Geosyntec has developed significant expertise in the design and implementation of gravel-bed bioreactors specifically for the treatment of dissolved inorganics, including perchlorate, chromium and selenium.
In just one example, our experience with the biosequestration of inorganics is being utilized at a limestone mine site in South Africa. At the Lyttleton Dolomite Mine, shallow, unlined dumps of sodium chromate and dichromate have contaminated the groundwater, the surface water, and the rock itself with hexavalent chromium. Shallow dumps still existing at the site continue to leach chromium to the groundwater, and hexavalent chromium has also impacted downgradient farming communities.
Geosyntec practitioners have demonstrated the use of an anaerobic, pilot-scale bioreactor to treat the chromium-impacted water that is pumped out of the Lyttleton mine. In collaboration with colleagues at South Africa's University of the Free State, our engineers used bench-scale columns filled with local limestone and water from the mine to test the various design components of the bioreactor. Bench-scale testing demonstrated that the bioreactor columns could remove concentrations of hexavalent chromium as high as 150 ppm from the influent water.
We then designed a horizontal flow, fixed-film bioreactor that uses the mine's existing high-water storage reservoir as a source of feed water. Our design also uses a local source of electron donor and reactor media to minimize costs and simplify logistics. Start-up data indicated that reducing conditions were established in the reactor as expected. Operations during the first 40 days resulted in reducing the effluent concentrations of hexavalent chromium by 85%. Subsequent minor modifications to the system have resulted in non-detect concentrations of hexavalent chromium in the effluent. Similar reactors are being developed for both uranium and acid mine drainage.
For the Irvine Ranch Water District in California, Geosyntec engineers designed a 250-gpm pilot gravel bioreactor to remove selenium, nutrients, and bacteria from surface water using a subsurface installation at the Cienega Filtration Project in Irvine. The novel system may eventually be used to treat a variety of constituents. Currently, a mass balance of the system is being prepared to analyze the behavior of various constituents, including selenium.