By MIKE LAMB | Desert Dispatch
HINKLEY — Pacific Gas Electric has received approval for a pilot test that will use a bioreactor for converting hexavalent chromium in Hinkley groundwater to solid trivalent chromium.
The Lahontan Regional Water Quality Control Board announced Friday that based on information PG E submitted, the project meets the required conditions under its General Discharge Requirements for the General Site-wide Groundwater Remediation Project.
The bioreactor project will be part of PG E’s cleanup process after the discovery that the company was responsible for toxic levels of chromium-6 contaminating the community’s groundwater.
The bioreactor process, according to the board, begins by diverting extracted groundwater from the Central Area In-Situ Reactive Zone operations and sending it to the first bioreactor — a 10,500-gallon vessel. Up to 585 gallons of acetic acid (vinegar) and up to 46 gallons of phosphoric acid, both at 20 percent concentration by weight, will be added to promote hexavalent chromium conversion to trivalent chromium.
As the trivalent chromium precipitates out, it’s removed by attaching to biofilms. Treated water then moves to the second bioreactor to undergo aerobic treatment using rock and sand filters and finally to a bag filter for removal of biomass, trivalent chromium and byproducts such as iron and manganese.
The effluent leaving the second stage vessel is expected to be depleted of chromium, nitrate, acids, biomass and byproduct metals. Final treated effluent will be dosed with ethanol and re-injected south of the Central IRZ in the South Central Reinjection Area. The board’s report said no impact to the groundwater quality is expected since total dissolved solids and pH are expected to be the same as the influent.
“The overall duration of the project is approximately 12 months from final system design and installation to system shut down and final sampling,” the Lahontan report said. “No change in groundwater levels are expected since all the project water will be returned to the aquifer and the added volumes of acids will be negligible in comparison.”
The project is limited to 14 months, but the Lahontan report said the discharger may propose to extend the project. All phases of this project are proposed on land owned by PG E or land with access agreements with PG E.