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

Selenium is a regulated drinking water contaminant. Excessive selenium in an aquatic environment is hazardous to living organisms because it accumulates in the food chain. Water quality 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 discharge limits 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 analytical detection limit of the total dissolved selenium was lowered to very low levels, ranging from 1 to 3 μg/L. The current performance-based NPDES limits reaching 50 μg/L may be reduced after the US EPA proposes new aquatic life criteria for selenium. Biological reduction of oxy-anions of selenium (selenite and selenate) to elemental selenium is well documented. The ABMET process is the most demonstrated biological reduction process and can produce an effluent containing less than 5 μg/L selenium. The ABMet process is an anoxic biological reduction process effective in reducing selenite and selenate to elemental selenium. The process involves growing special ABMet bacterial cultures on granular activated carbon (GAC) media. A treatability study was conducted using a bench-scale flow-through ABMet test system. The objective of this study was to evaluate potential of the ABMet process for removal of selenium from two refinery wastewater streams, including the refinery's activated sludge plant (ASP) effluent and sour water stripper bottom streams. While the ASP effluent contains mainly selenite with some selenate, the stripped sourwater (SSW) contains selenocyanate (SeCN). The anoxic selenite and selenate respiring bacteria are not capable of reducing SeCN. Therefore, SSW must be oxidized to convert SeCN to selenite and selenate. Laboratory testing for selenium reduction was performed in two bench-scale ABMet bioreactors, run in series. The bioreactors were initially loaded with GAC, which serves as a permanent substrate for the reductive biofilm, and is subsequently inoculated with the selected selenium culture. Each bioreactor was supplemented daily with an ABMet's molasses-based nutrient solution. The ABMet process was able to reduce the selenium concentration of the ASP effluent from 368 to 2.3 micrograms per liter (μg/L) and the selenium concentration of the pre-oxidized SSW from 2,510 to 11.7 μg/L.