Study On Pollution Control Effect Of Coal Gangue Based Bioretention Systems

With the rapid development of urbanization,the impervious area of city is increased,and urban waterlog and rainwater runoff pollution become common problems.Bioretention facilities are one of the stormwater management measures,which have an excellent control function on runoff rainwater.Coal gangue,as an industrial solid waste produced in the coal preparation process,has a high adsorption performance.In this study,coal gangue was selected to improve the bioretention facility,to study the removal effect of the facility on runoff pollutants and explore the relationship between the bioretention pollution control effect and biological characteristics（such as plants and microorganisms）.In addition,coal gangue-humic acid（C-H）was selected and applied in bioretention facility.We study on pollution control effect of C-H modified bioretention facility and explore the effect of the facility on plant and microbial characteristics.Research shows that:The fouling control efficiency and biological characteristics of coal gangue modified bioretention facilities（PFB,PB,FB）with different filler combination are better than traditional facility（Control）.In particular:（1）The removal rates of TN and TP in modified bioretention facilities are higher than traditional facility（Control）at different influent concentrations,which the removal rate of TP remained above 98%.With rainfall recurrence periods increase,the removal rates of TN and COD in the bioretention facilities generally show a downward trend and the removal rate of TP increased firstly and then decreased.The removal rate of various pollutants in the coal gangue modified facilities is higher than traditional facility in short drying period（2 d,4 d）,but the removal rate of TP and COD of the bioretention facility show a downtrend with the extension of the drying period because of the long-term drought reduces the microbial activity.The average effluent concentration of heavy metals Zn,Cu and Pb in modified bioretention facilities are lower than traditional facility.On the whole,PFB show the best runoff pollutant removal performance.（2）The order of Sedum sedum heavy metal content in bioretention facilities are PFB>PB>FB>Control,while the heavy metal content in substrate layer show the opposite trend.The BCF of heavy metals in Sedum sedum in coal gangue substrate is significantly higher than sandy soil,and the pollutants accumulated in substrate are absorbed to prolong the operating life of facility.The malondialdehyde（MDA）and the activity of peroxidase（POD）content of Sedum sedum in coal gangue modified bioretention are lower than that of traditional facility due to coal gangue alleviate heavy metal stress for Sedum sedum（3）Coal gangue not only improve the richness,diversity and stability of microbial community,but also promote the dominant bacteria（Proteobacteria,Bacteroidetes,etc.）which could improve the removal effect of runoff pollution.The microbial community of Proteobacteria,Chloroflexi and Bacteroidetes is dominate in coal gangue substrate layer.Compared with the traditional facility（Control）,the pollution control efficiency and biological characteristics of C-H modified bioretention facilities（15L,30L,15H,30H）with different filler ratios in different concentration runoff are significantly improve.In particular:（1）With the increase of rainfall return period,the removal rate of runoff pollutants by C-H modified facilities and traditional facilities generally show a downward trend.The removal rate of TP,TN,NH₄⁺-N by C-H modified facilities was better than that of traditional facility under different return periods,while the removal rate of COD was relatively low.The concentration of heavy metals in the effluent of the C-H improvement facility was lower than that of the traditional facility under different return periods.With the extension of rainfall drying period,the removal rate of conventional pollutants by bioretention facilities generally increased first and then decreased,and the pollution control efficiency reached the best in the 5 d drying period.The removal rates of TP,TN and NH₄⁺-N in C-H improved facilities were higher than those of traditional facilities in different drying periods.The concentration of heavy metals in the effluent of the bioretention facility increased with the extension of the rainfall drying period.The removal effect of the C-H modified facilities on the heavy metals is better and more stable in longer drying period.On the whole,the pollution control efficiency of modified facilities with the addition of 30%C-H show the best removal performance.（2）The plant height,leaves number,biomass and other growth indicators of buxus mircophylla in C-H modified facilities are higher compared with traditional facility,while the root-shoot ratio is lower.Compared with traditional facility,the contents of total chlorophyll,chlorophyll a and b,MDA,soluble sugar,soluble protein and proline in C-H modified facilities are significantly increased and further increased in high-concentration influent water,while catalase（CAT）and superoxide dismutase（SOD）activities are significantly reduced.The order of heavy metal content in the facility vegetation was 30L>30H>15L>15H>Control,while the order of heavy metal content in substrate layer is 30H>15H>30L>15L>Control.（3）C-H modified can improve the richness and diversity of microbial communities in bioretention facilities,while decrease the uniformity.The relative abundance of Bacteroides in the C-H modified facility is higher than traditional facility and nitrogen and phosphorus removal of Proteobacteria and Bacteroidetes are helpful for runoff purification.In conclusion,the application of coal gangue and C-H as coal gangue-based fillers in bioretention facilities can significantly improve the pollution control efficiency of bioretention facility,maintain the stability of the ecosystem and ensure the long-term operation of the facility.It not only provides creative ideas for the utilization of coal gangue resources,but also provides a new solution with economic,social benefits and environmental friendliness for improving the runoff control effect of bioretention facilities.