Synergistic Control Of Drinking Water Disinfection By-products And Opportunistic Pathogens Based On Modified Quartz Sand Filtration

Chlorine disinfection is an important way to kill fecal microorganisms in water,but chlorine also can react with natural organic matter(NOM)and new type organic pollutant in drinking water sources and produce toxic disinfection by-products(DBPs).Compared to the carbonaceous disinfection by-products(C-DBPs),nitrogenous disinfection by-products(NDBPs)are more biological toxicity and carcinogenicity.Bacteria in suspended biofilm and their secretions can act as precursors of N-DBPs,and the removal ability of conventional water treatment process is relatively limited.Opportunistic pathogens(OPs)have strong chlorine resistance and can grow and reproduce under adverse conditions.In this study,the maximum formation potential of N-DBPs in the effluent of the sand filter was greatly reduced by modifying the quartz sand filter media,and the modified quartz sand also had an inhibitory effect on the opportunistic pathogens.In this study,the microbial community structure and biofilm morphology in filter and effluent of sand with different interfaces were characterized in detail.The relationship between biofilm morphology and the maximum generation potential of disinfection by-products and the growth of conditional pathogens in effluent was studied.The mechanism of collaborative control of disinfection by-products and conditional pathogens in drinking water by modified quartz sand filtration was discussed.This study is of great significance to improve the effluent quality of sand filter and reduce the burden of subsequent water treatment process.The main research contents and conclusions are as follows:Firstly,iron modified quartz sand(Fe-sand)filter was synthesized,and a laboratory-scale simulated sand filter was operated,and compared with ordinary quartz sand.The formation potential of DBPs was measured by Krasner method,which confirmed that the addition of iron effectively inhibited the formation potential of N-DBPs in the effluent.The inhibitory effects of Fe-sand on the formation of halogenated nitromethane(HAc Ams),haloacetamide(HNMs)and haloacetonitrile(HANs)were increased by 51.51%,43.66% and 90.6%respectively.The gene copy numbers of OPs were detected via quantitative q-PCR,and it was found that Fe-sand had obvious inhibitory effect on OPs.By characterizing the extracellular polymer substance(EPS)on the surface of filter and in the effluent,it is found that the distribution of biofilms on the surface of the Fe-sand filter media was uniform,not likely to fall off,and more stable.The suspended biofilms in the effluent were more difficult to aggregate and the chlorine disinfectant can easily penetrate it.Fe-sand effectively reduces the risk of water quality in the effluent.Secondly,graphene oxide modified quartz sand(GO-sand)was synthesized,and the laboratory-scale sand filter was also operated.Compared with the effluent of sand,GO-sand reduced the activity of microorganisms in the effluent,the content of typical OPs in the effluent also decreased significantly.The protein content of EPS in GO-sand effluent was high,while the polysaccharide content was low.EPS was relatively cohesive and was an important precursor of N-DBPs,chlorine disinfectant is difficult to penetrate completely.Instead,the maximum generation potential of HAc Ams,HNMs and HANs in the effluent of GO-sand was greatly increased by 118 %,263.64 % and 132.27 %.GO-sand significantly altered the microbial community structure and the characteristics of the EPS in the effluent,resulting in significant chlorine consumption in the downstream drinking water treatment system,and therefore modified materials similar to GO-sand are controversial.Finally,based on Fe-sand,iron-graphene oxide co-loaded quartz sand(Fe GO-sand)was synthesized,and a laboratory scale sand filter was also operated.The effluent quality of Fe GO-sand was further improved,and the stress of microorganisms on the surface of graphene oxide was reduced.Fe GO-sand had the best inhibitory effect on the formation potential of N-DBPs in the effluent of sand filter.The iron element in Fe GO-sand counteracts the stimulation of graphene oxide to microorganisms,and the effluent quality of the reactor had been further improved.Fe GO-sand has the best inhibition effect on the maximum formation potential of N-DBPs in the effluent of sand filter.The formation potential of HAc Ams,HANs and HNMs in the effluent of Fe GO-sand was 85.21%,82.27% and 78.79%,lower than that of the effluent of sand,respectively.The EPS in the effluent of Fe GO-sand was relatively cohesive and the chlorine disinfectant can penetrate relatively easily,providing a more unfavorable growth environment for OPs,so Fe GO-sand also had an overall inhibitory effect on OPs in the effluent.Fe GO-sand filter had stronger ability to bound microorganisms.Compared with sand,Fe GO-sand had a certain shaping effect on the microorganisms in the effluent,and the microbial community in the filter of Fe GO-sand was closer to the raw water.It is a more reasonable technical idea to optimize the common quartz sand and then promote the subsequent water quality safety of drinking water.