Study Of The Quantitative Analysis Method For Intestinal Infection Disease Burden Regarding Water Reuse

In recent years,water reclamation and reuse has become a important method to relieve water crisis.Treated wastewater has been widely reused for industrial,agricultural,municipal,landscape recreation and so on.However,a variety of contaminants can be found in the reclaimed water,of which the existence of pathogens which are in wide range of types and may leading to significa nt hazards has become the major limitation of water reuse.In order ensure the safety reuse of reclaimed water,a quantitative assessment of health hazard caused by typical pathogens existed in treated wastewater need to be conducted.Traditional method for hazard evaluation is implemented by four typical steps,including:hazard identification,exposure analysis,dose-response analysis and risk quantification,through which the probability of health hazard attributed to typical pathogens,can be quantified,and on this basis,the safety of water reuse can be assessed.However,the probability of health hazard cannot be used to interpret the health loss as well as the severity of health damage,and also cannot be used to quantitative describing the comprehensive effect on human physical/mental health and social activities either.Therefore,on the basis of traditional risk probability assessment,other health indicators need be used to extend the evaluation of health hazard caused by typical environmental pathogens.Disability adjusted life year(DALY)is a health indicator proposed by the World Health Organization(WHO)to quantify disease burden.This indicator can be used to evaluate the healthy life loss due to specific health outcomes,including disability and death after disease outbreaks.Since the exposure of pathogens in the environment can also lead to morbidity outcome,DALY can also be used to quantify the disease burden caused by environmental pathogens,and the results obtained cannot only be used to determine major hazard factors as well as hazard transmission pathways,but also be used to quantitative evaluate the safety of water environment.In this study,the traditional risk assessment method as well as the disease burden quantification method is firstly reviewed,and on this basis,the evaluation method and theoretical system for quantifying infectious intestinal disease are put forward based on disability adjusted life year,specific to the health hazard caused by typical intestinal pathogens during water reuse.And meanwhile,for the absence of morbidity data during disease burden quantification,a morbidity quantification model that can be directly used for the calculation of DALY is further put forward.O n this basis,the morbidity quantification as well as the disease burden analysis method are applied for the quantification of health hazard caused by typical intestinal pathogens during practical water reuse case.Major research works and achievements are as follows:(1)The theoretical basis of disease burden quantitative analysis for waterborne infectious intestinal disease.Waterborne pathogens include:bacteria,virus,protozoan and parasite.The exposure and contact of environmental pathogens with human beings may lead to the outbreak and prevalence of waterborne diseases.The exposure dose for environmental pathogens can be calculated by a product of exposure concentration detected for specific pathogen,and the ingestion volume of pathogen carrier investigated for certain exposure route.The infection risk caused by pathogens can be estimated by using an exponential or Beta-Poisson model,and model parameters can be determined according to the result from fitting with experimental or disease outbreak data.The morbidity risk occurred after infection can be estimated by using the epidemiological investigatio n,human/animal experimental analysis,and model calculation methods,of which the model calculation method is used most widely and the result obtained is most reliable.The disease burden due to waterborne pathogen is usually calculated by ignoring the re lative rare death outcome,and only considering the health loss under morbidity outcome,therefore,disease burden can be calculated by a multiplication of morbidity,disability weight and disease duration.According to the WHO suggested disease burden acceptable threshold 10~-66 DALYs/person/year,the disease burden calculation result can be compared with this reference value,and therefore,the safety of water environment can be judged objectively.(2)Study of morbidity calculation method for infectious intestinal disease for water reuse.Several pathogens which are closely related to water reuse and studied sufficiently include:salmonella typhi,shigella and entervirus,of which salmonella typhi can lead to typhoid,shigella can result in bacterial dysentery and entervirus can cause respiratory tract,digestive tract,nervous system disease etc..Typical water reuse routes include:farm irrigation,urban landscaping,recreational replenishing and municipal reuses,as for which major contact pathways is inhalation.According to field survey result or referring to corresponding exposure standard,the method for exposure dose calculation for typical intestinal pathogen under various reuse pathways is discussed by using the inhalation exposure calculation model.Methodology for infection risk quantification for typical intestinal pathogen during water reuse is interpreted according to existed infection quantification models and corresponding parameters.Through disease development and disease affecting factors analysis,an morbidity model based on exposure dose is proposed and constructed in this study,and besides,model parameters for typical intestinal pathogens for water reuse are determined by using a maximum likelihood estimation method according to the obtained human experimental data.On this basis,the methodology for disease burden evaluation attributed to typical intestinal pathogens during water reuse is further discussed according to the DALY method and corresponding disease parameters.The quantitative calculation method constructed for the morbidity of infectious intestinal disease in this study can further extend the result of infection risk to morbidity risk for infectious intestinal disease,and therefore to realize the successful transforma tion of pathogen exposure data to disease burden quantification data.(3)Q uantitative analysis of disease burden for infectious intestinal disease for a practical water reuse case.Taking the secondary effluent of Xi'an Bei-Shiqiao wastewater treatment plant as research object,the exposure concentration and distribution characters for typical intestinal pathogens,including:salmonella typhi,shigella and entervirus in the secondary effluent are studied referring to previous research data,and results show that the concentration of salmonella typhi,shigella and entervirus in the secondary effluent are in logarithmic normal distribution,and the 50%accumulated concentrations for the above pathogens are 3.1×10~2 copies/L,1×10~2copies/L and 1.4 copies/L respectively.Major reclaimed pathways for treated wastewater in Xi'an include:urban landscaping and road dusting,and major contact routes is inhalation.Through exposure dose calculation,morbidity analysis and disease burden calculation,the disease burden for secondary water reuse in Xi'an is quantified in this study.Results show that disease burdens caused by salmonella typhi and entervirus in the secondary effluent are acceptable compared with the WHO threshold,while the disease burden caused by shigella are beyond the standard,therefore health hazard may arouse due to secondary water reuse,and disinfection treatment need to be further implemented.According to estimations,under 90%~99%safety levels,the log removal requirement for typical intestinal bacteria in the secondary effluent is 3~4-log,while for entervirus,the log removal requirement is 5~8-log.Besides,according to the disease burden quantification results for secondary effluent treated by different membrane separation effluents,the disease burdens caused by typical intestinal pathogens in ultrafiltration,nanofiltration and reverse osmosis effluent are all below the WHO threshold,while in microfiltration effluent,the disease burden is beyond the standard,therefore health hazard may arouse due to microfiltration water reuse,and disinfection treatment need to be further implemented.According to estimations,under90%~99%safety levels,the log removal requirement for typical intestinal bacteria in the microfiltration effluent is 3~4-log,while for entervirus,the log removal requirement is 3~5-log.(4)Application and perspective discussion for disease burden quantifica tion method.In previous studies disease burden analysis is mainly applied to global and national disease burden statistical calculations,ranking calculation of regional disease burdens,and tracking assessment of disease burden due to specific diseases etc..Since the exposure of pathogen in the environment can also lead to disease burden,DALY is also applied to the assessment of water environment safety in recent years.This method is conducted by taking pathogen exposure as starting point,and transforms the pathogen exposure data to disease burden quantification data through exposure dose calculation and morbidity analysis,and on this basis to quantitative evaluate the disease burden caused by specific environmental pathogens through DALY calculation.Compared with the results obtained from traditional risk analysis,disease burden assessment is a deepening and expansion of the traditional risk probability study.As for the DALY method,the safety of water environment is assessed by taking health loss as endpoint,therefore the results obtained from DALY analysis are much more reliable and rational.In future studies of environmental burden of disease,DALY can also be applied for the quantification of health hazard for other environmental areas,such as atmosphere and soil,as well as for other contaminants,such as chemical substances.According to the disease burden quantification results,a cost-benefit analysis specific to different interventions can be further conducted for calculating the economic cost for avoiding each case of disease burden under different interventions.And through comparison,the optimal resource allocation scheme can be finally determined,and on this basis to provide scientific guidance for environmental hazard management.