Nitrogen Removal Performance In Stormwater Runoff In Saturated Zones Of Bioretention And Microbial Mechanism

In recent years,the increasing nitrogen pollution in stormwater runoff has caused serious eutrophication.It investigated nitrate-nitrogen?NO₃^--N?removal performance in bioretention saturated zones under intermittent wetting and drying and analyzed denitrifying bacteria,which was of great significance to accelerate the practical application of bioretention in sponge city and reveal the denitrification mechanism of bioretention saturated zones.Denitrification performance was more stable using oak wood as carbon source,and average denitrification rate was 3 mg NO₃^--N/?L·d?.When influent NO₃^--N was 6mg/L,two drying days were enough for complete denitrification.When drying days were less than 2 days,there would be NO₃^--N leaking in the effluent.When drying days were more than 2 days,there would be accumulation of humic matters and volatile fatty acids,which resulted in high COD?the highest being 480 mg/L?in the effluent and caused secondary pollution.The increase of NO₃^--N would affect end time of denitrification,and the complete removal time of NO₃^--N were 4,8,12,16 days under12,24,36,48 mg NO₃^--N/L in the influent,respectively.Volatile fatty acids produced from hydrolysis acidification from woodchips were major carbon source of denitrification.There was no process of denitrification during wetting times.The direct method to promote denitrification ability of bioretention was to enlarge volume of saturated zones.The abundance of denitrification genes increased with depth from 0 to 50 cm of saturated zones and concentration of NO₃^--N.The abundance percentage of narG gene increased with depth increasing from 0 to 50 cm.The abundance percentage of narG,napA,nosZ,nirK and nirS gene increased with concentration and reached highest level at 36 mg/L,indicating system denitrification activity were promoted by increasing of NO₃^--N concentration.As for denitrifying bacteria,Citrobacter,Pseudomonas,Dechloromonas,Acidovorax,Diaphorobacter,Denitratisoma and Hyphomicrobium were abundant on the media by high-throughput sequencing analysis.The relative abundance of denitrifying bacteria increased as the NO₃^--N concentration increased.Besides denitrifying bacteria,Novosphingobium was found to be related to woodchip decomposition and production of humic matter.Lachnospiraceae and Lactobacillus were found to be related to formation of volatile fatty acids.