Passive Selenium Bioreactor--Pilot-scale Testing
The S&T Program funded a 2006-2007 "bench-scale" test of a selenium-reducing bioreactor. The "bench-scale" project was Proposal #394, Evaluation of Passive Bioreactors to Reduce Selenium Impacts from Reclamation Projects, 2006. The project was started in fiscal year (FY) 2006 and the final report was produced in Sept. 2007. Four 55 gallon barrels were filled with various types of agricultural wastes, including manure and rotting hay. The test was very successful, resulting in selenium removal rates of 92-98 percent under stable operational conditions and a 12-hour detention time. The final "bench-scale" report is due by October 2007.
This proposed project would take the technology to the next step (pilot-scale) and involve the construction and testing of a 50 by 50 foot bioreactor cell. The objectives of the study would be to:
* Demonstrate a high selenium removal efficiency at the pilot scale;
* Determine the relationship between selenium removal efficiency and detention time at this scale
* Determine the relationship between selenium removal efficiency and ambient air temperature at this scale
* Determine design and operating parameters for a full-scale system (i.e., one with a footprint on the scale of up to a few acres)
Need and Benefit
The lower Colorado and Gunnison Rivers in western Colorado and many of their tributaries are on the State of Colorado's 303(d) list of impaired waters for selenium, primarily due to irrigation drainage that mobilizes selenium from the local Mancos shale. These river segments also are critical habitat for four endangered fish species, and Reclamation is working with its partners to recover those fish. However, Section 7 Endangered Species Act (ESA) consultations could eventually place restrictions on water use by Reclamation projects and affect Reclamation's ability to deliver water.
A large portion of the source water for mobilization of selenium is delivered through Reclamation project facilities (Grand Valley Project and Uncompahgre Project). This same problem is occurring in other Reclamation project areas, including the lower Arkansas basin in eastern Colorado and many areas in California (San Joaquin Valley and Salton Sea).
Because of the diffuse and widespread nature of the source, a combination of selenium reduction efforts is needed locally, ranging from best management practices (lining/piping canals and laterals) for irrigation to treatment of selenium-contaminated ground-water at strategic locations. This proposed research effort aims to continue development of the technology and identification of optimal operating conditions for a passive selenium-reducing bioreactor, which could be a practical, low-cost treatment method with the potential to be used throughout western Colorado and other Reclamation project areas. It would likely be transferable to other locations where the presence of marine shales has led to selenium-impaired surface waters. Locally, it could be applied to drainages where State and Federal regulatory agencies are pushing for the reduction of selenium loading. Use of this technology could avoid future water conflicts with potentially substantial cost savings.
Although regulatory restrictions on water use are not in place at this time, the "writing is on the wall, - and this technology could lead to local entities (including gravel pit miners) undertaking remediation, thus benefiting Reclamation and project water users.
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