Bench-scale evaluation of drinking water treatment parameters on iron particles and water quality

Abstract

Discoloration of water resulting from suspended iron particles is one of the main customer complaints received by water suppliers. However, understanding of the mechanisms of discoloration as well as role of materials involved in the process is limited. In this study, an array of bench scale experiments were conducted to evaluate the impact of the most common variables (pH, PO4, Cl2 and DOM) on the properties of iron particles and suspensions derived from the oxygenation of Fe(II) ions in NaHCO3 buffered synthetic water systems. The most important factors as well as their rank influencing iron suspension color and turbidity formation were identified for a range of water quality parameters. This was accomplished using a 24 full factorial design approach at a 95% confidence level. The statistical analysis revealed that phosphate was found to be the most significant factor to alter color (contribution: 37.9%) and turbidity (contribution: 45.5%) in an iron-water system. A comprehensive study revealed that phosphate and chlorine produced iron suspension with reduced color and turbidity, made ζ–potential more negative, reduced the average particle size, and increased iron suspension stability. In the presence of DOM, color was observed to increase but a reverse trend was observed to decrease the turbidity and to alter particle size distribution. HPSEC results suggest that higher molecular weight fractions of DOM tend to adsorb onto the surfaces of iron particles at early stages, resulting in alteration of the surface charge of iron particles. This in turn limits particles aggregation and makes iron colloids highly stable. In the presence of a phosphate based corrosion inhibitor, this study demonstrated that color and turbidity resulting from suspended iron were lower at a pH value of 6.5 (compared to pH of 8.5). The same trend was observed in presence of DOM. This study also suggested that iron colloid suspension color and turbidity in chlorinated drinking water systems could be lower than non-chlorinated systems.