AWWA ACE59862

The United States Environmental Protection Agency has lowered the maximum contaminant level (MCL) for arsenic in potable water from 50 micrograms/liter (ug/L) to 10 ug/L. All water systems are expected to comply with the new MCL by January 2006. Similar to many other utilities in the United States, arsenic is of concern for System A. System A provides water to 30,000 people, and operates and maintains four drinking water wells (Wells 6, 7, 8 and 9). These wells are the sole source of water for System A. The capacities of the wells vary from 900 gallons per minute (gpm) to 1700 gpm, with approximately 7.5 million gallons per day of total well production capacity. Two of the wells (Wells 7 and 8) have high sulfate (400-500 mg/L) and hardness (400-450 mg/L) levels. These wells are under the influence of a mine-tailing plume that resulted in the deterioration of the water quality. The 400-500 mg/L of sulfate Wells 7 and 8 not only pose an aesthetic issue but may also cause public health issues (e.g., diarrhea in sensitive populations). System A initiated a planning study to develop strategies for arsenic removal from the four wells. The selection of arsenic mitigation strategies for Wells 7 and 8 is complicated by the presence of sulfate and other ions. Several arsenic treatment technologies including ion exchange (IX), adsorption, reverse osmosis (RO) and coagulation-assisted microfiltration (CMF) were evaluated for their feasibility. The concentration of sulfate determines the achievable run lengths (for regeneration) for the IX process. Sulfate can scale the RO membrane, if proper anti-scalant techniques are not used. There are no prior studies on the effect of high concentrations of sulfate on GIM and CMF treatment processes. Giving due consideration to water quality (arsenic and interfering ions concentrations), site constraints (availability of land, sewer and power), operational issues, residuals handling issues and economic feasibility, this study developed alternative treatment alternatives. These treatment alternatives considered granular iron media (GIM) adsorption for Wells 6 and 9 and GIM coupled with IX or RO for Wells 7 and 8. The sequencing of GIM and IX/RO at Wells 7 and 8 was developed in a manner to result in environmentally benign residuals. Site-specific recommendations for disposal of liquid and solid residuals were developed for each compliance alternative. Capital and operations and maintenance cost opinions were developed for all compliance alternatives. This paper presents the findings of this study that will be useful for all small and large water systems that are considering arsenic treatment for their impacted source waters. The paper presents the issues associated with selection of arsenic treatment strategies and assists water utility managers/operators in making some educated decisions for arsenic rule compliance. Includes reference, tables, figures.