Study On Rainwater Runoff Regulation Performance Of Excess Sludge Substrate Bioretention System

The rapid development of urbanization caused urban waterlogging to be increasingly serious,and the construction of sponge city was proposed.Bioretention system was one of the common sponge single technical measures,which had been widely used at home and abroad The safe disposal of waste sludge in wastewater treatment plants was the main bottleneck restricting the sustainable development of the water industry.In order to further expand the disposal way of excess sludge,the excess sludge was used to improve the substrate of bioretention systems,and the physical and chemical properties of the substrate under different sludge mixing ratios were measured.The effects of different sludge mixing ratio,filling depth,return period and pollution load on rainwater runoff regulation and water purification of excess sludge substrate bioretention system were investigated.In order to screen the range of sludge mixing ratio,the physical and chemical properties of the excess sludge substrate with different sludge mixing ratio were determined.The results showed that the permeability of sand substrate was improved when the proportion of mud mixing was less than 52.8%,and the permeability was the highest when the proportion of mud mixing was 40%,which was 159.9 mm/h.When the proportion of mud mixing was less than 44.8%,the organic matter content was less than 5%,which could effectively prevent the leaching of nutrients.When the mud-mixing ratio was 9.8%-48.8%,the p H of the substrate was in a weak alkaline environment,which had a good effect on removing pollutants.When the proportion of mud mixed was less than 48.5%,the EC was less than 570μS/cm,and the risk of salinization was low.Therefore,the mud mixing ratio≤44.8%was more suitable as the substrate of bioretention systems,and the mud mixing ratio of 0%-40%was finally selected as the follow-up experimental research.Investigated the inlet water with constant flow rate and non-constant flow rate（single-peak typeⅠ）,the effects of the excess sludge substrate bioretention system on rainwater runoff regulation under different sludge mixing ratio,filling depth and return period were investigated.The results showed that the reduction effect of 40%mud-mixing ratio,15 cm filling depth and P=0.5 a was better,when the inlet flow was constant,reaching 73.61%,84.73%and 100%,respectively.On the contrary,the peak runoff reduction rate enhanced from 53.12%to 67.45%with the increase of the proportion of mud.When substrate filling depth was 15 cm,the control effect of runoff peak improved to 60.16%.When the return period increased from 0.5 a to 5 a,the peak runoff reduction rate decreased by 7.29%.It was found that the regulation effect of stormwater runoff was better,when the inlet flow was constant.When the water enters at a constant flow rate,the removal effect of runoff pollutants by the excess sludge substrate bioretention system under different sludge mixing ratio,filling depth and pollution load was studied.The results showed that the removal rates of COD,NH₄⁺-N,NO₃^--N,TN and TP are 84.54%,87.78%,86.45%,87.46%and 99.15%,respectively,when the proportion of sludge was 10%.When the substrate was filled with 5 cm,the removal rates of COD,NH₄⁺-N,NO₃^--N,TN and TP were 87.93%,88.63%,88.43%,87.87%and 98.70%,respectively,which were better than those when the substrate was filled with 15 cm.Under high pollution load,the removal rates of COD,NH₄⁺-N and TP were 77.47%,59.60%and 98.05%,respectively.While under low pollution load,NO₃^--N and TN were 72.05%and 58.53%,respectively.