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Selenium

Selenium is an ongoing environmental concern because it can be toxic to aquatic wildlife and birds. The United States Environmental Protection Agency is currently updating the national recommended water quality criteria for selenium. The updated criteria will likely result in more restrictive effluent discharge standards for wastewater treatment plants.  Biological treatment is an effective way to remove selenium from water, and MicroC® serves as a supplemental carbon source and electron donor in such processes.

The Problem

Selenium is a naturally-occurring element that is present in certain rocks, soil, coal, and oil. Runoff, wastewater and emissions from mining operations, agricultural activities, power generation, and oil refining can mobilize and redistribute selenium throughout the environment. Selenium is used industrially as a glass additive and as a component of some pigments, photo cells, solar cells, and shampoos. Selenium is a trace element important for human and animal nutrition, but can be toxic at elevated levels.

Selenium’s potential toxicity to aquatic wildlife and birds has been a concern since at least 1982, when unusual mortality in animals at the Kesterson National Wildlife Refuge in California was attributed to selenium. Selenium bioaccumulates in the food chain, and its toxicity to wildlife is caused primarily by consumption of selenium-contaminated food rather than exposure to contaminated water. Selenium’s toxic effects in wildlife include increased embryo mortality and deformities.

In 1987, EPA set a national recommended chronic water quality criterion for selenium of 5 μg/L to protect aquatic life. In July 2015, EPA released updated, and more conservative, draft water quality criteria for selenium based on fish-tissue sampling and modeling. When the criteria are finalized, state regulatory agencies will begin incorporating the revised criteria into NPDES permits. In addition, EPA has begun including treatment goals for selenium in some industry-specific regulations, such as the proposed Effluent Limitations Guidelines released in 2013 for the Steam Electric Power Generating Point Source Category.  

Selenium can also impact human health. Long-term consumption of drinking water with high selenium concentrations could cause hair loss, nail brittleness, skin lesions, numbness in extremities, and/or circulatory problems. EPA has set a maximum contaminant level (MCL) for selenium in drinking water of 50 μg/L.

Selenium chemistry is complex. Selenium forms many organic and inorganic molecules; can be soluble, a particulate, or a gas; exists as six stable isotopes; and has four oxidation states (-2, 0, +4 and +6). Selenium chemistry in water depends on the water characteristics (temperature, pH, redox conditions, etc.) as well as any other chemicals present. In water with a moderate pH, most dissolved selenium occurs as inorganic selenate (SeO4²¯) or selenite (SeO3²¯), with the proportion determined by pH and redox conditions. Waters may also include insoluble particles of elemental selenium and/or suspended particles containing selenium. 

The selenium chemistry of wastewater varies greatly between industries, between treatment plants in the same industry, and even within different processes at a single treatment plant. Selenium’s complex chemistry makes it challenging to treat. Treatment technologies for selenium can be divided broadly into three categories: physical treatment such as filtration, chemical treatment such as precipitation or adsorption, and biological treatment. The most appropriate and cost-effective removal technology (or combination of technologies) depends greatly on wastewater flows, selenium forms and concentrations, and the presence of other chemicals such as metals, nitrates, etc. 

How EOSi can help

Biological treatment is emerging as a cost-effective option for removing dissolved selenium from wastewater. In this process, microbes use a supplemental carbon source such as MicroC® as an electron donor, and selenate/selenite as the electron acceptor. This reduces selenate and selenite to insoluble elemental selenium, which either precipitates or is trapped in the biomass. Microbes will only use selenate/selenite as electron acceptors if alternative, more preferred electron acceptors such as oxygen and nitrate are not present, so the process must take place under anoxic conditions and after denitrification. Learn more about EOSi’s experience with biological treatment of selenium in the mining and power generation industries.