Impact and management of iron corrosion by-products on drinking water quality in distribution systems

Abstract

Cast iron pipes were installed broadly in North American water utilities. Many of these cast iron pipes are corroded and are continuous sources of Fe(II) ions in drinking water distribution systems. Recent studies have reported that soluble or particulate iron decreases water quality in distribution systems.

In this study, an array of bench scale experiments were conducted to evaluate the impact of most common water parameters (e.g., pH, PO4, Cl2, and DOM) on the oxidation rate of Fe(II) ions, and on the formation of iron suspense in synthetic water samples. This was accomplished using a 24 full factorial design approach at a 95% confidence level. This study demonstrated that a lower content of iron suspension color, turbidity, and smaller particle size would appear to be obtained in presence of a phosphate based corrosion inhibitor at a pH value of 6.5 compared to a pH value of 8.5.

To investigate the impact of Fe(II) ions, phosphate, pH and reaction time, and their interaction on DBPs formation in water samples, this study was conducted following an experimental design approach. Considering all the significant (α = 0.05, p < 0.05) factors, mathematical models for HAAs and THMs prediction were developed using 80 experiments. The models’ adequacy was checked thorough the statistical and graphical diagnostics. Different sources of natural water samples collected from three main water treatment plants in Halifax, Canada, were used to validate the models. This study suggested that the models’ performance were found to be excellent under a wide range of studied variables. Consequently, the most predominant iron oxides (goethite and magnetite) were used to investigate their impact on chlorine decay and DBPs formation study. Goethite and magnetite were also used for the adsorption of DBPs precursor (DOM). The DOM adsorption data illustrated to fit well with the Langmuir adsorption isotherm, indicating monolayer coverage. Molecular weight (MW) distributions of DBPs precursor (DOM) revealed that the higher molecular weight fractions adsorbed preferentially onto goethite followed by magnetite surface. The change of MW distribution of DOM was found to be in reasonable agreement with the change of DBPs formation in iron-water systems.