| With the urban water crisis becoming more serious, multiple water supply has become a necessary means to solve the problem of urban water usage. However, water switching can bring new problems, mostly being the regional red water phenomena. In this study, two typical red water problems which exist in south and north China repectively has been selected to investigate the characteristics of red water and the reason why it comes out. Appropriate iron stability control strategies were then put forward based on different water qualities and causes. This study also proposed the red water risk analysis and sensitivity identification methods for the water diversion projects such as the South-to-North Water Diversion Project.A city in south China faced with red water problem after switching the water supply from underground water to surface water. The study in the iron stability of the effluent water showed that water after the switch has high concentrations of chlorides and sulfates, along with low pH value and low alkalinity. Larson index rose to 2.28 from 0.05, which means the water is highly corrosive to the iron pipes. The bacterial community in the urban drinking water distribution system during an occurrence of red water was also investigated. Variation in the bacterial community diversity and structure was observed among the different waters, with the predominance of Proteobacteria. The coexistence of Limnobacter as well as Sediminibacterium and Aquabacterium might contribute to the formation of red water.The effects of using Ca(OH)2 and NaOH to adjust the pH of the water on iron release were studied by experiments. It was found that the total iron concentration, turbidity and chromaticity were all greatly reduced while the pH gradually increased from 6.5 to 8.0. The effects of using lime(Ca(OH)2) were better, with the cost of about 0.02 yuan/m3. Therefore, it can be selected as a red water controling technology. After applying the pH control technology for over a year, the running results of the city water plant were evaluated. It was found that red water problems in the city were reduced by 62%- 80% compared to that in the last year during the red water problem peak period(April to June), which indicated that the pH control technology was effective and feasible.Another city in North China faced with a long-distance water diversion project which might cause red water problem. The sensitivity characteristics including sensitive controlling factor, sensitive period and areas were investigated in this study. Based on the residual chlorine, turbidity and total iron values of 63 sampling points from 2010 to 2013, studies had found that the total iron concentration and turbidity both had negative correlation with residual chlorine concentration. When the residual chlorine concentrations were lower than 0.3 mg/L in the pipe water, the average total iron concentration and turbidity both exceed the standard value. The residual chlorine is the key sensitive index for red water problem in this city. By water quality installment and data classification, summer was identified as the sensitive period and six sensitive areas needed to focus on were pointed out.After the actual water switch by water blending in the city of North China, the influences of water switch were evaluated. The results came out to be that the water quality actually improved after the water switch, with the qualified rates of residual chlorine, turbidity and total iron concentrations all rose to 100% from 99.7%, 98.6% and 97.5% respectively. The identification of the sensitive areas was proved to be effective. Sampling points which were found of low residual chlorine, high turbidity and high total iron concentration overlapped with those areas that were identied before the water switch by 100%, 100% and 75% respectively. |
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