Study On Runoff Pollution Control By A Constructed Ecological Ditch

Non-point source pollution is one of the reasons that cause the water quality deterioration of Shahe Reservoir in Beijing.It was observed that the rain water may collect in the ditches and then drain into the reservoir which leaded to reservoir pollution.Therefore,it is important to identify the ditch runoff characteristic and make effective control strategies to improve the reservoir water quality.At first,runoff from two typical ditches(roadside ditch,village side ditch)were collected for water quality monitoring.The results showed that the pollutants concentration of runoff in roadside ditches was higher than that in roadside ditch,which exceeded the Class V limit of "Environmental Quality Standard of Surface Water".Compared with that in natural rainfall,the particulate pollutants in the ditch runoff accounted for a relatively higher proportion.The particulate COD,particulate phosphorus and particulate nitrogen accounted for 50-55%,65%-85%and 44-51%of the total COD,total phosphorus and total nitrogen respectively.Heavy metals,such as Cr6+,Fe3+,and Cu2+,were not detected in the ditch runoff.Further analysis of dissolved organic matter(DOM)in the ditch runoff showed that the DOM with molecular weight<1kDa accounted for the highest proportion,mainly composed of fulvic acid-like substances.Then,ecological ditches with substrate units were designed to enhance the water purification efficiency.The canna lilly and calamus were chosen to plant in the ditches with the planting ratio of 1:1.Two substrate units were developed.The phosphorous removal substrate unit was composed of slag and zeolite with the ratio of 2:1,and the nitrogen removal substrate unit was composed of zeolite.According to the position of the substrates unit in the ditch,the ecological ditches can be classified to regular ecological ditches(no substrate units),"surface-enhanced" ecological ditches(substrate units on the planting soil)and "sunken-enhanced" ecological ditches(sunken substrate units in the planting soil).The pollutants removal efficiencies by three ditches were further investigated in hydrostatic and dynamic conditions.Experimental results showed that,in hydrostatic conditions,the performance of the"surface-enhanced" ecological ditches were better than other ditches.When the hydraulic retention time was 96 h,the removal efficiency of total nitrogen and total phosphorus were 72.09%and 75.83%,respectively.And the removal efficiency of particulate nitrogen and phosphorus reached to 91.77%and 78.5%,respectively,higher than that of regular plant ditches with 12%and 18.5%,respectively.While in the dynamic tests,surface type ditches on the removal rate of TN,TP and SS were 11.00%,19.17%,81.35%,,respectively,higher than that of regular plant ditches with 6.89%,6.87%,28.25%,respectively.And the pollutants concentration further reduced due to the interception and adsorption by substrates.At last,the ecological ditch renovation project plan,which was located in the north bank of Shahe Reservoir was put forward.The surface substrate units were applied in this plan.Substrate units were placed in the ditch every 25m.There were 98 substrate units in total with an alternating placement of phosphorus removal substrate units and nitrogen removal substrate units.The total project investment was 330,500 yuan.