| Stagnation condition often occurs in terminal water supply pipes, causing changes of chemical and microbial parameters in both pipe biofilms and bulk water, which would lead to iron release,even color water accident. Its mechanism and solution of control should to be studied in depth. The research relied on an experimental pilot platform constructed in terminal water supply pipes in JX City, Zhejiang Province. Taking pipes made of different materials with diverse diameters as research objects, we investigated that mechanism of stagnation and leakage on iron release by conventional method and modern molecular biology technology. Here are main conclusions:(1) Correlation of iron release change regularity in different pipe wall space positions and microbial communities in stagnation conditionMicrobial community diversity in HDPE (High density polyethylene pipe) was higher than that in DCIP (ductile carbon iron pipe). In addition, the quantitative variance of microbial community in HDPE, including different space positions was not distinct compared with that in DCIP. Quantitative variance of IOB (iron-oxidizing bacteria) and SRB (sulfate-reducing bacteria) on pipe walls manifested the increment in the middle and bottom regions was high, while that in the upper region was lower. Iron release was the most severe in the middle region of pipes, it mainly related to those bacteria relationship with corrosion. Biofilms of the region interact frequently with bulk water and it should pay more attention to the impact of the region biofilms on drinking water safety.(2) Change regularity and correlation of bulk water iron release and corrosive bacteria in pipes with different materials/diameters in stagnation conditionThe decline of water parameters like DO (Dissolved Oxygen) and residual chlorine caused total iron concentration increase in pipe water in stagnation condition. Quantity of corrosive microorganism,mainly referred to IOB and SRB,increased as stagnation continued. Proteobacteria,mainly containing Alphaproteobacteria, Betaproteobacteria and Actinobacteria, were the dominant species in water samples, whose percentage reached 86.69%-91.36% after stagnation, and the percentage increased as stagnation continues. From correlation analysis and RDA (redundancy analysis), it was discovered there are strong positive correlations between total iron release and bacteria communities including Proteobacteria, Actinobacteria and Acidobacteria, as well as Alphaproteobacteria, Betaprotebacteria and Bacilli in class level. Concerning environmental factors,parameters that influenced bacteria community growth included nitrate,nitrite,TOC,COD, ammonia nitrogen and DO.(3) Water quality change regularity under different seepage velocities and their correlation with microbial communities.Water parameters, like residual chlorine, DO and redox potential, often decline as seepage velocity increases. Meanwhile, the increase of pH, turbidity and total iron concentration would be restrained. When flow rate increased near the critical laminar flow velocity, these water parameters basically kept stable. Quantities of IOB and SRB decreased as seepage velocity increased, when the flow rate reached a certain value,the amount kept constant. For microbial communities in phylum,class or genus levels, microbial community structure basically kept stable with flow rate increased gradually. The amount and speed of total iron release reached the maximum threshold value as flow rate increased about the 40% of critical laminar flow rate.(4) Suggestions on control measures of iron release in stagnantion and low flow rate conditionOn the basis of mechanism study, the following measures are proposed to control the iron release in the condition of stagnant flow and small flow rate. a) Preferred HDPE instead of metal pipe; b) Stagnation time of Ductile iron pipe should not exceed 8 hours, HDPE should not be more than 8-16 hours; c) Water at the end of pipe in a certain degree of flow could effectively reduce the iron release. |
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