The Influence of Water Quality on Arsenic Sorption and Treatment Process Performance

A new regulation has been proposed that would lower the acceptable level of arsenic in drinking water from the current standard of 50 ppb. Therefore, research into effective arsenic removal treatment is important, especially for hard to treat waters with high concentrations of silica. The first phase of research was designed to determine if sand ballasted coagulation is a viable means of removing arsenic from drinking water, and if so, to identify the water qualities in which the technology performs best. A jar test protocol was developed and tested on a wide range of waters to compare microsand ballasted coagulation and other coagulation based treatment processes in terms of arsenic removal. Secondary impacts of the microsand process such as residual turbidity, iron, posttreatment membrane filter run length, and TOC removal were also considered as part of this evaluation. Microsand ballasted coagulation provided promising results for many of the simulated groundwater test conditions in which more than 80% of the arsenic regulation costs will be incurred. However, like conventional coagulation/sedimentation, microsand ballasted coagulation performed poorly in waters with high silica and high pH. Thereafter, a second phase of research more closely examined the kinetic behavior of arsenic sorption to amorphous and granular oxides in the presence of silica and calcium. At pH 8.5, calcium dramatically improved arsenic sorption to amorphous iron hydroxide in the presence of silica over short reaction times, but had no long-term advantage. This result could have considerable applications for treatment in that it suggests water quality controls the required reaction times. Additionally, batch tests indicated that activated alumina granular media was more sensitive to water quality than granular ferric hydroxide; however, calcium eliminated silica interference to arsenic sorption onto activated alumina.