Preparing for a new arsenic regulation: Arsenic occurrence and speciation in drinking water supplies, considerations in selecting treatment processes, defining interferences from silica, and predicting performance of sorption processes

Multiple phases of research were aimed at developing practical and scientific understanding for a more stringent arsenic maximum contaminant level (MCL) in drinking water. Arsenic occurrence and speciation in source water were examined to define the nature of the problem and to consider implications for treatment process selection. Although soluble arsenate was dominant, particulate arsenic is also surprisingly substantial, which implies the importance of treatments removing both soluble and particulate arsenic. A simplistic algorithm identifies the least-cost arsenic treatment option for individual utilities based on design flow and water quality, indicating that costs increase in the following order: modified conventional treatment << activated alumina or anion exchange < reverse osmosis. The most cost-effective treatment may not be best at individual utilities because secondary treatment benefits and residuals handling will also be taken into account.; To better understand the issue of particulate arsenic in water supplies, relationships between arsenate and iron, barium, copper, and zinc are examined. Although most particulate arsenic is associated with iron hydroxides, arsenate is also strongly adsorbed to copper and zinc hydroxides, which raises the concern that an elevated amount of arsenic may be released to tap water from pipe scales onto which arsenic is adsorbed and concentrated.; The least-cost treatment option for arsenic at many utilities is sorption to metal hydroxide surfaces, the efficiency of which depends on the presence of interfering substances. An improved surface complexation model is developed to account for strongly sorbing soluble dimeric silica species that were previously neglected, and to allow consideration of impacts from sulfate, natural organic matter, and hardness. The final model provides an explanation for several perplexing trends noted in previous research regarding simplified Langmuir isotherms, sorption column run length variation, and the relationship between pH, arsenic, and silica interference.; This effort provides a sound foundation to support decision-making and also provides direction to future work necessary to meet the burden posed by the new MCL.