| Chlorine has been used in drinking water treatment for more than a hunderd years because of its high sterilization efficiency,perfect stability and low cost.It is still the most widely used disinfectant in the world and has made an important contribution to the water supply safety.Although chlorination can effectively inactivate most pathogenic bacteria,some bacteria are with strong resistance to chlorine and can grow even in the presence of residual chlorine.These bacteria are called chlorine-resistant bacteria.In recent years,the seasonal risk that the standard plate-count bacteria in finished water exceed the standard limit occurred in the drinking water plants of a city in southern China for many times,all of which were caused by chlorine-resistant bacteria.Some chlorine-resistant bacteria are pathogenic and then pose a serious threat to drinking water safety.Therefore,how to control the growth and reproduction of chlorine-resistant bacteria and reduce the risk of exceeding the standard limit of standard plate-count bacteria is a problem to be solved urgently at present.In this study,in response to the seasonal water quality risk caused by chlorine-resistant bacteria,the bacterial community and spore number in a water supply system of a city in southern China were analyzed,in order to identify the composition and distribution of chlorine-resistant bacteria.Three spore-forming bacteria isolated from a drinking water plant were used as typical chlorine-resistant bacteria,and their biological characteristics were analyzed.Based on that,the control technologies for chlorine-resistant bacteria were studied.In addition,relying on a drinking water plant reconstruction project in southern China,the formed key technologies were applied.The results showed as follows:The bacterial community indices,standard plate-count bacteria and spore number in wet season were higher than those in dry season in the water supply system.They displayed a decreasing trend along the water treatment processes,but a increasing phenomonon in the sand filter and carbon filter was observed.In the water treatment processes,the process units that reduced the bacterial community indices,standard plate-count bacteria and spore number included coagulation sedimentation,ozonation,ultrafiltration and disinfection.The number of main chlorine-resistant genera gradually increased from the water sources to the drinking water plants.Pathogenic chlorine-resistant genera were mainly Mycobacterium and Bacillus.Non-pathogenic chlorine-resistant genera were mainly Sphingomonas,Stenotrophomonas and Methylobacterium.The three typical chlorine-resistant bacteria(Bacillus alvei,Bacillus cereus and Lysinibacillus fusiformis)could be completely removed by the ceramic ultrafiltration membrane.In addition,Lysinibacillus fusiformis and its spores were respectively with the highest cell surface hydrophobicity in the three bacteria and correpsonding spores,and it was with the strongest resistance to chlorine(Cl 2)and chlorine dioxide(Cl O2).The Cl2 and Cl O2 pre-oxidation could effectively stimulate the spore-forming bacteria to transform into spores,and the spores could be insentively removed in the enhanced coagulation sedimentation process.A high ultraviolet(UV)dosage was required to efficiently inactivate the spore-forming bacteria using UV alone.The inactivation efficiency of the spore-forming bacteria was effectively enhanced by the UV based advanced oxidation processes(UV-AOPs),especially the combination of UV and potassium monopersulfate(UV/PMS),and the combined processes of UV and Cl2,especially the UV-Cl2.After UV inactivation alone,the bacterial cell surfaces were with slight shrinkage,and the bacterial cells shrank in size.The intracellular structures were disordered,and the partial intracellular edemas with low electron density were observed.However,the bacterial cells were intact,and UV inactivation did not lead to the leakage of intracellular materials.By contrast,after the UV-AOPs and combined processes of UV and Cl2 treatment,the bacterial cells were damaged to varying degrees and shrank significantly.The cell walls and cell membranes were damaged,and the cell surfaces were sunken.The cells showed moderate or severe edema,and large intracellular edemas with low electron density were observed,and the cells were disintegrating.In a word,the UV-AOPs and combined processes of UV and Cl2 treatment resulted in the leakage of intracellular materials,and the amounts of leakage were proportional to the inactivation efficiencies.The chlorine-resistant bacteria control technology application project,integrating the enhanced conventional treatment,ozone-carbon sand filter advanced treatment,and UV-Cl2 combined disinfection,was established in a drinking water plant in southern China.After the engineering reform,the standard plate-count bacteria and spore number in the finished water were barely detected,suggesting that the level of microbiological security safeguards was greatly improved.The 106parameters of the finished water met the requirements of the Standard for Drinking Water Quality GB 5749-2006 and the Standard of Drinking Water Quality DB4403/T60-2020,the more stringent local standard in Shenzhen.The infrastructure cost of the project was 5 563 128.21 yuan,and the unit water production cost increased by 0.057 2 yuan/m3,which suggests that the control technologies are economically feasible.The chlorine-resistant bacteria control method proposed in this study can provide a reference for the solution of similar water quality problems in other regions in China,and provide the theoretical guidance and technical support for the drinking water biological security safeguards. |
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