Removal Effect And Mechanism Of Pollutants In Oyster Shell Bioretention System Using Conch Shell As Water Conservation Area

With the rapid development of urbanization and industrialization,urban inland inundation and urban runoff pollution have become common problems.Bio-retention devices could reduce stormwater runoff and purify pollutants of stormwater runoff effectively.Based on the water quality characteristics of runoff in Qingdao,this study selected oyster shells and conch shells as the main filler for these can be obtained easily from coastal city common waste,and constructed new bio-retention devices to investigated removal efficiency of common pollutants in storm runoff.On this basis,high-throughput sequencing and three-dimensional fluorescence technology were used to explore the mechanism of pollutants removal.The main research conclusions are as follows:（1）The pollutants removal efficiency of the five bio-retention devices was investigated by simulated rainfall for 15 times.It was found that bio-retention device with conch-oyster shells could effectively reduce the concentration of total phosphorus（TP）in runoff,and the effluent concentration range of the device was 0.02-0.06 mg/L,which was less than 0.1 mg/L and up to the requirements of the Environmental Quality Standard for Surface Water（Ⅱ）.Compared to the control group（BK）,the addition of conch shells had a positive effect on nitrogen removal in runoff.The ammonia nitrogen（NH₄⁺-N）removal efficiency of CM and CB groups（conch shells and oyster shells located in the middle and bottom of devices,respectively）was still relatively high as the NH₄⁺-N pollution load was increased.In addition,nitrate nitrogen（NO₃^--N）was undetected in the bottom effluent of the four groups which containing conch shells.The COD removal rate also tends to be a stable tendency in the later period of devices under the different influent concentration.（2）The drought during the early filling stage（1-day drought,3-day drought,6-day drought,9-day drought,15-day drought,and 30-day drought）were set to explore the influence of antecedent dry days（ADD）on the removal of pollutants by conch-oyster shells bio-retention devices.The results showed that the change of ADD had no significant effect on the removal efficiency of TP,and the removal efficiency of NH₄⁺-N showed a trend of first decrease and then increase with the increase of ADD.For the bio-retention system with conch shells,the removal efficiency of NO₃^--N was almost unaffected by dry time.However,the removal efficiency of NO₃^--N in the bio-retention system decreased when ADD exceeded 30 days,but the CM and CB groups were stable compared with the other groups.Therefore,the addition of conch shells can effectively reduce the impact of dry on the operation of bioretention devices.（3）Three-dimensional fluorescence technology and fluorescence parameter analysis were used to investigate the source,composition and change of dissolved organic matter（DOM）in the effluent during the operation of the bioretention devices.Correlation analysis was conducted between water quality and volume integral of DOM,fluorescence parameters of each fluorescence region.The results showed that the effluent DOM of bio-retention devices were mainly endogenous.In the NO₃^--N removal process,microbial metabolites would accumulate in the 30-50 cm packing layer,and the upper metabolites could be used in the 50-75 cm packing layer of denitrification process to reduce the DOM fluorescence peak intensity.（4）High-throughput sequencing was used to analyze the diversity and structure of microbial communities and the mechanism of pollutant removal,samples collected from different devices and the pathway of different devices.The results showed that Acinetobacter,Pseudomonas and Flavobacterium were the dominant genera in the devices with conch shells,and all of them were proved to be functional bacteria for denitrification,which could remove NO₃^--N from the system effectively.There were significant differences among microbial communities in different levels of bio-retention systems.At the phylum and genus level,the dominant species in each layer were different.The dominant bacteria in soil layer（CM-S）can degrade organic matter and denitrifying phosphorus.Shell packing layer（CM-O）was benefit to denitrifying reaction,while gravel layer（CM-B）could effectively degrade organic matter.（5）In practical engineering,it is suggested that conch shell layer should be placed in the middle of oyster shell filler,so as to enhance the anti-impact ability of oyster shell bio-retention systems and extend the service period of bio-retention devices while forming effective submerged area（denitrification）.