The Formation Mechanism And Regulation Of Biofilm In Drinking Water Distribution System (DWDS)

The existence of biofilm in drinking water distribution system?DWDS?will lead to series of drinking water hygiene problems including the deterioration of drinking water quality,pipeline corrosion and the recovery of damaged microorganisms.Therefore,the regulation of biofilm in DWDS is really important for the water security of urban residents.To explore the formation mechanism of biofilm and find economic and efficient control measures in DWDS,heterotrophic plate count?HPC?,adenosine triphosphate?ATP?assay,flow cytometry?FCM?and high-throughput 16S rRNA sequencing were used for analyzing the formation mechanism and active microbial distribution of biofilm on different pipelines.What's more,the regulation mechanism of biofilm by quorum-sensing?QS?was studied.Studies on biofilms of different pipelines showed that the active microbial biomass per unit area of PVC pipe wall reached the peak first,followed by PPR and STS.The total number of microorganisms,total ATP density and culturable microbial biomass per unit area of the three pipes all showed PVC>PPR>STS,in other words,STS was the most difficult pipeline for microorganisms to grow.However,the proportion of active microorganisms was the highest on STS,it was up to 95.52%.The content of ATP per living cells was 4.7 10^-1919 g·cell^-1?RSD=0.02?and stable in all biofilm samples.Compared with PVC and PPR,the microorganisms on STS had the least total and active microorganism species,the lowest diversity and the simplest community structure.The higher proportion of active cyanobacteria and actinomycetes on PVC and PPR is more likely to cause odor problems in the process of distribution.Compared with RNA-based high-throughput sequencing,conventional DNA-based sequencing methods can identify more species categories,but the information contains partial inactive extracellular free DNA,so it is unable to accurately assess the risks caused by active microorganisms in drinking water.The RNA-based high-throughput sequencing method can accurately obtain the composition and structure of active microorganisms on the biofilm,and determine functional value of active microorganisms to drinking water.Studies on biofilm removal by different flushing methods showed that both water flushing and gas rich water flushing could remove 99%of biofilm and the drinking water quality could be improved accrodingly,but it could not be maintained for a long time.The mechanical removal of the biofilm on the pipe wall could be maximized by medium rate flow combined with gas-water mixed flushing so that the risk of drinking water caused by the biofilm could be reduced.Results of isolation and identification showed that the bacterium with flushing-resistance in DWDS were Bacillus sp.,Pseudomonas sp.,Microbacterium sp.,Mycobacterium sp.and Rhodanobacter sp.The extracellular polymers?EPS?contents of these strains and the ratio of extracellular polysaccharide to extracellular protein were both much higher than that of the isolated without flushing-resistance.In addition,C6-HSL was detected in all flushing-resistant but not in flushing-nonresistant strains using HPLC-MS.The effects of exogenously added C6-HSL and vanillin?quorum-sensing inhibitors,QSI?on EPS showed that C6-HSL and vanillin could promote and inhibit the production of extracellular proteins and extracellular polysaccharides of flushing-resistant bacteria respectively,what's more,extracellular polysaccharides could sense the lower concentration and produce more sensitive and significant changes.Therefore,C6-HSL is a key signaling molecule that regulates the synthesis of EPS and thereby affects the microbial tolerance on flushing in DWDS was speculated in this study.