Removal Of Nitrogen And Phosphorus From Secondary Effluent By Constructed Wetland With Granular Biochar

With the continuous increase of urban sewage secondary treatment plants in China,Nitrogen and phosphorus form secondary effluent has become one of the important pollution resource of adjacent surface water bodies which cause eutrophication.Therefore,it is necessary to further remove nitrogen and phosphorus from the secondary effluent.Constructed wetland is popular as one of economic and ecological treatment processes,but there are still problems such as low efficiency of nitrogen and phosphorus(especially nitrogen)removal.In order to improve the nitrogen and phosphorus removal efficiency in constructed wetland,this paper studied the effect and mechanism of constructed wetland with granular biochar on nitrogen and phosphorus removal from secondary effluent.The main contents are as follows: The characterization of granular biochar and the release of nitrogen and phosphorus from granular biochar;the effect and mechanism of nitrogen and phosphorus removal from NH4+-N dominateds econdary effluent by constructed wetland with granular biochar;the effect and mechanism of nitrogen and phosphorus removal from NO3--N-based secondary effluent by constructed wetland with granular biochar.The main results are as follows:(1)Binder addition and drying treatment increase the specific surface area of granular biochar.In this study,the specific surface area of granular biochar is 22.18m2/g,which is 4.11 times higher than that of powdered biochar.Granular biochar contains C elements(more than 60%)and some organic matter,and can slowly release COD.The release amount is 1.64 times that of powder biochar(14.83 mg/g),and it has the potential as a denitrifying carbon source.However,there is also a very small amount of ammonia nitrogen(0.03 mg/g)and a certain amount of nitrate nitrogen(1.05 mg/g)and TP(1.01 mg/g)release.(2)The removal efficiency of TN and NH4+-N in NH4+-N-based secondary effluent by constructed wetland with granular biochar(GB)were 78.61% and 93.03%,the removal rate of TN and NH4+-N in the non-granular biochar wetland(CK2)were 60.60% and 85.12%,the removal efficiency of TN and NH4+-N were13.12% and 52.11% without biochar and plant system(CK1).The nitrogen removal efficiency of GB was the best,and the NO3--N concentration in the effluent was the lowest,1.69 mg/L.The effluent TP concentration of GB fluctuated greatly,accompanied by phosphorus release.However,the removal efficiency of TP was the best in the later stage(90.01%).Granular biochar promoted the growth of reeds.The plant height and biomass of reeds communis increased by 7.17 cm and 0.29 g per plant,respectively,compared with the constructed wetland without granular biochar.Granular biochar significantly increased the amount of organic acids secreted by the roots of reeds in constructed wetland,which was 1.84 times greater than the amount of organic acids secreted by roots of reeds in constructed wetland without added granular biochar.In addition,granular biochar increased the diversity of denitrifying microorganisms and maintained the existence of various denitrifying bacteria(Exiguobacterium,Bacillus,Pseudomonas,Rhodobacer,Steroidobacter).(3)Initial start-up phase,the removal rate of TN,NO3--N and TP in NO3--Nbased secondary effluent from constructed wetland with granular biochar(GB)were 82.19%,95.38%,and 87.92%.The removal rate of TN,NO3--N and TP by constructed wetland with granular straw(GS)were 60.27%,72.60%,and 76.52%,respectively.The removal rates of TN,NO3--N and TP from secondary effluent by constructed wetland only with reeds(CK)were 48.66%,56.08%,and 62.71%,respectively.The effect of GB and GS on nitrogen and phosphorus removal was better than that of CK,and GB had the best effect.The application of external carbon source could improved nitrogen and phosphorus removal efficiency in constructed wetland.Granular biochar was more suitable for purifying nitrogen and phosphorus from secondary effluent than granular straw.Granular biochar could play a long-term role in constructed wetland.Restart phase,GB still maintained high removal rates of 94.88%TN,99.84%NO3--N,and 90.51%TP.Granular biochar promoted the release of root exudates from reeds,the amount of organic acid secretion was 4.10 times that of the organic acid secretion which was secreted from constructed wetland with granular straw.Granular Biochar increased diversity of wetland microbial community and the abundance of denitrifying bacteria(Gemmatimonas,Longilinea).In summary,granular biochar is a good carbon source for denitrification,which can increase the nitrogen and phosphorus efficiency of secondary effluent purification in constructed wetland.Its application can reduce the problem of water eutrophication and low utilization rate of straw resources.