| The treatment and disposal of excess sludge as the by-product of sewage treatment plants is the major challenge for the sewage treatment industry nowadays,and the cost of treatment and disposal is high,accounting for more than half of the operating costs of sewage treatment plants.The use of excess sludge modified bioretention facilities substrate can improve its physical properties,increase its porosity,permeability and water retention capacity,and improve its performance of stormwater infiltration and pollutants removal.The thesis takes the excess sludge substrate bioretention facility as the research object,analyzes the static risk of heavy metals in the substrate,studies the removal effect of the bioretention facility on heavy metals in stormwater runoff,and discusses the longitudinal transport and accumulation law of heavy metals and propose maintenance suggestions to extend its service life.While solving the way out of excess sludge,it can improve the control efficiency of the runoff water quality of bioretention facilities,which provides a certain theoretical basis for the application of substrate improvement of bioretention facilities to practical engineering.The average contents of typical heavy metals Pb,Cr,As,Cu,Zn and Ni in the excess sludge substrate were 16.11,85.36,13.06,24.65,218.69 and 17.78 mg/kg.Respectively,the contents of all heavy metal elements were lower than the risk standard value,it indicated that the heavy metal content of the excess sludge substrate met the standard requirement.The occurrence forms of Pb,Cr,As,Cu and Ni were mainly in the residue state,and the environmental stability was good;while the risk assessment index method showed that As,Cu,Zn and Ni were susceptible to transport and transformation,but the content were much lower than risk standard value,environmental risk could be ignored.The geoaccumulation index method,the Nemerow comprehensive pollution index method and the potential ecological risk index method evaluated all heavy metal elements as safe levels or slight ecological hazards,and the excess sludge could be used to improve the substrate of bioretention facilities.The removal efficiency of heavy metals from stormwater runoff by the excess sludge substrate bioretention facility weakened with the increase of pollution load and hydraulic load.Through the simulated stormwater runoff experiments with different combinations of working conditions,it was found that the removal effects of other heavy metals except Cr were good.Among them,the removal rate of As was close to 100%,and the removal effects of Pb,Cu and Ni were more stable,the removal rate ranged from 95.55% to99.81%,96.86% to 99.36% and 94.09% to 99.74%.The Zn removal rate ranged from77.84%～99.69%.However,the removal effect of Cr was relatively poor,and the removal rate ranged from 50.31% to 95.24%,which was related to its easy transport characteristics.The comparison of the content of heavy metals at the bioretention facility substrate before and after the simulated stormwater runoff experiments showed that the most heavy metals accumulated significantly on the surface of the substrate,and the accumulation degree was Cu>Pb>Zn>As> Ni,and the accumulated amounts were 44.01,33.21,200.22,12.92 and 12.84 mg/kg.Not only Cr accumulated on the surface of the substrate of the bioretention facility,but also accumulated at the sampling port 1 to sampling port 5 to different degrees,which indicated that the interception and adsorption stability of Cr in the excess sludge substrate bioretention facility was poor.It was estimated that its service life could reach 5 years,and it was recommended to plant super-enriched plants to maintain and extend the service life of the bioretention facility. |
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