carbon source systems was not significant,indicating that the addition of carbon source had little effect on the removal of TP.?2?The hydraulic retention time could significantly affect the removal of the pollutants.The longer the HRT was,the better the pollutant removal effect was.The influence of HRT on the different parts of the combined system was analyzed,and it was found that when the hydraulic retention time is 10h and 5h,the pollutants removal were mainly occurred in ecological ditch.When the hydraulic retention time was changed to 2h,the contribution of bioretention ponds to the pollutants removal was obviously increased,and the removal rate of TN and TP was higher than that of the ecological ditch.?3?The rule of the removal of pollutants after different periods of drought was similar between the control group,the rice straw group and the sawdust group.That is,the removal rate of the pollutants increased first and then decreased after 2 days of drought.After 1 and 3 weeks of drought,the removal rate of pollutants decreased first and then increased.The removal rates of pollutants in the three systems were all improved after the dry and wet alternate operation.?4?The results of high throughput sequencing of different packing layers of ecological ditches and bioretention ponds showed that the Alpha diversity of different packing layers in bioretetention ponds was higher than that in ditches,and the Alpha diversity of the zeolite layer of straw and sawdust group was higher than that of control group.In the three groups,Thiobacillus and Pseudomonas with denitrification were detected,and Nitrospira with nitrification were detected in the ditches.The results were consistent with the contribution of TN and NH₄⁺-N removal in each segment of the system under the hydraulic retention time of 2h.?5?The demonstration project has a good removal effect on pollutants in three random samples.The removal rates of COD,ammonia nitrogen,total nitrogen and total phosphorus were73.0±3.0%,78.5±2.0%,75.0±1.8%and74.8±0.6%,respectively,and the average reduction amounts of pollutants were 53.6g,6.06g,8.80g and 1.17g,respectively.