Dissertation > Medicine, health > Preventive Medicine,Health > Environmental health,environmental medicine > Water and water supply sanitation

Regrowth Characters and Risk Assessment of VBNC (Viable But Non-culturable) Pathogens in Reclaimed Water System

Author LinZuoZuo
Tutor HeMiao
School Tsinghua University
Course Environmental Science and Engineering
Keywords viable but non-culturable (VBNC) pathogen reclaimed water regrowth risk assessment
CLC R123
Type Master's thesis
Year 2013
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Guarantee of reclaimed water quality has been a key issue to promote the use ofreclaimed water and to achieve the sustainable utilization of water resources.Pathogens can enter into a viable but non-culturable state (VBNC) under disinfectionstress, which is unlikely to be detected by conventional culturable method, but stillretain the original virulence factors and pathogenicity. The pathogens in VBNC statecan regrow and reproduce after releasing disinfection pressure, resulting in microbialhealth risks of reclaimed water.This study has established two detection methods of viable pathogen respectivelybased on reverse transcription activity and cell membrane integrity, investigatedcharacteristics of existence, removal and regrowth of pathogens across reclaimed watersystem, explored the key factors related to the regrowth, analyzed the variations ofpathogenic community by454Pyrosequencing technique, and evaluated the microbialhealth risk of reclaimed water for municipal uses. Based on this, control methods ofVBNC pathogen regrowth were proposed.The results have demonstrated that the influent concentrations of culturable E.coli,Enterococcus spp., Salmonella spp. and Mycobacterium spp. were3~5Log CFU/100mL, the effluent concentrations of E.coli, Enterococcus spp. and Salmonella spp. inVBNC state after tertiary treatment were about1.5~3.8Log CE/100mL, accountingfor40~100%of total viable pathogens. Mycobacterium spp. showed a relativelystronger resistance to disinfectants and had a concentration of1Log CE/100mL inVBNC state, accounting for20%of total viable cells. Ultrafiltration membrane processhad1Log removal rate for both culturable and viable pathogens, Disinfection processhad2~3Log removal rate for culturable pathogens and only had <1Log removal ratefor viable cells. Besides, this study indicated that disinfection has little removalefficiency for Mycobacterium spp.A large number of pathogens have regrown thought the storage and distributionsystem, the regrowth rate of four pathogens has reached0.3~2.9Log. Pathogenconcentrations correlated with the disinfectant residual, and chlorine residual of>1.5mg/L possessed stronger inactivation for E.coli, Enterococcus spp.and Salmonella spp.Organic matters and nutrients promote the regrowth of pathogens in distribution system; however, there is no remarkable correlation between nutrients and regrowth.The diversity and variation of biological community in reclaimed water systemwere investigated by means of454Pyrosequencing. The results indicated these areconsiderable changes of microbial communities across disinfection process, storageand distribution. In the end of distribution system, Proteobacteria is the dominantphyla including a great number of pathogens such as Burkholderiales. Based on theresults of Canonical Correspondence Analysis (CCA), it was found that TN, TP,NO3-N and TOC are important impacts on the microbial community.The health risks assessments of reclaimed water for municipal use were carriedout. The relationship between annual health risks value and pathogen concentrationswas identified. In the effluents of reclaimed water, the potential risk values inconsideration of VBNC pathogens is likely to be2orders of magnitude higher thanthat by conventional culturable detection method. After storage and distribution,potential health risk have become real health risk, thus microbial health risks ofrecycled water will be increased.

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