Research On Removal And Mechanism Of Pathogenic Microorganism By Bioretention System In Urban Surface Runoff

The rapid development and disorderly expansion have led to an increase in impervious areas in cities.On the one hand,they have changed the original ecosystem hydrological cycle process and the characteristics of the water text,increased the total amount of surface runoff,and caused problems in urban floods;on the other hand,the surface of a lot of pollutants have been accumulated under the action of atmospheric deposition and human activities,so that the runoff rainwater after surface erosion contains a lot of heavy metals,nitrogen,phosphorus,organic matter and pathogenic microorganisms and other pollutants.In particular,pathogenic microorganisms in surface runoff have the potential for public health and human health due to their universality and variety of characteristics.Therefore,the search for economical,effective,and convenient surface runoff pathogenic microorganism pollution control technology is one of the topics that we will focus on in the future.Low impact development(LID)is to reduce runoff peaks through decentralized,smallscale source control measures,purify runoff water quality,and achieve the purpose of controlling surface runoff pollutants.As a typical source control measure in low-impact development technology,bioretention system can purify surface runoff pollutants through interception,adsorption,degradation and filtration.The biological retention system can effectively remove the conventional pollutants such as COD,nitrogen and phosphorus in surface runoff.However,there are few studies on the removal characteristics of pathogenic microorganisms in surface runoff,and the relevant mechanism is not yet clear.Based on the above problems,this paper takes simulation of surface runoff as the research object,and through simulation experiments,it examines the control of total surface runoff by different media types of biological retention systems and the system’s representation of surface runoff COD,nitrogen,phosphorus and E.coli.The removal effect of pathogenic microorganisms,the influence of design conditions and process parameters on the removal of pollutants,and the removal mechanism of pathogenic microorganisms in biological retention media were analyzed.The main research contents and conclusions are as follows:(1)Study about the effect of biological retention system on runoff volume and water quality control.The study found that biological retention system can effectively reduce the total amount of runoff and delay the peak arrival time.The bioretention system has a higher removal rate of total phosphorus and ammonia nitrogen,while there is a certain fluctuation in the removal rate of COD and total nitrogen.The type of composite media,return period,rainfall duration and concentration of pollutants all have an impact on the removal efficiency of each pollutant.The return period and rainfall duration have a great influence on the removal efficiency of COD,total phosphorus and ammonia nitrogen,but have little effect on the removal of total nitrogen.The total nitrogen and ammonia nitrogen concentrations in the influent water have a great influence on the removal effect,while the total phosphorus concentration in the influent water has little effect on the removal efficiency.(2)Study on the removal effect and influencing factors of pathogenic microorganisms in biological retention system.E.coli can be removed efficiently by the bioretention systems and the average removal efficiency is above 55%.The concentration of pollutants,return period,rainfall duration,drying period,media type,medium height and inundation zone settings have a certain impact on the removal of E.coli.Among them,the bio-char composite media had the highest removal effect on E.coli;when the media height was 70 cm,the E.coli removal efficiency was the best;the E.coli effluent concentration and the influent concentration had a linear correlation;the return period was 0.5 a,the large intestine The removal efficiency of Bacillus was the most stable;the removal rate of E.coli gradually increased with the rainfall time,and the removal rate gradually stabilized after 35 minutes of outflow;the removal rate of E.coli decreased under dry conditions;the setting of the inundation area can relieve the drying of the system.(3)The removal mechanism of pathogenic microorganisms in bioretention system media was studied.The removal of E.coli by bioretention systems is mainly based on filtration and retention.The E.coli in the influent water mainly remains in the surface layer of the composite medium,and the concentration decreases with the increase of the depth of the medium.The breakthrough curve of E.coli in the medium was 0.003-0.004,most of the E.coli was retained in the medium,and a few of them migrated under the action of the influent water flow.There is no obvious accumulation of E.coli in the bioretention system.The removal of E.coli by the bioretention system can maintain long-term stability.