Research On The Cause And Control Of Blockage In Bioretention System

Bioretention system is an important facility for the management of sponge city rainfall-flood,but the problem of matrix blockage will inevitably occur in the long-term operation of the system,which will significantly reduce the operating effect of the bioretention system.Therefore,it is of great practical significance to study the factors affecting the blockage of the bioretention system.In this paper,the cause and process of blockage in bioretention system are analyzed by combining laboratory test and demonstration project.In the laboratory study,five groups of bioretention systems were designed,and the effects of carbon source,plant,microorganism and dry-wet alternating on the blockage process of bioretention systems were investigated by measuring the suspended matter(SS)cumulants,permeability coefficient and void fraction of each system.A demonstration project of bioretention system was built in Jiandong Village,Lishui,Nanjing,to investigate the permeability and purifying effect of the system for a long time.The main research results are as follows:In the bioretention system,SS was mainly trapped by the system surface media,resulting in a decrease in the permeability coefficient and voids of the system surface media.The media layer with a depth less than 25 cm retained more than 85% of SS and other obstructions,and the SS accumulation decreased with the increase of depth.The addition of carbon source,the presence or absence of plants and microorganisms in the system had no significant effect on SS accumulation.SS cumulants were related only to substrate depth.The system with added carbon source(sawdust)had higher permeability coefficient,which can effectively alleviate media blockage,but had less impact on system porosity.Compared with the three plants of Juncus,Canna and Calamus,it was found that the plant species significantly affected the permeability coefficient and voids of the upper media of the system,but had little effect on the permeability coefficient and voids of the lower media of the system.The Canna system maintained the highest percolation coefficient at all times,and at the same time,the voids in the system decreased the least and had a high voids at the end of the stage.Canna was more conducive to delaying system blockage.Microorganism had a significant negative effect on the permeability coefficient and porosity of the upper media,but had little effect on the permeability coefficient and porosity of the lower media.The alternation of dry and wet had a significant effect on the permeability coefficient and porosity of the bioretention system.Proper dry period could effectively improve the permeability of the system,while longer wet period had a significant negative impact on the overall permeability of the system.The permeability coefficient of the upper and lower substratum of the system showed an opposite trend with the alternation of wet and dry.The overall permeability of the system was determined by the upper layer.During the operation of the system,the void of the upper media fluctuated alternately with the wet and dry conditions,and the overall void decreases.The porosity of the lower media was higher and the trend was stable.The continuous monitoring of the demonstration project showed that the permeability coefficient of each bioretention system was relatively small in rainy days(wet period)and larger in sunny days(dry period),which was consistent with the laboratory results.In winter,the lateral compaction of plant roots on the surface media was weakened after the wilting and harvesting.The permeability coefficient of bioretention system was obviously increased.Therefore,the regular maintenance of biological retention system plants can not be ignored.Both laboratory and demonstration engineering studies had found that the change of system permeability did not affect the removal effect of pollutants.COD removal rate was about 80%,TN removal rate was about 85%,and TP removal rate was about 70%.Through laboratory tests and demonstration project monitoring,it was found that the blockage of bioretention system could be controlled by adding carbon sources such as wood chips into the media of bioretention system,planting plants with strong root dredging effect such as Canna,and timely updating plants and surface media after plant death in winter.