Selenium Removal from Oil Refinery Wastewater Using Advanced Biological Metal Removal (ABMet?) Process

摘要

Selenium is a regulated drinking water contaminant. Excessive selenium in an aquaticenvironment is hazardous to living organisms because it accumulates in the food chain. Waterquality is impaired due to refinery selenium discharges in selected regions of the U.S. (e.g.,North San Francisco Bay). The current National Pollutant Discharge Elimination System(NPDES) effluent limits for refineries are interim performance-based limits. The new dischargelimits for refineries might be required by the ongoing Total Maximum Daily Load (TMDL)process that considers the fish tissue-based objectives for selenium. In recent years, the analyticaldetection limit of the total dissolved selenium was lowered to very low levels, ranging from 1 to3 μg/L. The current performance-based NPDES limits reaching 50 μg/L may be reduced after theUS EPA proposes new aquatic life criteria for selenium. Biological reduction of oxy-anions ofselenium (selenite and selenate) to elemental selenium is well documented. The ABMET processis the most demonstrated biological reduction process and can produce an effluent containingless than 5 μg/L selenium. The ABMet process is an anoxic biological reduction processeffective in reducing selenite and selenate to elemental selenium. The process involves growingspecial ABMet bacterial cultures on granular activated carbon (GAC) media. A treatability studywas conducted using a bench-scale flow-through ABMet test system. The objective of this studywas to evaluate potential of the ABMet process for removal of selenium from two refinerywastewater streams, including the refinery’s activated sludge plant (ASP) effluent and sour waterstripper bottom streams. While the ASP effluent contains mainly selenite with some selenate, thestripped sourwater (SSW) contains selenocyanate (SeCN). The anoxic selenite and selenaterespiring bacteria are not capable of reducing SeCN. Therefore, SSW must be oxidized toconvert SeCN to selenite and selenate. Laboratory testing for selenium reduction was performedin two bench-scale ABMet bioreactors, run in series. The bioreactors were initially loaded withGAC, which serves as a permanent substrate for the reductive biofilm, and is subsequentlyinoculated with the selected selenium culture. Each bioreactor was supplemented daily with anABMet’s molasses-based nutrient solution. The ABMet process was able to reduce the seleniumconcentration of the ASP effluent from 368 to 2.3 micrograms per liter (μg/L) and the seleniumconcentration of the pre-oxidized SSW from 2,510 to 11.7 μg/L.