Removal of arsenic from waters with elevated concentrations of silica using adsorptive processes

In 2006, the USEPA reduced the maximum contaminant level (MCL) for arsenic from 50 micrograms per liter (mug/L) to 10 mug/L, requiring many municipalities to implement additional treatment methodologies to comply with the new regulatory standard. The communities impacted most by the new standard are small and rural. These communities are now faced with increased capital costs and additional operation and maintenance requirements.;The primary objective of this study was to evaluate the performance of four commercially available adsorptive media to determine if adsorption was a viable treatment method for arsenic removal. Waters tested during this study were collected from the Te-Moak Tribe of the Western Shoshone Indian Colony in Battle Mountain, Nevada and the Utu Utu Gwaitu Paiute Tribe of the Benton Paiute Reservation in Benton, California. These small, rural communities are faced with evaluating new treatment methods for their source waters with arsenic concentrations ranging from 23 mug/L to 26 mug/L. The silica concentrations were 79 mg/L and 66 mg/L, for the Battle Mountain, NV and Benton, CA waters, where silica plays a significant role in adsorptive removal technologies due to its ability to compete for adsorption sites on the media.;Pilot scale studies of arsenic removal technologies often require lengthy operation times to determine the best available technology to comply with new drinking water standards. This study used the rapid small-scale column test (RSSCT) method to evaluate the performance of four commercially available adsorptive media for arsenic removal. RSSCT technology accelerated the test method used to evaluate the performance of media and generated preliminary data for sizing large-scale systems.;Three of the media were granular ferric oxide (GFO) (Bayoxide RTME33, Kemira CFH-12 and Kemira CFH-18) and one media was granular titanium dioxide (GTO) (DOW Adsorbsia). The RSSCT method employed two parallel columns for each media type and one blank column in order to obtain duplicate data throughout the study. Water was pumped through each of the columns at varying pH conditions to determine the quantity of water that could be treated in each column before the concentration of arsenic in the effluent exceeded 10 mug/L, at which point breakthrough was considered to have occurred. Separate tests were performed under pH conditions of pH 8.1, pH 7.5, pH 6.8 and pH 6.5.;Results indicated that by reducing the pH from 8.1 to 6.5, the bed volume capacity increased considerably for each of the media. The bed volume (BV) capacities reached for water conditions at pH 8.1 were less than 5,000 BV, while the bed volume capacities at pH 6.5 were approximately 30,000 BV. These results indicated a clear relationship between pH and arsenic removal efficiency. Several dimensionless mathematical equations may be applied to size a large-scale system using results obtained from the RSSCT methods. Proportional diffusivity scaling approaches are commonly used to determine effectiveness in a full-scale system.;The results of this study indicated that the Kemira CFH-18 would most likely provide the greatest bed volume capacity for the two source waters that were studied. However, bed volume capacity of the BayoxideRTM E33 also increased markedly with the decreasing pH conditions.