A Study Of Potential Microbial Risk Of Drinking Water In One Of China’s Eastern Coastal Cities

The safety of drinking water plays an important role in the development of city.The new standard for drinking water(GB5749-2006)published by the Ministry of Health was implemented on 1st,July in 2007.According to the standard,the number of water-quality index has increased from 35 to 106 and the microorganism items have increased from 2 to 6.The total coliform group indicator was revised for the higher safety demand of drinking water.So,it is necessary to do the urban drinking water microbiological risk assessment study and then organize effective risk management.In this study,the total coliform group was selected as indicator bacteria and the microbial potential risk in a city waters supply system in a southeast of China was investigated by using kinds of methods including quantitative microbiological risk assessment(QMRA),log-normal distribution and Monte Carlo simulation.The evaluation model in this study is useful for the risk assessment of pathogenic bacteria of drinking water pipe network and the results provide good ideas for the risk management of drinking water in the city.The conclusions are the following:i)The microbial potential risk in the city drinking water supply system investigatedThe QMRA method was used to analyze the microbial potential risk in different drinking water treatment systems.The results showed that the water during the whole drinking water treatment systems had some microbial risk based on results of the total coliform group.The risk degree varied in different systems and different seasons.The microbial risk happened in the in ozone generating pool is the highest.The treatment efficiency of the pretreatment process is higher in winter than summer,while the treatment efficiency of the conventional treatment process is opposite.The total coli group was not detected in the finished water.Moreover,some other pathogenic bacteria including epidermis staphylococcus and bacillus canthracis were also detected in the treatment systems.It suggested that the city water supply system has some potential infection risk and it is helpful to perform some measures to control the epidermis staphylococcus and bacillus anthracis.ii)The effect of pipeline characteristics of urban pipe network on the drinking water microbial potential risk investigatedThe effect of pipe material(ductile iron pipe,galvanized pipe,stainless steel pipe,grey cast iron,plastic pipe),pipe age and pipe diameter on the potential risks of drinking water in pipe network was analyzed through the QMRA method.The results showed that:the ranking of annual infection risk of pipe material are:ductile iron pipe>galvanized pipe>stainless steel pipe>grey cast iron>plastic pipe.The pipe age also played a great role in the annual total coliforms group infection risk and the potential risk increased with the rising of pipe age,especially for the pipes with age of over 10 years.The pipe diameter also had effect on the potential risks of drinking water in pipe network.The infection risk was stable when the pipe diameter was lower than 80mm,while the infection risk decreased with pipe diameter when the pipe diameters were higher than 150mm.iii)The microbial potential risk of drinking water pipe network system of the southeast coastal city determinedThe log-normal distribution and Monte Carlo simulation with QMRA method were used to assess the microbial potential risk in drinking water pipe network system in normal working and adverse conditions.The results showed that the percentage of samples reaching to standard was accounted for 83.3%.The samples with lower water quality were all from pipes with high pipe age.The whole condition of drinking water pipe network simulated by the 30 samples can reduced the randomicity of sampling and the cumulative probability curve indicated that the percentage of samples reaching to standard is 75%.The annual risk of Coli group infection was simulated by Monte Carlo simulation based on the annual risk limitation of lower than 0.01%and the attainment rates were 70%.However,under the retention and hydraulic erosion condition,the attainment rates were less than 20%and the potential risk significantly increased.The average annual risk of Coli group infection under hydraulic erosion condition up to 7.1×10-2,caused 1-2 orders of magnitudes higher than retention condition which value is 2.8×10-4,which had more great impact on the drinking water safety in pipe network.