Performance And Mechanism Of Biological Contact Oxidation Using Modified Composite Fillers For Enhanced Removal Of Nitrogen And Phosphorus In Micro-polluted River

Strengthening the ecological functions of water bodies and the deep improvement of river water quality is a national strategy for the construction of Beautiful China.This study aimed to investigate the performance of biological contact oxidation technology packing with different conventional combined filters(i.e.,polyurethane,sponge iron and fiber balls)for treating river water from the micro-polluted section of the Fuyang River.Based on the performance of different biological systems with different filters on the target pollutants,the selected fillers were further modified,and the biological contact oxidation systems constructed by the modified fillers was investigated for their removal ability of target pollutants,ability of resistance to water quality shocks and temperature changes.In addition,the microbial population structure and diversity was investigated to explore the mechanism of the removal of target pollutants.ASM3-SMP model was established to underline the conversion and utilization of organic matter by the biological contact oxidation system.The main conclusions obtained are as follows:M-K model and single factor analysis methods were used to analyze the water quality distribution and temporal and spatial characteristics of the test reach.The results showed that the COD_Cr concentration of each monitoring section of the test reach had a large interannual change from 2018 to 2020,and the ammonia nitrogen(NH₄⁺-N)and total nitrogen(TN)concentrations had small inter-annual changes,showing an overall upward trend.The chemical oxygen demand(COD_Cr),NH₄⁺-N,TN and TP pollutant indicators exceed the"Surface Water Environmental Quality Standards"(GB3838-2002)IV standards for water bodies,and the pollution degree of the main pollutants is:TN>TP>NH₄⁺-N>COD_Cr.Biological contact oxidation systems composed of fiber balls,polyurethane and sponge iron,respectively,combined with polyethylene was used to remove nitrogen and phosphorus pollutants from the micro-polluted river.The results showed that the polyurethane composite packing system had the best denitrification performance,with the removal efficiency of NH₄⁺-N and TN of 85.28%and 65.51%,respectively;the sponge iron system achieved the highest removal rate of TP(88.79%).The polyurethane and polyethylene materials were modified by chemical oxidation-gelatin protein grafting method and chemical oxidation-iron ion loading method respectively.Modification analysis results showed that ions are loaded on the surface of the polyethylene tennis ball,and the surface roughness,specific surface area and structural depth of the polyurethane material increase,and the hydrophilicity of the two fillers is enhanced.Compared with before the modification,the systems with modified polyurethane and polyethylene shortened the start-up periods from 20days to 13 days.Under the conditions of the filling ratio of 50%and dissolved oxygen(DO)concentration of 4 mg/L,the removal efficiency of NH₄⁺-N,TN,TP and COD_Crwere increased by 10.08%,9.89%,36.93%and 17.84%,respectively,compared with those before the modification.The TN concentration of effluent was only 1.60 mg/L.The ability of the system to resist water quality shocks and temperature changes was also enhanced.Compared to the microbial diversity of filters before modification,the microbial diversity on filters after modification was obviously higher,with the relative higher Ace,Chao and Shannon indexs.Proteobacteria,Firmicutes and Nitrospirae were the dominat phlya.The microbial interactions of functional microbes and microbial population structure were investigated.Results showed that the abundance of functional bacteria,e.g.,ammonia oxidation bacteria,nitrite oxidation bacteria,denitrification bacteria and phosphorus-accumulating bacteria,of modified polyurethane and polyethylene fillers increased obviously.The electron transfer processes and pathways of biofilm extracellular polymer(EPS),microbial metabolites(SMP)and X_STO are coupled to construct an ASM3-SMP model of organic matter degradation.The sensitivity of the simulation values of different components to parameter changes is arranged as follows:?_H>k_UAP>Y_H>K_O in UAP;khyd in BAP;?_H>k_EPS>Y_H>K_O>?_EPS>Y_EPS>K_EPS,?_H>k_STO>Y_Hin EPS.The test results with glucose as the substrate are calibrated with the model(r²>0.85).The main flow of external carbon sources during the removal of nitrogen and phosphorus pollutants by microorganisms attached to biofilms was:UAP of 9%,EPS of 33%,and X_STO of 21%,DO of 7%,and new cell synthesis of 30%.This result showed that the proportion of microbial organic matter synthesis and metabolism was high,and the assimilation and carbon source storage capacity were strong,which was beneficial to promote the removal of nitrogen and phosphorus.The paper has 62 pictures,43 tables,and 212 references.