| Unstable corrosion scales might trigger “red water” issues when water quality enteringdistribution systems changed significantly. More serious and larger scale red water mayoccurre in the condition of water source switch. It’s been confirmed that corrosion productsrelease was one of the main water quality problems caused by water source switch.Water source switch to Danjiangkou reservoir (the middle route source area of theSouth-to-North Water Diversion Project in China) water might cause increased iron release indistribution systems of water receiving areas. In order to ensure the safety of water supply, it’snecessary to investigate the degree of possible water quality changes and to reserve applicablecontrol technologies.Cast iron pipes transporting groundwater were harvested from three sites of existingdistribution system of a northern city, and pipe-loop distribution systems were established inDanjiangkou. Pilot study was conducted to observe the outcomes of source water switch, andthe effects of sulfate, chloride, different disinfectants, dissolved oxygen, alkalinity anddifferent pH adjustment measures on iron release behaviors were investigated.Results show that the corrosion scales on the test pipes are relatively thin and less stable;At the early stage of water source switch, apparent “red water” occurs. The effluent ironconcentration and turbidity increase accordingly with the increase of sulfate and chlorideconcentration.The change of iron release with free chlorine concentration in the range of0.3~3.6mg/Lhas no clear trend. While iron release increases significantly as chloramine concentrationraised from1.3to2.0mg/L; When dissolved oxygen (DO) is increased appropriatly, ironrelease decreases significantly, but excess DO may increase iron release. DO of15mg/L isthe optimal in this study; The iron release decreases significantly with alkalinitysupplementation (through NaHCO3addition) in the range of90-250mg/L (as CaCO3) andcould ensure total iron concentration meeting the drinking water standard (<0.3mg/L), andkeep the stability of pH; While when the pH is elevated by adding Ca(OH)2in the range of7.8-9.0, the iron release decreases obviously, but the total iron concentration is always higherthan0.3mg/L, and the turbitity is incerased, the stability of pH is not easy to keep; Under thecondition of low sulfate concentration of feed water, the iron release do not increase when alkalinity adjustment and Ca(OH)2addition discontinue, but under high sulfate condition, ironrelease could increase again with the termination of alkalinity and pH adjustments; Whenwater pH is elevated by addition of NaOH in the range of7.8-9.0, the iron release is notinhibited significantly but enhanced to some extent.Identifying the area where “red water” could occur is important before water sourceswitch, which could be achieved through systematic evaluation of the chemical stability ofboth new and old source waters and the corrosion scale characteristics. When switching towater with higher corrosivity (e.g. high concentration sulfate or chloride water), blending newand old water source with an approperiate ratio could be feasible. Control measures includingincreasing dissolved oxygen, alkalinity and adjusting pH by adding Ca(OH)2, can be taken toreduce risk or control the iron release for those areas where “red water” is prone to occur.Based on this study, alkalinity elevation by adding NaHCO3is the most effective measure tocontrol iron release. |
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