| Rainwater is one of the important components of urban water resources.However,with the acceleration of China's urbanization process,the impervious surface increases,the rainfall infiltration decreases,and the runoff increases.A large number of pollutants carried by rainwater have become the main cause of urban water pollution.It has great signilficance to study the control of urban rainfall runoff pollution to solve the problem of urban water environment.As one of the typical low impact development(LID)technologies,bioswale can combine runoff regulation with pollution control,which has important practical significance for rainwater resource utilization in arid and water-deficient areas.Firstly,through static adsorption test and Langmuir adsorption isotherm curve fitting,the adsorption performance of spec:ial filler in bioswale was studied,which provided a basis for the selection of bioswale fillers.Subsequently,six types of 30 bench-scale columns and six pilot-scale grooves were designed.Through bench-scale and pilot-scale tests,and through mathematical statistics and m athematical simulation,the internal factors such as the type and combination of fillers,the thickness of fillers,vegetation conditions,the depth of submerged area and the amount of carbon source added,and the external factors such as hydraulic load,pollution load and simulated rainfall interval were comprehensively studied.Finally,a set of ecological filter ditch design method was proposed based on water balance,and a field test device was designecd and constructed according to this method,and the effect of runoff regulation and polfution control was studied.The main research conclusions are obtained as follows:(1)T he adsorption capacity of zeolite and activated carbon on ammonia nitrogen,total nitrogen and total phosphorus was higher,and the adsorption capacity of activated carbon and fly ash on COD was better.In terms of total phosphorus adsorption,fly ash is slightly better than activated carbon and COD is slightly better than zeolite.The adsorption of slag in ammonia nitrogen and total phosphorus is not different from activated carbon and fly ash.From the economic and environmental point of view,fly ash and slag can be selected as special filler of ecological groove system.(2)In the aspect of runoff water reduction,soil layer is the key factor that restricts the water reduction of bioswale due to its low permeability coefficient.Sand and slag have obvious effects on water control due to large void fraction.The effect of fly ash filler on water control is poor.The root system of vegetation can make the soil layer relatively fluffy and can promote water reduction.It is necessary to consider the water purification comprehensively to set the flood area to restrain the rate of water reduction.Under the experimental conditions,the delay time of bioswale to runoff could be up to 15-25min,the total reduction rate of runoff could be up to about 84-91%,and the flood peak reduction rate could be up to 85-93%.(3)In terms of the purification effect of pollutants,with the increase of runoff pollution load,the purification effect of ecological filter trench decreased gradually.Overall,the introduction of special fillers can significantly improve the purification effect of pollutants,the thickness of fillers increases,the water quality is improved,and the effect of fly ash on the purification of COD and TP is obvious.In the short time of rainfall,the effect of vegetation purification is not obvious.The addition of carbon source and the area with internal flood can improve the purification of TN in runoff wastewater,but the addition of carbon source has disadvantages for COD purification.The bioswale can achieve 10%-40%of TN,40%-80%of TP and 30%-90%of ammonia nitrogen.(4)The results of multiple regression analysis showed that there is a linear positive correlation between filler type and filler thickness with TN removal rate,and a linear negative correlation between the water inflow quantity and the pollution degree of runoff with the removal rate of TN.It also showed that there is a linear positive correlation between filler type and filler thickness with TP removal rate,and a linear negative correlation between the water inflow quantity and the pollution degree of runoff with the removal rate of TP.The simulated values of the standardized regression equation between TN and TP removal rates with the influencing factors obtained by stepwise regression are close to the measured results,and the deviation is small,which indicates that the simulation value has high credibility and has guiding significance for the test design.(5)When using hydrus-1d software to simulate the purification effect of runoff pollution,the simulation value of TP concentration of pollutants in the outflow from the ecological channel is close to the measured value.At medium and high load,the simulated value of TN outflow concentration is close to the measured value,while at low load,the simulated value is slightly lower than the measured value.(6)Based on the principle of water balance,a quick calculation method of the confluence ratio of the bioswale is proposed.This method takes many factors into account and covers 80 typical working conditions.Abioswale designed according to this method and effectively monitored 7 times rainfall from May,2015 to June,2016.The rainfall was between 2.5 mm and 30.4 mm.For a return period nearly three years and short-time strong rainfall,the runoff reduction rate can reach 83%,main pollutants load reduction rate is above 80%,and the device for rainfall runoff reduction and main pollutant load reduction effect is more significant,for small and medium-sized rainfall runoff control effect is better,which fully reflects the good effect of impact development technology on urban road runoff and pollution control.From the perspective of the mitigation of heat island effect in the sponge city pilot area of XiXian New Area,it can also prove that the green rainwater infrastructure which is mainly based on bioswale has ecological value. |
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