Microorganisms Risk And Control In Drinking Water System

Providing high-quality and safe drinking water is essential to protecting public health.In the drinking water supply system,the drinking water is usually purified through the treatment processes such as sedimentation,filtration,and disinfection to meet“standards for drinking water quality”（GB5749-2006）before entering drinking water distribution system,and finally reaches the end tap of the user.A series of physical,chemical and microbiological processes will occur in drinking water distribution system,which will change the water quality.An important process in drinking water distribution system is that microorganisms use organic matters to regrowth and form biofilms.This research explores effect of different water treatment processes on removal of different microorganisms and influence of different water quality parameters on the growth of microorganisms in drinking water distribution system.Basis on the above,measures to control microbial risks are proposed.The main research contents and conclusions are as follows:（1）Water samples collected from the water treatment process of 5 water plants,the concentration of microorganisms was analyzed by using q-PCR quantitative and plate counting methods to revealing the removal of microorganisms by different water treatment processes and the influence of water quality parameters on them.The results show that:microbial growth will occur after the filter tank,which may be due to the fact that the filter media not only adsorbs organic matter,but also provides the substrate and carrier for microbial growth.Microorganisms multiply and grow in large quantities with organic matter as nutrients to form biofilm.In addition,the water treatment process to reduce microorganisms is mainly sedimentation and disinfection process.Microorganisms have a positive correlation with the particles number and turbidity.The sedimentation process was mainly to reduce the turbidity and destroy the attachment points of opportunistic pathogens（OPs）in the water by removing particulate matter in the water,so that the OPs can be reduced.Both coagulation sedimentation and mechanical pond sedimentation can effectively remove the total bacterial 16S rRNA,while coagulation sedimentation can better remove Legionella spp.,and mechanical pond sedimentation can better remove Mycobacterium spp.and Acanthamoeba spp..There was a negative correlation between microorganisms and disinfectants.Disinfection is an effective measure to control different microorganisms,including OPs.Disinfection is mainly to destroy the cell wall and cell enzymes of bacteria through disinfectants,thereby destroying its original system structure and causing it to lose activity and die.（2）Through the comparison of the microbial concentration of finished water with tap water,it was found that even though the high-quality water was obtained,the phenomenon of microbial regeneration will appear after the distribution system.In the tap water,7.14%（7/98）of the samples appeared HPC>500 CFU/mL,and 16S rRNA,Mycobacterium spp.,Legionella spp.and Legionella pneumophila increased significantly（P<0.05）.In addition,the detection rates of Legionella spp.,Pseudomonas aeruginosa and Harmanella vermiformis were all 100%.The OPs of samples with HPC>500 CFU/mL were higher than those with HPC≤500 CFU/mL.However,through the annual risk assessment of Mycobacterium avium infection,the annual risk infection rate was lower than the WHO benchmark value of 10^-3.（3）Maintaining residual disinfectant is a main method to control microorganisms in the drinking water distribution system.In the sodium hypochlorite disinfection area,the concentration of HPC was less than 500 CFU/mL for samples with free chlorine residual greater than 0.05 mg/L.In the chloramine disinfection area,the concentration of HPC was less than 500 CFU/mL for samples with total chlorine residual greater than0.35 mg/L.Maintaining the free chlorine residual between 0.15 and 0.20 mg/L,and the total chlorine residual between 0.35 and 0.50 mg/L,can control the opportunistic pathogens growth.The particles number should be controlled below 300 in order to control M.avium,L.pneumophila,and Acanthamoeba spp.growth.The corresponding turbidity should be controlled at 0.25～0.35 NTU.