Study On The Application Of Granular Materials Made Of Coal Bottom Ash And Coal Gangue In Bioretention System

With the rapid development of urbanization in China,impervious ground in cities is increasing,which leads urban waterlogging,runoff pollution and groundwater level decline to become more and more prominent.Bioretention system can promote rainwater infiltration and remove pollutants in stormwater runoff,becoming an effective control measure for urban stormwater management.However,traditional bioretention system fillers have many problems such as rapid reduction of permeating speed,filler blockage and unstable removal effect of pollutants（nitrogen and phosphorus）.Coal bottom ash（CBA）and coal gangue（CG）are solid wastes in coal industry,and the annual production of them in China are hundreds of millions of tons,which becomes a serious challenge in the reutilization of coal industry.CBA and CG,with volcanic ash characteristics and good adsorption properties,have a high content of metal oxides,which makes them to be widely researched.In this study,the basic properties and reutilization risk of CBA and CG were first assessed.On the basis of ensuring the safety,granular materials were prepared by using CBA and CG,and then the produced granular materials were combined with soil and river sand as fillers to form different bioretention systems.Becides,we researched theses systems’effects of storage-infiltration and pollution control to select the best performance bioretention system.The main conclusions of the study are as follows:（1）CBA in this study had environmental risk of reutilization,so it should not be applied in bioretention system alone.However,the concentration of heavy metals and inorganic salt in the leachate of granular materials with CBA/CG mass ratio of 3:7 were in accordance with the national standard,and the granular materials showed the best physiochemical performance according to this ratio.The specific surface area of the granular materials with CBA/CG mass ratio of 3:7 was 2.06 m²g^-1,which is high among the commonly used equal size fillers.Besides,the granular materials with CBA/CG mass ratio of 3:7 had good porosity which was mainly made up of macroporous and mesoporous.In addition,the granular materials were hard and had good resistance to compression and support.（2）The bioretention system BS-C（upper layer:35%soil and 65%river sand with a thickness of 30 cm;lower layer:granular materials with a thickness of 20 cm）and BS-D（upper layer:35%soil,55%river sand and 10%granular materials with a thickness of 30 cm;lower layer:granular materials with a thickness of 20 cm）performanced better in infiltrating than the conventional bioretention system BS-A（35%soil and 65%river sand with a thickness of 50 cm）.Compared with the control group BS-A,BS-C and BS-D had better resistance to high-intensity rainfall floods.After the experiment,the permeability coefficient of BS-C and BS-D were 46.90 mm h^-1and 46.09 mm h^-1,which were much higher than BS-A(30.39 mm h^-1).In addition,the fillers of BS-C and BS-D had better support capability,which could effectively reduce the porosity reduction rate and delay the lifespan of fillers.（3）The granular materials made of CBA and CG could effectively enhance the ability of bioretention system to remove TP,NH₄⁺-N,and TN and BS-D had the best removal effect.In the early stage of rainfall,the average removal rates of TP,NH₄⁺-N and TN by BS-D were95%,91%and 77%,which were higher than those of BS-A（92%,89%and 58%）;in the later stage,the removal rates of TP,NH₄⁺-N and TN by BS-D were 62%,91%and 52%,which were much higher than those of BS-A（41%,73%and 7%）.（4）The rainfall interval（2d,4d,10d）had little effect on the removal rates of COD_Mn,SS,and TP.The increase of interval could effectively improve the removal rate of NH₄⁺-N,but with it increasing,the removal rates of BS-A and BS-C for TN showed a gradual decrease,while BS-B（35%soil,55%river sand and 10%granular materials with a thickness of 50 cm）and BS-D showed a trend of increase and then decrease.Among them,BS-D maintained high removal rates of COD_Mn,SS,TP,NH₄⁺-N,and TN at all rainfall intervals.（5）The structure of microbial community was related to the composition of fillers,and the relative abundance of functional microorganisms was consistent with the removal efficiency of bioretention system.The granular materials made of CBA and CG could significantly increase the types and relative abundance of denitrifying bacteria in bioretention system,in which the relative abundance of denitrifying bacteria Povalibacter in BS-B,BS-C and BS-D were 63.65%,21.12%and 38.26%,much higher than that in BS-A（3.64%）.Besides,phosphorus accumulating organisms were detected in BS-A,BS-B,BS-C,and BS-D,indicating that there was microbial phosphorus removal in these bioretention systems,and the abundance of phosphorus accumulating organisms in BS-C and BS-D were higher than that in BS-A.In summary,the application of granular materials made of CBA and CG in bioretention system was feasible,and it could keep the bioretention system operate stably for a long time and delay the lifespan of fillers.BS-D exhibited the best infiltration performance and efficient pollution removal rate among all bioretention systems in this study.This reaech provided a new idea for the improvement of bioretention system performance and resource utilization of coal bottom ash and coal gangue.