Optimization Of Efficiency And Mechanisms Of Nitrogen Removal On Stepped Bioretention System

To combat the impact of urban waterlogging and degradation of the river ecosystem,low impact development(LID)and sponge city construction have been recommended in China.The bioretention system,one of green infrastructures of the LID measure,has been widely adopted.However,the effect of nutrient removal was unstable,and the denitrification mechanisms were still unclear,which need to be addressed.In this research,we designed the bioretention system with two-stage series stepped.The effluent quality characteristics of bioretention systems and design parameter optimization were investigated.Nitrogen transformation characteristics in bioretention systems were studied by 15N isotope tracer and three-dimensional fluorescence technique.Futhermore,the hydrology and water quality performance of pilot bioretention systems were evaluated by detecting the actual condition.Water quality surveyed from the bioretention systems showed that the NH4-N removal effect was obvious,but NO3-N removal rate was low,which lead to TN increase as the volume of effluent increases.The experiments of optimization on design parameter showed that arrangement of soil:sand?4:6 was better to remove TN(87.14%)with low permeation rate.Juncus roemerianus was conductive to remove NO3-N(the removal rate up to 82.03%),and the removal rate of TN was 84.50%in Radermachera Hainanensis Merr system.Flow regime with U could increase the removal NO3-N(up to 94.83%).Meanwhile,there is a positive correlation between NO3-N and soil content.The Nitrogen balance model showed that 37.25%of NH4-N was adsorbed by soil,about 60.64%was be nitrificated,plants and microbial assimilated.About 1.48%of NH4-N and 27.26%of NO3-N was detained in saturated layer.About 80.48%was denitrificated,plants and microbial assimilated.Within 60 hours,NH4-N adsorbed by soil was nitrated of 0.1 245 mg/(kg h)and the denitrified or other removal way was of 0.0925 mg/(kg·h).NO3-N in the saturated layer was denitrified of 0.0613 mg/(L·h).The results of three-dimensional fluorescence indicated that sponge iron in the saturated layer could contribute to organic matter removal(the removal rate up to 84.52%).The biometabolism and protein-like substances were the main ingredient in effluent water.The study of pilot bioretention systems showed that the system could retain the rainwater of rainfall within 0.34 mm.The system was less effective at removal NO3-N?DON and DOC,and the effect of TN by system of Medicago sativa were better than system of Ophiopogon Japonicus.The mainly organic component of effluent was protide,which could provide carbon for denitrification and be translated into NH4-N.In a word,Bioretention systems with stepped two-stage series could removal nitrogen pollution in rainwaterand retain the runoff.The design parameter optimization by the application area could improve the performance of water quality purification and runoff retention.