Water quality characteristics and corrosion potential in blending zones in X city drinking water distribution system

Blended water, always existing in a drinking water distribution system (DWDS) with different sources, can cause some unintended results, including corrosion and/or release of corrosion by-products. Although some studies have specially focused on the blended water in DWDSs, the water quality characteristics, variations, and mechanisms for corrosion and metal release have not been fully understood. This study aims to examine the characteristics and evaluate the corrosion potential of blended water in X city DWDS using four indices of Langelier saturation index (LSI), Ryznar stability index (RSI), Puckorius scaling index (PSI), and calcium carbonate precipitation potential (CCPP). Physical and chemical analysis showed that the values of pH, total dissolved solids (TDS), sulfate (SO₄^2-), and chloride (Cl^-) in blended water were always at acceptable levels, while some free residual chlorine concentrations fell outside the regulatory standards (≥ 0.05 mg/L) with the minimum of 0.01 mg/L. Most parameters except pH varied in large ranges with maximum to minimum ratios (MMRs) over 2.25. The mean values of the LSI, RSI, PSI, and CCPP indices were - 0.44, 8.65, 8.79, and - 1.95 mg/L CaCO₃, respectively, indicating that the blended water was slightly corrosive. For the three zones, Z2 had the highest mean levels of TDS (320.84 mg/L), alkalinity (188.70 mg/L CaCO₃), SO₄^2- (13.69 mg/L), Cl^- (36.37 mg/L), calcium hardness (Ca²⁺) (28.99 mg/L), and magnesium hardness (Mg²⁺) (15.22 mg/L) and the lowest mean level of dissolved oxygen (DO) (6.72 mg/L). Thus, the corrosion potential in Z2 was the lowest with the LSI, RSI, PSI, and CCPP values of - 0.17, 8.11, 8.08, and 2.87 mg/L CaCO₃, respectively. During a year, the corrosion in blended water was more serious in winter with the LSI, RSI, PSI, and CCPP indices of - 0.79, 9.25, 9.37, - 7.54 mg/L CaCO₃, respectively. The water corrosivity reached the minimum level in summer (LSI - 0.12, RSI 8.05, PSI 8.03, and CCPP 5.22 mg/L CaCO₃) owing to the decrease of DO concentrations and the increase of temperature and groundwater supplies with higher alkalinity. During rainy season, the concentrations of TDS, alkalinity, SO₄^2-, Cl^-, Ca²⁺, and Mg²⁺ in blended water were reduced by 41.05%, 40.48%, 35.83%, 47.48%, 23.47%, and 55.73%, respectively, resulting in the increase of water corrosivity. More decreases of water parameters were recorded in Z2 (TDS, 221.80 mg/L; alkalinity, 139.50 mg/L CaCO₃; SO₄^2-, 9.97 mg/L; Cl^-, 13.74 mg/L; Ca²⁺, 7.10 mg/L; and Mg²⁺, 11.37 mg/L), because most groundwater from No. 5 WTP was pumped paretic water with more variations of water quality by rainfall. Moreover, it was suggested that Mg²⁺ should be considered in the corrosion indices, and the corrosion tendency of blended water could be reduced by adjusting the levels of pH, alkalinity, Ca²⁺, and Mg²⁺. The results of this research may pave the way for several opportunities to improve the management and corrosion prevention of blended water.