Pilot-Scale Treatment of Selenium in Gravel Pit Seepage Water Using Biochemical Reactor Technology

Abstract

Water quality in the lower Colorado and Gunnison Rivers in western Colorado, and many of their tributaries, is impaired by selenium, which originates from the local Mancos shale. Because of the diffuse and widespread nature of this source, there are limited opportunities to reduce selenium inputs. One option is to treat selenium-contaminated surface water at strategic locations, such as point-source discharges. Gravel extraction is common along these rivers, and treatment of discharges from pit dewatering presents an opportunity for reducing selenium loading. The pilot test goals were: 1) demonstrate that a passive BCR can accomplish high-efficiency selenium removal at the pilot scale 2) determine the relationship between selenium removal efficiency and detention time 3) assesses the influence of seasonal temperature fluctuations on treatment performance 4) determine design parameters for a full-scale system (i.e., one with a footprint on the scale of up to a few acres). A single 4,380 cubic foot pilot BCR was constructed to treat flows ranging from 2 to 24 gallons per minute. The vertical-flow reactor media contains cow manure, hay, sawdust, wood chips, and limestone. Influent was drawn from a dewatering trench in a gravel pit next to the Colorado River near Grand Junction, Colorado. The pilot operated, with varying detention times, over a thirteen-month period from September 2008 until October 2009. The pilot achieved maximum selenium removal rates of 98% with a hydraulic retention time of 2.4 days and a minimum effluent concentration of 0.0005 mg/L (0.5 µg/L). The highest mass removal rate achieved by the BCR was 73 mg/day/m 3 and the cumulative mass of total selenium removal was 600 grams. The BCR treatment process was effective throughout the cold winter months during which total selenium removal rates remained greater than 90%. The total capital cost for the pilot BCR was $39,200 or $0.50 per 1,000 gallons treated. The operations and maintenance costs for a passive BCR system are minimal. Based on the successful operation of the pilot, including high rates of selenium removal and consistent year-round operation, the BCR technology appears to be an effective, low-cost selenium treatment option. This project was funded by the US Bureau of Reclamation Science and Technology Program, Project #4414.

Three-Year Pilot Case Study of Biochemical Reactor Treatment of Selenium

“…The BCR technology has been developed over the past two decades for treatment of acidity and metals in mining-influenced water. Bench, pilot and demonstration-scale testing of BCR technology for selenium treatment in recent years has demonstrated effective rates of selenium removal (Pahler et al 2007, Blumenstein et al 2008, Rutkowski et al 2010.…”