Technologies and Strategies for the Treatment of Selenium as a Microconstituent in Industrial Wastewater

Selenium is a naturally occurring component of coal, petroleum, and some soils and is present in the wastewater streams of mining, refining, and coal burning power generating facilities, as well as in some landfill leachates and agricultural drainage. Selenium, particularly in its dissolved forms, is not significantly removed from wastewater through conventional treatment and, when discharged to receiving waters, is an environmental concern due to bioaccumulation effects. Many regulatory agencies have driven selenium discharge limits to very low levels.Regulatory discharge limits of 5 μg/L and lower have presented challenges to many industrial facilities. Consistently achieving these effluent selenium concentrations is difficult with currently available selenium removal technologies. This is largely due to the complex aquatic chemistry of the element and the different approaches required for treatment based on the selenium species present. Chemically, selenium is similar to sulfur and is found in four common oxidation states in the environment, namely: Se (+VI) (i.e. selenate), Se (+IV) (i.e. selenite), Se (0) (i.e., colloidal elemental selenium), and Se (-II) (i.e., selenide species). The behavior of selenium in wastewater treatment processes depends critically on its chemical form and on concentrations of other contaminants in the wastewater. Knowledge of the speciation of selenium in wastewater is therefore important for the development of adequate treatment strategies specific to selenium.This paper describes several strategies for the removal of selenium in the power and refining industries. It discusses current practices as well as new emerging technologies that may be considered for meeting strict effluent discharge limits. The paper will also discuss selenium chemistry and speciation and their importance in the selection of successful treatment strategies.