Industrial selenium pollution: Sources and biological treatment technologies

Selenium (Se) discharge into the environment is becoming a matter of increasing concern because it induces toxic effects to biota at low concentrations (several micrograms per liter). Industrial activities that include energy generation, metal and oil refining, mining, and agriculture irrigation generate effluents contaminated with selenium. Biological treatment of these effluents is gaining in popularity in recent years. Microbial reduction of selenium oxyanions to particulate elemental Se⁰ can be achieved in a number of bioreactor systems that are emerging as a viable bioremediation option because of their favorable cost, footprint, and treatment efficiency. Traditionally, granular sludge bioreactors (e.g., upflow anaerobic sludge blanket, UASB) have been tested for the treatment of selenium-laden wastewaters. Fluidized-bed bioreactors (FBBR) and packed-bed bioreactor systems were later adapted for Se treatment. The hydrogen-based hollow-fiber membrane biofilm reactor (MBfR) is a technology that delivers H₂ gas as the electron donor by diffusion to the biofilm formed on non-porous hollow-fiber membranes. A hybrid electro-biochemical reactor (EBR) which uses electrons that are delivered from an external power source through electrodes to selenium-reducing bacteria growing on electrodes has been developed. Constructed wetlands may be useful when the wastewater is produced in large volumes, but they are sensitive to temperature fluctuations and seasonal variation of the vegetation, and they have a large footprint. If Se⁰ colloids are not captured efficiently within the bioreactor, a challenge is removing colloidal Se⁰ from the effluent. When properly recovered, Se⁰ can be a valuable product due to its photo-optical, semiconductive, and adsorptive properties.