Removal Mechanisms Of Opportunistic Pathogens In Secondary Effluent By Slow Filtration

The secondary effluent water of urban sewage treatment plant contains a large number of opportunistic pathogens,which pose a potential threat to human health.In this paper,the secondary effluent was taken as the treatment object to explore the removal efficiency of the slow filtration process on opportunistic pathogens（Pseudomonas aeruginosa,Legionella and Mycobacterium avium）,E.coli and organic matter under different operating conditions(different filtration speed,influent C/N ratio,Ca²⁺ concentration,pH value)in water.Quartz crystal microbalance technology was used to monitor the deposition process of pollutants on the surface of slow filtration biofilm,and the community structure of microorganisms on slow filtration surface under different conditions was analyzed.The results showed that under different filtration rates,slow filtration of biofilm had better removal effect on opportunistic pathogens than that without biofilm,and the optimal filtration rate was 5 cm/h.Under the conditions of optimal filtration rate and changing influent water quality,the removal effect of slow biofilm filtration on opportunistic pathogens was greater than that without slow biofilm filtration,and there was an optimal value of c/N ratio,Ca²⁺ concentration and pH value in the influent of slow biofilm filtration.When c/N ratio was 10,Ca²⁺ concentration was 60 mg/L and pH value was 7,slow filtration of biofilm had the best removal effect on opportunistic pathogens and E.coli.In addition,under different operating conditions,the removal effect of DOC in water by biofilm slow filtration is greater than that without biofilm slow filtration,and under different operating conditions,the removal effect of different types of organic matter was different.At the optimal filtration rate,the removal effect of biofilm slow filtration was mainly microbial metabolites and humic acid.The main substances removed were tryptophan,fulvic acid and humic acid.When the optimum Ca²⁺ concentration was 60 mg/L,the main substances removed by slow filtration were tryptophan and humic acids.At the optimum pH value,the main substances removed by biofilm slow filtration are tryptophan,fulic acid and humic acid.Under the optimal operating conditions,the content of opportunistic pathogens and DOC were positively correlated.Except for the Ca²⁺ concentration in water,the removal of opportunistic pathogens by slow filtration of biofilm was positively correlated with E.coli under other operating conditions.When the filtration rate was 5 cm/h and the C/N ratio was 10,the deposition of pollutants in secondary effluent on the surface of slow filtration biofilm was mainly due to the adhesion of extracellular polymer.When the concentration of Ca²⁺ in water was 60 mg/L and pH value was 7,the deposition process of pollutants on the surface of biofilm was mainly due to microbial adhesion.Under the conditions of lower filtration rate,lower C/N ratio of influent water,lower Ca²⁺ concentration and neutral pH value,the diversity of biofilms on filter material surface was higher and the species composition was more uniform.Under different filtration rate conditions,the aerobic bacteria such as Planctomycetes,Nitrospira and Novosphingobium were dominant in the slow filtration surface biofilms,and the proportion of these microorganisms was the highest when the filtration rate was 5 cm/h.When the influent C/N ratio was 10,the dominant bacteria,such as Rhizobiales and Planctomycetes,accounted for a large proportion;When Ca²⁺ concentration was 60 mg/L,dominant genera such as Anammox,Proteobacteria and Bacteroidetes accounted for a larger proportion.When the pH value was 7,the dominant genera such as Nitrospira and Novosphingobium accounted for a larger proportion in the biofilms on the filter material surface.Under different operating conditions,the bacteria such as Anammox,Nitrospira,Novosphingobium,Rhizobiales,Bacteroidetes and Acidobacteria in the biofilm of filter material surface can inhibit the growth of opportunistic pathogens to a certain extent,which was conducive to the reduction of opportunistic pathogens.