Study Of The Reduction Of Urban Stormwater Runoff Thermal Pollution By Bioretention

The thermal pollution of stormwater runoff to the receiving water body is one of the outstanding rain-flood problems in cities.It is generally believed that solar radiation and industrial high-temperature wastewater discharge are the direct factors that cause the receiving water body to heat up.At present,China has formed a corresponding monitoring and management system for chemical pollution.However,it is ambiguous that the pollution problem caused by the shortterm sudden rise in water temperature caused by stormwater runoff.Existing research has proven that green stormwater infrastructure(GSI)can effectively reduce thermal pollution caused by stormwater runoff.However,the related research in China is relatively superficial and lacks research on the mechanism of thermal pollution generation and the effectiveness of GSI in reducing thermal pollution.Similarly,the prevention and control measures for stormwater runoff thermal pollution in China are still in their infancy,and relevant regulations,policies and emission standards are lacking in varying degrees.Taking bioretention as research object,the effectiveness of bioretention in reducing urban stormwater runoff thermal pollution under different rain patterns,rainfall temperature,duration,volume and recurrence period was analyzed.The common parameters for evaluating thermal pollution load,the policy standards and technical methods for controlling thermal pollution were analyzed,and quantitative calculation methods for thermal pollution reduction rate at different scales were proposed.The applicability analysis of current domestic stormwater source facilities' control efficiency of thermal pollution was analyzed,and the countermeasures and suggestions for the prevention and control of stormwater runoff thermal pollution were given based on the actual situation in China.The main conclusions of this article are as follows:(1)Bioretention has a significant reduction effect on urban stormwater runoff thermal pollution,and the average heat reduction rate can reach 22.65%~47.54%.The reduction effect of the experimental fillers is sorted from large to small,and the result is: slag> quartz sand> ceramsite> zeolite.(2)The influence of rainfall characteristics on thermal pollution reduction rate of bioretention is analyzed.The results show that thermal pollution reduction rate increases with the increase of rainfall duration.Prolonging the water outlet time or reducing the internal water flow velocity can effectively improve the thermal pollution reduction rate of the bioretention.In experiments,the higher the runoff temperature of bioretention,the higher the thermal pollution reduction rate.When the rainfall temperature differs by 6?,the maximum difference of thermal pollution reduction rate can reach 10.16%.The correlation between the reduction rate of thermal pollution and rainfall characteristics is ranked from large to small: rainfall duration> rainfall volume> rainwater runoff temperature.(3)The contribution ratio of volume reduction and heat exchange to thermal pollution reduction under different rainfall conditions is analyzed,and volume reduction has been found to be the dominant influencing factor for thermal pollution reduction by bioretention.The average volume reduction contribution ratio is 72.59%,and the average heat exchange contribution ratio is 27.41%.The response relationship between the thermal pollution reduction rate and the contribution ratio of the two effects under the conditions of uniform rainfall and Chicago rainfall pattern is analyzed.Under the experimental conditions of uniform rainfall and Chicago pattern rainfall,keep other conditions unchanged.When the heat reduction of volume reduction changes by 100 kJ,the thermal pollution reduction rate changes by 1.02% and 0.77%,respectively;keep other conditions unchanged,when the heat reduction of heat transfer changes by 100 kJ,the thermal pollution reduction rate changes by 0.97% and 0.39%,respectively.(4)An index for evaluating the effectiveness of GSI in controlling stormwater runoff thermal pollution called the load reduction rate of stormwater runoff thermal pollution is proposed.According to different scenarios,the detailed indicators are the reduction rate of field and annual stormwater runoff thermal pollution load reduction rate of a single facility,and the average annual stormwater runoff thermal pollution reduction rate of a project or area.The calculation formula of stormwater runoff volume corresponding to the amount of heat that can be controlled by GSI under certain conditions is proposed,and the method for determining the size of bioretention based on thermal pollution reduction is proposed.(5)The applicability analysis of the control effectiveness of stormwater runoff thermal pollution by current domestic stormwater source reduction facilities is presented,and the countermeasures and suggestions for the prevention and control of stormwater runoff thermal pollution are given in combination with China's actual situation.