Research On Phosphorus And Nitrogen For Purification Of Paddy Field Runoff By Red Soil Constructed Wetland With Matrix Optimization

Agricultural non-point source pollution in China has diversity and regionalism source.However the removal of nitrogen/phosphorous pollutants is critical for agricultural runoff purification in southern China,especially at the irrigation paddy fields in the red soil plain,there is still a noticeable research gap in developing an effective treatment method.The loss of nitrogen and phosphorus is serious in red soil area.Due to high content of iron and aluminum present,poor permeability red soil which leads to higher fluctuation in water quality of the constructed wetland effluent in southern China.Focus on the higher fluctuation of nitrogen/phosphorous concentration in the red soil constructed wetland effluent,this study focused on using Fe-modified medical stone as the promoter for N/P removal.Moreover investigate the removal law and adsorption mechanism of nitrogen/phosphorous in the enhanced wetland.The hydraulic load and residence time of the trial device were subsequently optimized based on a real-time pilot plant study on the field.Based on the removal rate of the main pollutants,the economic benefit of the constructed wetland was evaluated,and the relevant maintenance plan was proposed accordingly.This study provides a firm scientific background and guidance for the pollution mitigation of agricultural runoff in the paddy field of southern China.The key conclusions were listed below:(1)The optimal preparation conditions of Fe-modified medical stone were decided by single-factor experiments.The best conditions were concentration 2.0 mol/L,pH 3.0,and modification time 9.00 h.Concentration during modification has the most dominant impact,followed by pH and impregnation duration.The NH3-N and TP theoretical saturated adsorption capacity of Fe-modified medical stone were 1206.07 mg/kg and 2351.42 mg/kg.were higher 29.66%and 11.17%than medical stone without modification.(2)During the operation period of enhanced constructed wetland,the removal of total phosphorus,total nitrogen,ammonia nitrogen and nitrate nitrogen which average removal rates were 85.27%?69.96%?61.25%,55.02%,which were higher 118.57%,18.47%,12.50%,8.79%than ordinary constructed wetland.Enhanced constructed wetland has adsorb 4304.30 mg of phosphorus.Fe-modified medical stone showed the most notable phosphorus adsorption(1592.60 mg)among various substrate constituents.(3)During the field pilot test,the average removal efficiency of total phosphorus,total nitrogen,ammonia nitrogen and nitrate nitrogen were 65.3%,64.77%,66.38%,and 60.93%by enhanced constructed wetland.which were higher 13.48%,11.68%,15.32%,14.77%than ordinary constructed wetland.The best optimal HRT and hydraulic loading were determined to be 16 h and 0.4 m3·m-2·d-1.(4)Principal components analysis was introduced to assess the economic of reducing nitrogen and phosphate for the constructed wetland.For the treatment of agricultural runoff in southern China,the cost of enhanced constructed wetland turned out to be 0.5970 tons/RMB.Compared to the lowest sewage treatment cost in Nanchang,it is 59.13%lower.The economic benefit was the highest when targeting total nitrogen and ammonia nitrogen pollutants.