The Safety Status And Risk Assessment Of Microorganisms In The Drinking Water In A Province

Objectives: The aims of this study were to detect indicator bacteria(total coliforms, Escherichia coli, thermotolerant coliforms and heterotrophic plate counts), pathogenic microorganisms(Salmonella and E. coli O157) and some physicochemical indices(pH, turbidity, conductivity, COD, dissolved oxygen and residual chlorine) in the raw water, finished water and tap water of Z and X drinking-water plants in a province in central country, and to analyze the correlation between indicator bacteria and pathogenic microorganisms. The safety status of indicator bacteria was evaluated and the infection risk assessment was calculated on the basis of the level of Salmonella and Escherichia coli. The quantitative microbial risk assessment(QMRA) was applied as a tool for prediction and prewarning of microbiological safety of drinking-water.Methods: In this study, FilmplateTM Salmonella test and FilmplateTM E. coli O157 test were applied to detect the concentration of Salmonella and E. coli O157 in the raw water, finished water and tap water of Z and X drinking-water plants respectively; The pH, turbidity and conductivity were detected by the methods described in?GB/T5750.4-2006 Standard examination methods for drinking water-Sensory and physical parameters?; The dissolved oxygen was detected by luminescence dissolved oxygen(LDO) method; The residual chlorine was detected by N, N-diethyl-p-phenylenediamine(DPD) method; The COD was detected by USEPA digestion and colorimetry; The level of total coliforms, Escherichia coli, thermotolerant coliforms were detected by enzyme substrate technique described in ? GB/T5750.4-2006 Standard examination methods for drinking water-Microbiological parameters?, the heterotrophic plate counts was detected by the plate count method. These results were compared with ?GB3838-2002 environmental quality standard for surface water? and ?GB 5749-2006 Standard for drinking water quality?, the sample would be unqualified if any results of the tested parameters failed to meet the standard. The correlation between the indicator bacteria and pathogenic microorganisms was determined by Spearman rank correlation analysis; the method of quantitative microbial risk assessment(QMRA) was used to calculate the infection risk of Salmonella and Escherichia coli.Results: 1. The raw water for X drinking-water plant attained the grade ? of ?GB3838-2002 environmental quality standard for surface water? based on the result of thermotolerant coliforms ? pH?dissolved oxygen and COD. The raw water from Z drinking-water plant sampled in July, September, October and November 2015 attained the grade ?, while samples in June, August, December 2015 and January 2016 attained the grade ?, and samples in February 2016 attained the grade ?.2. 3~52 CFU/ml of Salmonella were detected in the raw water for X drinking-water plant collected in June 2015~December 2015; 18~1.02×102 CFU/ml of Salmonella were also detected in the raw water for Z drinking-water plant collected in June 2015~February 2016, and the Salmonella were not detected in the finished water and tap water from X and Z drinking-water plants. Meanwhile, the E. coli O157 were not detected in the raw water, finished water and tap water from X and Z drinking-water plants.3. According to the?GB 5749-2006 Standard for drinking water quality?, the failure rate of the finished water and the tap water from X drinking-water plant was 44.44% and 94.44% respectively, the failure rate of the finished water and the tap water from Z drinking-water plant was 11.11% and 22.22% respectively. The main indices that failed to attain the standard were total coliforms, residual chlorine and turbidity.4. By Spearman rank correlation analysis, the positive correlations were found between Salmonella and total coliforms(n=18, r=0.570, p<0.05), between Salmonella and heterotrophic plate counts(n=18, r=0.484, p<0.05), and the positive correlation was also found between total coliforms and heterotrophic plate counts(n=18, r=0.885, p<0.01) in the raw water from X drinking-water plant. There was a significant positive correlation between thermotolerant coliforms and Escherichia coli(n=18, r=0.515, p<0.01) in the raw water from Z drinking-water plant.5. On the assumption that there was no secondary pollution, the probabilities of Salmonella infection by drinking the unboiled tap water would be 0~2.97×10-4 and 0~2.18×10-4 for Z and X drinking-water plants respectively at the disinfection efficiency of 99.9999%. When the disinfection efficiencies of the two drinking-water plants were lower than 99.99999%, the probabilities of Salmonella infection by drinking the tap water from Z and X drinking-water plant would fail to attain the standard proposed by EPA(10-4). The probabilities of E.coli infection by drinking the unboiled tap water would be 0~3.19×10-4 and 0~2.41×10-4 for Z and X drinking-water plants respectively at the disinfection efficiency of 99.9%. When the disinfection efficiencies of the two drinking-water plants were lower than 99.99%, the probabilities of E. coli infection by drinking the tap water from Z and X drinking-water plant would fail to attain the standard proposed by EPA.Conclusions: During the sampling period, the level of Salmonella is 0~102 CFU/ml in the raw water from Z and X drinking-water plants, meanwhile, E. coli O157 are not detected. The quality of the raw water from X and Z drinking-water plants meets the requirements of surface source for central drinking-water, and the raw water for X drinking-water plant is classified into grade ?, the raw water for Z drinking-water plant changes between grade ?~?, the quality of the raw water for X drinking-water plant is better than that for Z drinking-water plant. There are positive correlations between Salmonella and total coliforms, Salmonella and heterotrophic plate counts and between total coliforms and heterotrophic plate counts in raw water for X drinking-water plant. Escherichia coli is positively correlated with heterotrophic plate counts in raw water for Z drinking-water plant. The qualification rate of the finished water from Z drinking-water plant is higher than that from X drinking-water plant. There may be some problems in the distribution system of X drinking-water plant, resulting in a higher failure rate of the finished water from X drinking-water plant. When the disinfection efficiencies of the two drinking-water plants are lower than 99.99999%, the probabilities of Salmonella infection by drinking the unboiled tap water from Z and X drinking-water plant would fail to attain the standard proposed by EPA. When the disinfection efficiencies of the two drinking-water plants are lower than 99.99%, the probabilities of E. coli infection by drinking the unboiled tap water from Z and X drinking-water plant would fail to attain the standard proposed by EPA. The results of quantitative microbial risk assessment could be used to predict the microbial risk effectively and help to protect population health by taking preventive measures.