Construction And Pollutant Removal Performance Of Dual-Mode Stormwater-Greywater Bioretention System

A large number of old neighborhoods exist in China’s urban areas and have become a key renovation task for urban construction management.The Opinions of the Central Committee of the Communist Party of China and the State Council on Further Strengthening Urban Planning and Construction Management clearly states that the renovation of old residential communities should incorporate low-impact development facilities such as rain gardens（i.e.,stormwater bioretention systems）.However,the widely adopted bioretention systems lack the necessary resilience to cope with weather extremes and usually require artificial watering and maintenance to avoid plant death and pollutant drenching caused by perennial drought,resulting in high O&M costs.At the same time,the phenomenon of balcony laundry wastewater mixing into the stormwater system commonly exists in old residential communities,resulting in incomplete diversion of stormwater and sewage sources and aggravating water pollution,which has become one of the key tasks of sponge transformation in old residential communities.Therefore,this thesis innovatively constructs a storm-gray water dual-mode bioretention system around the problem of drought watering maintenance and balcony wastewater mixing discharge of bioretention system.Firstly,the solid carbon source species and media soil composition of the storm-gray water dual-mode bioretention system were studied,based on which the influence of different plants on the system’s decontamination performance was investigated,and the plant species were comprehensively evaluated by hierarchical analysis based on multi-objective pollutant removal;for the characteristics of low carbon to nitrogen ratio of laundry wastewater,the influence of the solid carbon source addition ratio and flooding zone setting height on the system’s nitrogen removal performance was explored.In order to provide a theoretical basis for the application of storm-gray water dual-mode bioretention technology in the sponge transformation of old residential communities.The main research findings are as follows.（1）The effect of different cellulosic solid carbon sources（wood chips,corn residue,rice husk,rice hull,wheat straw）and different additions（0%,3%,5%,7%,mass ratio）on the decontamination performance of the system was investigated using a bioretention system to treat balcony laundry wastewater and rainfall runoff.The results showed that all cellulose carbon source materials could release a large amount of organic carbon to meet the denitrification demand,among which wood chips had the strongest denitrification ability with NO₃^--N removal rate up to 95%.Under the dual mode of stormwater and graywater,the added carbon source can effectively improve the nitrogen removal capacity of the bioretention system,especially for NO₃^--N removal,but too much carbon source will cause the elevation of NH₄⁺-N in the effluent;the added carbon source can significantly improve the phosphorus removal capacity of the system,and it will be enhanced with the added amount;the addition of carbon source will lead to the elevation of COD concentration in the effluent and the phenomenon of organic matter leaching.Considering the pollutant control objectives,the addition of 3%wood chips to the submerged zone media soil can meet the demand for carbon source from the storm-gray water dual-mode bioretention system for treating laundry wastewater and rainfall runoff from balconies in old residential communities.（2）The bioretention system was used to treat balcony laundry wastewater and rainfall runoff,and the effects of different sand-to-soil ratios（BSM1～BSM9）of fillers within the planting layer on infiltration coefficients under graywater intake were investigated to screen out the best filler ratios.The results showed that the BSM1 group and BSM2 group were significantly higher than the specification requirement of permeability coefficient of bioretention system in Chongqing,and the permeability coefficients of BSM6,BSM7,BSM8 and BSM9 groups were already close to or lower than the lower boundary of the specification,and the fillers collapsed to different degrees,while the remaining three groups of BSM3,BSM4 and BSM5 still met the specification requirement after the decay of permeability coefficient occurred.As the longer the operation time after the formal operation of the device,the stronger decay of the permeability coefficient inside the system will occur,the group with higher permeability coefficient should be selected as much as possible.Considering the actual operation of the bioretention system,the BSM3 group was selected as the filler ratio of the planting layer of the storm-gray water bioretention system in this project and the subsequent study was carried out.（3）Six plants,namely Cyperus alternifolius L.（Cyp.）,Trachelospermum jasminoide（Tra.）,Hedera nepalensis var.sinensis（Hed.）,Canna indica L.（Can.）,Pennisetum alopecuroides（Pen.）and Iris tectorum Maxim（Iri.）,were selected and their removal of multiple target pollutants under storm-gray water stress was investigated.The results showed that the removal effect of each pollutant index was poor at the beginning of the experiment,among which NH₄⁺-N,TN and TP removal by graywater and COD removal by stormwater were especially obvious,and the removal rate of each plant for the above indexes only ranged from 20%to 50%at the beginning.With the passage of time,the removal rate of each index basically rose gradually,and the system gradually stabilized,in which the average removal rate of each index basically rose to between 60%and 90%,and the removal rate of NO₃^--N even approached 100%;the difference between the removal effect of each plant was small,and the difference between different plant groups and blank samples was also small;the overall removal effect of stormwater was significantly better than that of graywater The overall removal effect of stormwater is significantly better than that of graywater,and the stability of the removal effect of stormwater is also significantly better than that of graywater;through Pearson correlation analysis to analyze the influence of pollution removal indexes,there is no significant correlation between two indexes in the stormwater bioretention system before the stormwater is fed;there is a significant strong correlation between COD and ammonia nitrogen in the system under the condition of feeding graywater,and a significant strong correlation between TP and TN;through the hierarchical analysis method The six plants were evaluated comprehensively by hierarchical analysis,and the comprehensive ranking of all plants was Cyp.>Tra.>Hed.>Can.>Pen.>Iri.（4）Four different concentrations of wood chips at 0%,3%,5%and 7%were selected as the carbon source addition in the submerged layer,and two different submerged zone settings of 0 mm and 300 mm were selected to study the removal of multiple target pollutants by this cross-change parameter under storm-gray water stress.The results showed that the NH₄⁺-N removal rate decreased slightly with the rise of carbon source,but the submerged zone setting had no significant effect on it;the addition of carbon source and the submerged zone setting both had significant promotion effect on the NO₃^--N removal;the addition of carbon source and the submerged zone setting had significant effect on the TN removal under gray water conditions,but both had no significant effect on the TN removal under rainwater conditions,even under the setting of the removal rate of TN decreased gradually with the increase of carbon source ratio under the condition of inundation zone;the effect of carbon source on COD removal under dual-mode inlet water was not obvious（or slightly decreased）regardless of the existence of inundation zone;the addition of carbon source had a significant enhancement effect on TP removal,while the setting of inundation zone reduced the system’s effect on TP removal.