Iron Release from Unlined Cast Iron Pipes in Drinking Water Distribution Systems: Effect of Sulphate and Alkalinity Zi-long Mi, Xiao-jian Zhang, Chao Chen, Jun Wang School of Environment, Tsinghua University, Beijing, P.R. China, ( Many cities in north China use multiple water sources in recent years due to the water shortage. The switch among multiple water sources, which located in quite different geographical area, brought forth new challenge of maintaining the water chemical stability. One of the most common concerns on chemical stability in China is the iron release in distribution system. Iron release from unlined cast iron pipes is the principal cause of “red water”. Introduction This research work was supported by the Science and Technology Program of China (No. 2009ZX ), the High Technology Research and Development Program (863) of China (No. 2009AA06Z308). Results & Discussion We usually use Larson Ratio to judge the corrosive ability of source water. But the effect of sulphate and alkalinity on iron release were not understood clearly. The objective of this study was to evaluate the effect of two key water quality parameters, sulphate concentration and alkalinity change on iron release from old unlined cast iron pipes removed from drinking water distribution systems. The results showed that the sulphate concentration increase could lead to the iron release quickly and the rate of iron release increased when sulphate concentration elevated. The deterioration of water quality also accompanied the iron release phenomenon. Iron concentration, turbidity and color of water had good correlation with sulphate concentration. Generally, the water quality was stable when sulphate concentration were in the range of mg/L; The iron release rate, turbidity and color of water increased obviously when sulphate concentration increased to mg/L. When sulphate concentration increased to 180 mg/L and the Larson Ratio increased as high as 1.40, those parameters dramatically increased to 2.6 mg/L, 16 NTU and 80 Co-Pt units respectively. Conclusions Figure 2: pipe section reactor Iron release rates of cast iron pipes under different sulphate concentrations were showed in Fig 3. Iron release rates of cast iron pipes under different alkalinity were showed in Fig 4. Figure 4: Effect of alkalinity change on iron release Sulphate was one of the most important water quality parameters that lead to red water phenomenon in distribution system and should be paid high attention during the water source switch. The effect of sulphate and alkalinity on iron release are converse, that is to say, sulphate accelerates the iron release, while alkalinity decreases the iron release. Figure 1: corrosion cast iron pipe Figure 3: Effect of sulphate concentration change on iron release It was found that sulphate was the dominant factor that led to iron release in contrast to alkalinity. Under the condition of low sulphate concentration, alkalinity change had no effect on iron release. While under the condition of high sulphate concentration, iron release from corrosion scales decreased when the alkalinity elevated. Based on our studies, it was proposed that sulphate concentration should be controlled as low as possible in order to minimize iron release from distribution systems, and increasing alkalinity can be considered as an effective strategy for reducing iron release from corrosion scales under the condition of high sulphate concentration. The effect of different water quality conditions on scale breaching and iron release was evaluated using the “pipe section reactors” which were designed to simulate the flow mode in distribution system. The simulated water qualities in the reactors are showed as follow. Parameter Sulphate (mg/L) Chloride (mg/L) Alkalinity (mg/L) Hardness (mg/L) Calcite (mg/L) pH Larson Ratio 1# # # # # # # # # Table 1: Water quality in the pipe section reactors (b) sulphate concentration = 100mg/L (a) sulphate concentration = 75mg/L