Research On Enhanced Removal Of Nitrogen And Phosphorus By The Amended Bioretention System

With the development of modern cities,the impervious area such as road hardening increases,which breaks the original hydrological cycle function links such as infiltration,stagnant storage and purification,and aggravates the occurrence of urban waterlogging and non-point source pollution.As a mainstream application technology in sponge city low-impact development technology（LID）,bioretention technology has multiple effects of rainwater utilization,runoff peak cutting and non-point source source control.The bioretention pond has a good effect on the removal of most pollutants,but the removal of nitrogen and phosphorus is easy to fluctuate,and even appear negative removal phenomenon.With vermiculite and water treatment residue as the main component,to strengthen the removal of N and P in rainwater,prepared hydroxy-aluminum vermiculite sludge particles（HAVSP）,through the material phase analysis,the influence of nutrient loss,p H change,different dosage,different concentration,isothermal adsorption line experiment,adsorption dynamics test and ammonia nitrogen adsorption test to explore the adsorption characteristics and removal mechanism.Using composite packing,bi-layer medium,different flow direction and filter saturation area combination setting,construct the simulated bioretention experiment column,and set the filter saturation area combined drainage system,explore the simulation of different filler under different water concentration,packing height and drought conditions,the effect of phosphorus retention and denitrification enzyme activity in the column.In addition,two bioretention systems of direct and folding current（DC and FC）type were constructed to compare the removal effect of nitrogen and phosphorus in the rainwater,and comprehensively analyze the optimal submerged height of the improved bioretention system.The study shows:（1）HAVSP has a rough appearance,rich void structure and active group,which can provide a better living environment for the growth of biofilm,improve the biological load and ion exchange capacity,and promote the adsorption and precipitation of nitrogen and phosphorus on the surface of the medium.The removal of ammonia nitrogen mainly occurred in the upper medium of the experimental column,and the NH₄⁺-N adsorbed by the upper medium was finally converted to nitrate nitrogen by nitrification.（2）The bi-layer medium structure and the filter saturation region were more stable in removing total nitrogen.The extension of the dry period favors the removal of total phosphorus and nitrate nitrogen,but hinders the removal of ammonia nitrogen and total nitrogen.Composite packing and double layer media added with hydroxy-aluminum vermudite sludge are more suitable for the rainy season of dry and wet alternation,and have strong resistance to environmental changes.（3）The HAVSP modified DC bioretention system has good effect on total phosphorus removal,and obvious removal effect of total nitrogen and ammonia nitrogen.The removal effect of nitrate nitrogen fluctuates greatly,but the average removal rate of total nitrogen,ammonia nitrogen,nitrate nitrogen and total phosphorus is higher than that of traditional filler DC bioretention system.（4）The reduction effect of HAVSP modified FC bioretention system on nitrogen and phosphorus is more obvious than that of traditional filler and modified filler DC,and the best reduction effect on N and P pollutants at350 mm.In conclusion,the results show that the use of HAVSP can improve the removal effect of nitrogen and phosphorus and improve the hydraulic process.The application of HAVSP can solve the problem of poor nitrogen and phosphorus removal in traditional bioretention pond,double layer medium and broken flow structure can also optimize the flow production process and relatively reduce the land area;in addition,this technology expands the application direction of vermiculite and water treatment residue,and provides a new idea for the reuse of waste.