Study On Hydrological Effect And Water Quality Change Of Bioretention

Bioretention,as one of the core measures for Low Impact Development?LID?,have significant effects in purifying runoff water quality and rainwater utilization.As the main carrier to undertake dry and wet deposition of the atmosphere,the roof has an increasing proportion of impervious underwater surface in the city,which makes the roof runoff pollution not to be underestimated and the roofing runoff pollution is more serious due to the aging and corrosion of the roofing materials.This paper takes a roof of Tianjin University of Technology in Xiqing District of Tianjin as the research area.Firstly,the law of hydrological and water quality changes of roof runoff with rainfall time is studied.Secondly,the variation law of roof runoff water quality with storage time is studied.Thirdly,the effects of bioretention columns of different plants and structures on the water quality of the accumulated roof runoff is studied.Finally,the effects of different soil depths and influent methods on the hydrology and water quality of the bioretention cells is studied,in order to provide a scientific basis for the study of regional runoff control,and to provide reference for the construction of bioretention.The main results are as follows:?1?The study area is flat and the shallow soil is the carrying soil.The soil bulk density of the surface layer of the study area was 1.21¹.48 g/cm³,and the infiltration rate was0.61mm/min.The average annual rainfall in Tianjin is 411⁷16 mm,the number of rain days is 63⁷0 days,and the rainfall in June,July and August accounts for 75%of the whole year.?2?A total of 9 rains were collected from 2017 to 2018,including light rain,moderate rain,heavy rain and torrential rain.The roof runoff changes with the change of rainfall intensity,and reaches the flood peak about 10³0 min after the start of rainfall.Roof runoff pollutants gradually decrease with rainfall time.The roof runoff pollution of the four rains in 2018 is relatively high,especially the SS,COD,TN and NH₃-N pollution is relatively serious,and worse than Grade V.The TP content in the roof runoff is higher than the five rainfalls in 2017,and the content is higher,which is worse than Grade V.When the rainfall is over,there is a backwatering process in the roof runoff.Under the stable storage conditions,the SS,COD and Cr were significantly reduced with the increase of the storage time,and the TN and TP contents increased significantly after the storage conditions were changed?temperature<0°C?.?3?After the roof runoff collected and stored by the bioretention columns treatment,it was found that the bioretention column of H.Ohba were best treated in different plants,and the removal rates of COD,TN,TP and Cr were 41%,-23%,-64%,and 90%,respectively.The removal of bioretention column without geotextiles in of different structures is better,and the removal rates of COD,TN and TP are 41%,-23%and-64%,respectively.However,there was no significant difference in the Cr removal rate of two structures.The removal rates of the without/with geotextiles were 90%and 87%,respectively.From the vertical distribution of Cr in the soil layer of the bioretention columns,the Cr content gradually decreased with the increase of soil depth,and the soil Cr content in different depths of the bioretention columns of the planting of H.Ohba was lower than that of other plants,and The Cr content of the 15cm soil has been reduced to the background value?50.9 mg/kg?,while the no plant is 28 cm.The vertical distribution of heavy metals in the soils of two different structures of bioretention columns is almost identical.?4?For bioretention cells with different soil heights,the bioretention cell filled with 20 cm soil has better control of water volume.The average effluent time is 12 min,the flood peak is reduced by 45.5%,and the peak is delayed by 30⁴4min.And the concentration of effluent pollutants in bioretention cell filled with soil of 20 cm is lower than that of bioretention cell filled with 15 cm of soil.The removal rate of each pollutant in the bioretention cell filled with soil 20 cm was better than that of the bioretention cell filled with 15 cm soil.The average removal rates of COD,TN,NH₃-N,NO₃^--N,NO₂^--N and TP in the four experiments were91.7%,93.8%,96.1%,95.7%,97.4%and 70.1%,respectively.?5?For bioretention cells with different influent modes,the bioretention cell with surface water inflow has better control of water volume,with an average peak reduction of 23.9%and a peak delay of 7 to 19 min.In general,The removal rate of pollutants in bioretention cells with surface and lateral influent was similar.The removal rates of COD,TN,NH₃-N,NO₃^--N,NO₂^--N and TP were 91.7%and 90.4%,93.8%and 86.5%,96.1%and 91.3%,95.7%and92.6%,97.4%and 88.3%,70.1%and 72.7%.The removal rate of SS by the bioretention pool is always above 90%,which is more stable than the above pollutant removal rate.