Factors affecting scale formation in water distribution networks of Semnan city and preventive measures

Background and Objective: Corrosion and precipitation are important indicators of water quality assessment. The aim of this study was to analyze water stability water in Semnan distribution networks. Materials and Methods: To achieve the purpose, samples were taken from 5 points (3 points of storage tanks and 2 distribution network points) and analyzed for physical-chemical analysis. The concentration of common anions and cations was determined by ion chromatography (IC). The sediment formed in the water distribution network was also investigated for studying its structure and main elements by using X-ray fluorescence spectroscopy (XRF) and induced coupling plasma spectroscopy (ICP). The corrosion and scaling indices, including the Ryznar stability index (RSI), Langelier index (LI), and calcium carbonate precipitation potential (CCPP) were also studied. Results: According to the obtained results, the quality (physically-chemically) of most of the taken samples lies within the permissible limits of Iran's national standard (1053). However, in terms of total harness all sampling point showed values beyond WHO guidelines for drinking water. Magnesium and sulfate concentrations were also exceeded in one of the reservoirs. The results also showed the main sediment structures were calcium carbonate (calcite) and silica oxide. Moreover, According to the results, the average values of LSI and CCPP represented the scaling due to carbonate calcium nature of water. Water stability analysis indicated major roles of calcium and magnesium bicarbonate formation as temporary hardness and minor roles of magnesium sulfate and chloride as permanent hardness. Conclusion: According to the results, to prevent water scaling in pipes it is recommended to use lime softening at pH=10 which not only removes temporary hardness but enhances silicate ions removal by co-precipitation with Mg(OH)2 formed followed by filtration.