Field evaluation of arsenic and selenium removal by iron coprecipitation

The priority trace elements arsenic and selenium were removed from ash pond effluent by iron coprecipitation in a field study at the Carolina Power and Light Company's Roxboro Station. The study's broad objectives were to verify previously‐derived laboratory results, confirm the technology's technical and economic feasibility, and develop information that could be used to make preliminary designs and cost estimates for full‐scale treatment plants. The technology was tested at pilot scale in a continuous‐flow system under field conditions. A chemical coagulation system was used of the kind typically employed for municipal and industrial water treatment. Arsenic was strongly removed (90 percent and above) with moderate iron dosages (14 mg/L to 28 mg/L Fe). Selenium removals were lower (56 to 80 percent) under comparable chemical conditions. Arsenic and selenium removals were strongly affected by solution pH. Testing indicated that process sludges can be gravity thickened, then dewatered with filter presses to 25 to 35 percent solids. Dewatered pilot sludges were determined to be not hazardous, as defined by Extraction Procedure (EP) toxicity test criteria. Total process costs for the Roxboro site are estimated to be 9.3 cents/m3 ($353 per million gallons). This cost is for a facility that processes an average flow of 1.1 m3/sec (26 mgd) and uses an iron dose of 14 mg/L, as Fe. The facility is sized to process a peak flow of 1.5 m3/sec (35 mgd).