Biological selenium treatment reduces environmental impacts
Selenium is a naturally-occurring essential nutrient. In excessive amounts, however, it can be detrimental to natural wildlife, especially birds and fish. Selenium can be released into water resources through natural sources, but high concentrations can also be released through mining and energy plant processes. Stormwater runoff high in selenium must be treated to reduce harm and meet federal and state regulations. The EPA federal guideline for selenium in natural waters is 5 ug/L or ppb. In the past, meeting this level has presented technical treatment challenges. Today we have effective treatment systems available to achieve ultra-low levels, thus reducing harmful environmental impacts.
At mines and industrial sites, the approach to selenium treatment is based on chemistry, the operating environment, permit requirements and economics. Biological treatment has emerged as an efficient means of removing oxidized selenium. Most selenium-containing mine runoff and surface waters dissolve selenium in the oxidized form, selenate. Some waters can also contain dissolved selenite or a mixture of both anions.
Biological selenium treatment systems harness the natural ability of bacteria to precipitate the soluble selenium species to the insoluble form of elemental selenium. Biological treatment is most efficiently carried out in a confined bioreactor or biofilter system, where optimal conditions can exist to facilitate selenium precipitation and removal from the water stream.
Ideally, the precipitated selenium particles are retained in the system and are therefore not exposed to the receiving downstream water body. Various selenium treatment equipment configurations exist today, such as fixed bed bioreactors and fluidized bed bioreactors.
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