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Investigation on the Microbiological Safety of O_ 3-BAC Process in Drinking Water Treatment

Author: ZhuLing
Tutor: LiuWenJun
School: Tsinghua University
Course: Civil Engineering
Keywords: biological activated carbon particles disinfection microbiological safety
CLC: TU991.2
Type: Master's thesis
Year: 2010
Downloads: 317
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Abstract

With increasing pollution to the source of drinking water, the O3-BAC advanced treatment which combines the effect of ozone oxidation, activated carbon adsorption, biodegradation and ozonization has been used extensively worldwide for its effective removal of micro-pollutants. However, potential microbilogical safety problems of the product water arise because the number of particles and bacteria increases in the carbon bed effluent and protozoa was also found in the effluent. To evaluate the microbiological safety problems that may be associated with O3-BAC treatment, a comprehensive study was performed to determine the impact of particles and bacteria released into the product water. Samples were collected from four water treatment plants throughout China and analyzed for (1) number of particles released before and after carbon bed, (2) number of heterotrophic plate count (HPC) bacteria released with homogenization technique and (3) identification of microorganisms in raw water and treatment units effluent, as well as on carbon particles and biofilms of the distribution system.The number of particles in the carbon bed effluent of water plants in the north was larger than that of water plants in the south. Moreover, the increase in particle numbers from the sand filtration effluent to the carbon bed effluent rose from water plants in the north to water plants in the south. Average HPCs showed little change or a slight decrease between carbon bed effluent and sand filtration effluent in the northern water plants. On the contrary, the HPCs of carbon bed effluent was 9- to 99- fold greater than that of sand filtration effluent in the southern water plants. The HPCs of carbon effluent in the northern water plants showed no appreciable increase after homogenization while that of the southern water plants was 3-4 times higher than the hand-shaken count.Traditional microbial cultivation and molecular biological approaches were combined to identify the microbial community in raw water and effluent of treatment units as well as those on the carbon particles and biofilms. The results show that the dominant bacteria and common bacteria of Nanzhou Drinking Water Plant differed a lot from those of Tian Cunshan Drinking Water Plant. There were massive numbers of microorganisms in the effluent of biological activated carbon bed of Nanzhou Drinking Water Plant, most of which were the common bacteria on activated carbon bed. Some species of the frequently detected mycobacterium in the finished water are opportunistic pathogens. The number and type of microorganisms detected in raw water and process water of Tian Cunshan Drinking Water Plant were much less comparing with that of Nanzhou Drinking Water Plant. One strain of staphylococcus haemolyticus was found in the finished water and it had potential pathogenicity, therefore it was of great significance to realize effective disinfection of drinking water and maintain sufficient residual chlorine in the distribution systems.The effect of activated carbon particle size on the disinfection of Bacillus Subtilis Spores and E. Coli was also investigated in this research. The results indicate that activated carbon does inhibit chlorine and UV disinfection of the two organisms. The <2-μm fraction and the >10-μm fraction had a pronounced protective effect against chlorine inactivation. For Bacillus subtilis spores, the inactivation rate for suspended bacteria and homogenized bacteria after the addition of activated carbon was decreased by 0.03~0.08-log and 0.07~0.12-log respectively; for E. coli, the inactivation rate for suspended bacteria and homogenized bacteria after the addition of activated carbon was decreased by 0.64~1.77-log and 0.30~1.87-log respectively. The 2~10-μm fraction reduced chlorine and UV disinfection of the two organisms, but there were no appreciable effect on the inactivation rate of suspended bacteria and homogenized bacteria.

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