Feasibility Assessment And Performance Exploration Of Algal-bacterial Synergistic Treatment For Municipal Wastewater

China’s water shortage and water environment pollution problems are becoming increasingly prominent,and wastewater reuse is an important way to alleviate these problems.The traditional municipal wastewater treatment technology is difficult to find a balance between green energy saving and efficient pollutant degradation.The development of wastewater treatment technology based on the symbiosis of bacteria and algae has more advantages,but its application still requires low intensity aeration and harsh operating conditions,which limit the development of this technology.In this paper,the activated sludge was mixed with algae,and an open biofilm reactor without external oxygenation was established based on combined filler,polyurethane filler and K2 polypropylene filler,and the optimum biofilm carrier filler was selected by examining the pollutant removal efficiency and biomass accumulation.Single-factor experiments were conducted to analyze the effects of different light-to-dark ratios,influent concentrations and hydraulic retention times（HRT）on the pollutant removal efficiency and operating conditions of the reactor under the optimum carrier filler.The mechanism and operation strategy of bacterial-algal symbiosis were investigated in combination with the changes of biofilm microbial community structure and composition of extracellular polymer（EPS）.The results of the study showed that:（1）At the end of the start-up phase,all three carriers could successfully hang the film,and the highest biomass,the best pollutant removal performance and the most stable operation status were achieved in the reactor of group A（combined packing）.The accumulated biomass and chlorophyll concentration on the surface of carrier packing reached 0.00978 g/cm³and 1698.69μg/g·VSS,respectively.90.03%,70.58%and 92.11%of TOC,TN and TP were removed,respectively,and the dissolved oxygen concentration ranged from 2.5-3.5 mg/L.（2）In the experiments of optimizing the operating conditions of group A reactor,it was found that increasing the light duration and extending the HRT within a certain range could improve the pollutant removal efficiency,and the best pollutant removal effect could be obtained for low concentration feed water（COD:150mg/L,TN:25mg/L,TP:3mg/L）at HRT=24h and light-dark ratio of 12h:12h,TOC,TN,NH₄⁺-N and TP steady operation removal efficiency mean values were 93.00%,79.31%,94.20%and 95.31%,respectively.The mean values of removal efficiencies for TOC,TP,TN and NH₄⁺-N in low concentration influent water were 89.43%,89.66%,58.52%and 78.32%,respectively,at HRT=12h and light-dark ratio of 6h:6h.In contrast,increasing the influent pollutant concentration for high quality concentration influent water reduces the reactor removal efficiency,the average removal efficiency of TP and TN is as low as 42.25%and 42.45%,respectively.（3）In a characterization study of bacterial-algal biofilm EPS,it was found that polysaccharides were the main components in EPS and that the operating conditions changed the composition of EPS.The shortening of the light duration resulted in a significant increase in EPS concentration.The richness of the functional group composition of EPS increased after the bacteriophage symbiosis,representing an increase in the infrared characteristic peaks of proteins and polysaccharides,and the increase in the proportion ofα-helical protein secondary structures promoted the formation of bacteriophage biofilms.In addition,increased tryptophan-like protein substances（Ex/Em=230/325-330）were added to the bio EPS fluorescence spectra.Correlation analysis showed that the percentage ofα-helical structure in the secondary structure of EPS protein was positively correlated with the removal efficiency of TP and TN,while the irregularly curled structure was negatively correlated with the removal efficiency of TP and TN;there was a negative correlation between the increase of PN,TOC（EPS）and TN（EPS）and the removal rate of TN;the percentage of tyrosine-like and tryptophan-like substances was positively correlated with the removal rate of TOC There was a positive correlation between the percentage of tyrosine-like and tryptophan-like substances and the removal rate of TOC,while humic acid-like substances had the opposite effect.（4）In the microbial community analysis,S.obliquus were no longer the only algae species in the reactor,and the microalgae species were greatly enriched.The biofilm Alpha diversity was significantly higher than that of the initial inoculated sludge,and the biofilm-forming species（Terrimonas,Nakamurella）were formed.Meanwhile,the abundance of relevant functional strains in the system varied with the change of operating conditions.There was an increase of photosynthetic mycorrhizal algae（Roseifiexaceae,Chloroplast,etc.）and functional bacteria regarding nitrogen degradation（Nitrospira,Azospira,etc.）in the biofilm.This all indicates a more diverse pathway of pollutant removal in the system.In conclusion,the experimentally constructed bacterial-algae symbiotic biofilm system achieves efficient operation of the open reactor without external aeration conditions and can achieve efficient degradation of pollutants in municipal wastewater.