Microbial As(III) oxidation in water treatment plant filters

Abstract

The effectiveness of arsenic (As) removal from water is largely dependent on the oxidation state of the arsenic. Efficient arsenite [As(III)] oxidation requires a strong oxidant such as free chlorine or permanganate. There are also microorganisms capable of oxidizing As(III) to arsenate [As(V)] although they have not been reported to be important in drinking water systems. The objectives of this study were to monitor and evaluate As(III) oxidation and arsenic removal in a full‐scale iron‐removal plant with microbially active granular media filters and determine how to most efficiently regain microbial properties in new filters. Results from the full‐scale plant showed that in raw water with average concentrations of 2,300 μg/L iron and 46 μg/L arsenic, removal rates of > 99% for iron and 80% for arsenic were achieved, and the plant regularly met the arsenic standard. The majority of arsenic in the raw water (37 μg/L) was in the reduced As(III) form. Arsenic was removed without the addition of a strong chemical oxidant to convert As(III) to the As(V) form. Although some As(III) may be removed during iron removal, the observed arsenic removal through the plant was far greater than expected. Bench‐ and pilot‐scale investigations concluded that microbial oxidation of As(III) and ammonia took place within the filters, which explained the greater‐than‐expected arsenic removal. Pilot‐test results indicated that different seeding approaches did not affect the time frame to regain microbial activity in new filters. Microbial analysis using traditional and molecular techniques confirmed the presence and identity of oxidizing bacteria.