| The drinking water supply system in one city in hot and humid South China wasselected as the investigation objective. This water supply system has the representativeprofile of multi water sources, multi water treatment processes, multi pipe materials,different pipe ages and super huge networks, which make this investigation highlycomplex, representative and challenging.In this paper, the bio-stability of different sub networks in this city was compared,the principal index of bio-stability judgment was identified, the capacity of differentwater treatment processes to control bio-stability was analyzed and the comprehensivemeasures suitable for this city in South China was presented to improve the bio-stability.The results and achievements of this paper were given as below:According to the long term monitoring of water quality in the networks all overthis city, certain parts of the network bear obvious bacteria re-growth problem. Residualchloramine concentration and hydraulic retention time were identified as the principlefactors influencing the bacteria re-growth by the analysis with software SPSS. Thehigher chloramine concentration was maintained in networks, the fewer bacteria couldsurvive; the longer hydraulic retention time was, the more bacteria existed in networks.Emphasis was given to the sub-network fed by the water treatment plant with goodsource water (Ⅱ) and advanced treatment process (ozone and biological activatedcarbon). As far as the author knew, it was the first time that the Biostability Curve wasapplied in real drinking water distribution system. The quantitative boundary wasdetermined by the Biostability Curve for achieving the bio-stability in the networks:maintaining residual chloramine at1.11-1.73mg/L which is practical for water industry,or keeping the AOC concentration as low as7-47μg/L which is far beyond the drinkingwater plant’s capacity. Therefore, the bio-stability issue in the distribution system withhigh water temperature in South China is dominantly affected by residual chloramine,which is quite different with the previous report that the bio-stability in networks inNorth China is controlled by both residual chloramine and organic substrate. Furtherinvestigation indicated that the detachment of biofilm from pipe wall by flushing wasthe main source of bacteria in bulk water. The inactivation characteristic of biofilm was studied by the simulative biofilm annular reactors. The results indicated that1.0mg/L ofmonochloramine or0.5mg/L of free chlorine could successfully inactive the biofilmbacteria to the level of100CFU/cm2.Further investigation was conducted on the re-grown bacteria. It was found that thechloramine-resistant bacteria survived pervasively in the distribution systems all overthis city. Three kinds of chloramine-resistant bacteria, i.e. Mycobacterium mucogenicum,Methylobacterium fujisawaense, Sphingomonas sp.(new strain) were identified in thedistribution system. Mycobaterium, one well-known opportunistic pathogen, was highlyresistant to disinfectant; Sphingomonas was moderately resistant to disinfectant whileMethylobacterium was relatively lowly resistant to disinfectant. Free chlorine of0.4mg/L or chlorine dioxide of0.4mg/L could effectively inactivate Mycobateriumwhile chloramine could hardly inactivate Mycobaterium. Therefore, the free chlorine orchlorine dioxide was necessary to eliminate the resistant bacteria.Based on the aforementioned results, the control strategy was established toguarantee the bio-stability in the high-temperatured distribution system in South China.The high residual chloramine was necessary to maintain the bio-stability in thedistribution system, the free chlorine or chlorine dioxide should be applied if theresistant bacteria existed. To maintain the sufficient residual chlorine, esp. free chlorineor chlorine dioxide, the finished water quality should be improved by switching to goodwater source and updating the water treatment process with biological pre-treatment oradvanced treatment process of ozone and activated carbon. |
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