Study On Optimization Of Nitrogen And Phosphorus Removal In Bioretention Systems Modified With Water Treatment Residuals

Aim at the disagreement about the removal efficiency of N and P in bioretention system with saturated zone?SZ?,this study designed two kinds of bioretention system with sand media and sand-WTR?sand and 15%water treatment residual?media,respectively.The removal effect of soluble N and P in bioretention with SZ was further clarified by setting different SZ depths?40cm,60cm,80cm?.At the same time,the removal mechanism was revealed according to the variation of N and P along the system depth.The effects of antecedent dry period?ADP?on the removal of N and P,and the microbial community structure and species abundance in the bioretention column were also investigated.The main research contents and results are as follows:?1?Both sand and sand-WTR columns effectively removed COD and NH₄⁺-N without setting saturated zone,the average removal rate of COD was above 80%and the average removal rate of NH₄⁺-N was over 90%.However,the removal rate of NO₃^--N and TN was not high,and the effluent concentration of them fluctuated greatly.The removal efficiency of phosphorus by sand-WTR column was significantly better than that of sand column?P<0.05?.By incorporation of the 40cm SZ at the bottom of the system,the denitrification reaction was promoted and the removal rate of NO₃^--N and TN was increased effectively.One way-ANOVA showed that there was a significant difference in the removal rate of NO₃^--N and TN between the two columns with saturated zone?SZ 40 cm??P<0.05?.There was no significant difference in the removal rate of NH₄⁺-N and TP with or without SZ?P>0.05?.?2?The concentration analysis along the system depth shows that the vegetation layer has obvious effect on the removal of N and P,and the vegetation layer removed about 40%of NO₃^--N and TN,50%of NH₄⁺-N and 20%of TP in bioretention with or without SZ.There was almost no effect on the change of NH₄⁺-N and TP along the column depth.After the upper 20 cm media layer,the average removal rate of NH₄⁺-N by sand and sand-WTR column was 88%and 94%,respectively.The removal mechanism of NH₄⁺-N in bioretention system was mainly achieved by plant absorption and surface medium adsorption.The removal of TP in sand column mainly occurred in the upper madia layer and about 99%of TP was removed by upper 20 cm filling layer.The removal efficiency of P in sand column was poor,the concentration of TP decreased gradually with the increase of the depths of packing layer,and the average removal rate of TP in the final outflow was about 55%.The removal of P is directly related to the adsorption properties of fillers.Setting saturated zone can promote denitrification reaction and effectively improve the removal efficiency of NO₃^--N and TN in the system.The experimental results showed that the removal mechanism of NO₃^--N in the biological retention system was mainly plant uptake and biological denitrification.?3?The effluent concentration of NO₃^--N in both columns decreased gradually with the depth increase of saturation zone,and the removal rate increased with the increase of height.Setting SZ can prolong retention time,promote denitrification and improve the removal rate of NO₃^--N.When the height of SZ increased from 0 cm to 40cm,60cm,80 cm,the average removal rate of NO₃^--N in sand column increased from 35%to 55%,70%and 75%,respectively,and that of NO₃^--N in sand-WTR column increased from 41%to 70%,75%and78%,respectively.The growing rate of NO₃^--N removal rate decreased with the increase of height.When the height was higher than 40 cm,DO content and residence time were no longer the main factors affecting denitrification.The saturated zone can promote the nitrification-denitrification alternating process,thus improves the removal rate of TN.When the height of saturation zone increased from 0 cm to 40cm,60cm,80 cm,the average removal rate of TN in sand column increased from 53%to 68%,73%and 74%,respectively,and that of TN in sand-WTR column increased from 58%to 73%,74%and 77%,respectively.The height of saturation zone had no significant effect on the removal of NH₄⁺-N and TP.At different SZ height,the removal rate of NH₄⁺-N by sand column was over 89%,and that by sand-WTR column was more prominent,with an average removal rate of more than 94%.In the process of operation,the adsorption effect of sand filler on P was unstable and the phosphorus removal ability was poor.The removal effect of TP by bio-retention column with WTR was very prominent.From the perspective of controlling TN and treatment time,it is advisable to control the saturation zone at a height of 40 cm during engineering design.The purpose of enhancing the removal efficiency of nitrogen and phosphorus can be achieved by setting the internal saturation zone and using the water sludge to improve the bioretention system.?4?After the inflow in the antecedent dry period?ADP?was restored,the outflow rules of nitrogen in the sand and sand-WTR columns were basically the same.The average removal rate of NO₃^--N and TN in the two-column effluent increased first and then decreased with the extension of the dry period,while the removal rate of NH₄⁺-N decreased first and then increased.In the short-term ADP?1-15d?system,the average removal rate of NO₃^--N in the two columns increased with the prolongation of dry period,and the average removal rate of NH₄⁺-N decreased slightly.While the removal rate of TN does not differ much with ADP elongation due to the combination of NO₃^--N and NH₄⁺-N.In the 30d and 40d ADP systems,the system is in a dry state and the internal saturation zone does not exist,and the average removal rate of NO₃^--N in the two columns drops significantly,even below 0,while the treatment effect of NH₄⁺-N is impoved,the average removal rate of NH₄⁺-N in both columns can reach 90%or more;since NO₃^--N accounts for a large proportion in the effluent TN?up to80%?,the outflow of TN in the two columns is similar to that of NO₃^--N,and its average removal rate also dropped significantly.The length of the dry period before the rain had no significant effect on the removal rate of TP in the sand column and the sand-WTR column,the removal of phosphorus from the sand-WTR column was more stable and efficient.In terms of the removal rate of TN and TP,the incorporation of SZ in bioretention column and the addition of WTR for improvement are more resistant to the adverse effects caused by drought.During the dry period before 1-15d,the dissolved oxygen content in the saturated zone of the system gradually decreased with the extension of the drought period,and the oxidation-reduction potential also decreased rapidly.The saturated zone gradually became an anoxic reduction state.In the 15d dry period,the saturated environment is conducive to the denitrification reaction,the NO₃^--N concentration gradually decreases,and the NH₄⁺-N concentration gradually rises,causing the water quality in the saturated zone to deteriorate.?5?The composition of microbial community is complex and the diversity of community distribution is high in the bioretention system.The main microbial communities of the two columns were similar,and their distribution along the column depth was similar.The dominant bacteria in sand column and sand-WTR column were all Proteobacteria,with relative abundance of 79.1%and 55.71%,respectively.The dominant bacteria in the sand columnwereThauera,andinthesand-WTRcolumnwere Saccharibacteria_genera_incertae_sedis.Theaerobicgenus?eg.Luteimonasand Sphingomonas?had a high abundance in the upper layer of the system and can degrade organic matter;Nitrogen-fixing bacteria and denitrifying bacteria?eg.Thauera,Azoarcus and Ensifer?had higher abundances in the middle and lower fillers.The composition of microbial community indicated that pollutants in runoff can be removed by biological reaction in the bioretention system.