Study On Filter Media In Road Runoff Bioretention Systems

With the acceleration of urbanization, the hardened road area grows rapidly. As a result, rainfall runoff coefficient increases and convergence time accelerates, increasing the risk of urban flooding. Bioretention measure as the representative of runoff source reduction control technology has become an important measure to solve the problem of stormwater. In controlling runoff pollution and covering the groundwater, bioretention technology also has significant ecological effect. Bioretention technology is a better management of stormwater runoff. In bioretention system, filter media play an important role. In this study, we combine with the actual situation in Beijing, screening suitable filter media type for bioretention systems to provide the basis for a comprehensive study of bioretention technology.Firstly, a screening test by filling a single species, the multi-objective decision method and AHP is used to screening better performance of a single species filter media through the study of screening single filter media. Then, through the study of screening combination filter media, using multi-objective decision method and AHP screened better performance of combination filter media, the conclusions as follows:(1) The permeability coefficient of loam did not reach the permeability coefficient of bioretention facilities10-5m/s, so other materials need to be added to the loam. Considering a single species filter permeability coefficient, water holding capacity and decontamination effect, through screening and evaluation analysis, loam, sand, gravel, perlite, vermiculite and peat soil is to be a combination of filter media. Gravel is used in the bottom of the gravel drainage layer, and the remaining materials with different proportions to screen combination filter media.(2) Preparing the filter media, if the content of sand and loam is certain, the ratio of perlite and vermiculite should be one to one. If the content of perlite and vermiculite is certain, the loam content should be reduced and the sand content should increase to ensure the permeability coefficient of filter media meet the requirement.When perlite and vermiculite as variables, field capacity increases with the increasing of vermiculite content; When sand and loam as variables, field capacity increases with the increasing of loam content; When perlite or vermiculite as variables, field capacity increases with the increasing of perlite or vermiculite content. When the content of sand and vermiculite is certain, field capacity is almost unaffected with the variation of loam and perlite content.(3) Kostiakov infiltration model is used to simulate infiltration of the filter media, resulting in good effect. The correlation coefficient was over0.87. When the loam content is30%, sand content is40%or30%, the permeability coefficient of combination filter media is larger. When the content of loam and sand is certain, and the ratio of perlite and vermiculite is one to one, the permeability coefficient of combination filter media is larger. The results fall in line with the laboratory results.(4) The total nitrogen content of each combination filter media was below1000mg/kg, within the range specified in Facility for Advancing Water Biofiltration (FAWB). The content of total phosphorus was over80mg/kg, without the total phosphorus range specified in FAWB. The organic matter content is less than3%, without the range of3%to5%specified in FAWB. Cation exchange capacity is greater than10cmol/kg, within the range specified in University of North Carolina.Most combination filter media have better pollutant removal on conventional pollutants of Total Nitrogen (TN), ammonia, Total Phosphorus (TP), Chemical Oxygen Demand(COD), Suspended Solids(SS). The removal efficiency of TN, ammonia, TP, COD, SS is over60.5%,66.6%,85.1%,58.5%,77.3%respectively. The maximum reached82.3%,82.8%,91.5%,91.8%,93.9%. And the COD removal efficiency of some combination filter media is unsatisfactory.(5) In these combination filter media, the highest score is the21st combination filter media, and the ratio of loam:sand:perlite:vermiculite=30:40:15:15; The following is the26th filter media, and the ratio of loam:sand: perlite:vermiculite=30:30:20:20; Then were the20st,24th,19th filter media. When considering permeability the effects of water quality, the21st,26th,20th,24th,19th combination filter media are feasible.