Stability evaluation of Tigris River raw water and treated drinking water from main water treatment plants within Mosul City

Corrosion and scaling are the main factors affecting raw water supply resources and treated water produced from Water supply treatment plants (WTPs), resulting in operational, economic and health issues. Mosul City, North of Iraq, depends on Tigris River as the only surface water supply source for its municipal uses. Water quality parameters were taken from eight locations on Tigris River that represent the intake of the main WTPs providing water for Mosul City; and for the water produced from these plants. Langelier and Ryznar indices were calculated to evaluate water stability. Results showed that the average value of Langelier Saturation Index (LSI) for raw water was 0.23 (+/- 0.31) and 0.08 (+/- 0.30) for treated water. Approximately 78% of raw water samples and 59% of treated water samples were classified as scale forming according to LSI, although Tigris River raw water and treated water are not potentially aggressive even when LSI value was less than zero. The average value of Ryznar Stability Index (RSI) for Tigris River raw water was 7.25 (+/- 0.34) and for produced water was 7.42 (+/- 0.33). Both raw and treated water were aggressive and likely corrosive according to RSI classification. Duncan analysis of variance (ANOVA) statistical analysis for RSI showed non-significant differences among the means of most of Tigris River locations. However, water is more saturated with CaCO3 at the river downstream. Paired t-test analysis of RSI revealed a significant difference between raw water and treated water stability. Again, Duncan ANOVA statistical analysis of treated water RSI showed significant differences between produced water from WTPs plants; which reflect the deference of their operational style. All statistical tests were made at the threshold for significance at 95% confidence.