Selenium is a metalloid present in the Earth’s crust, often occurring in higher concentrations with sulphur containing minerals. Weathering of these minerals can lead to high selenium concentration in groundwater. The mining and power production industries can produce wastewater high in selenium, along with the agricultural industry by the irrigation of selenium rich soils. Selenium can present in different chemical forms in water, typically selenate, selenite and organoselenium. Each of these have different toxicities and treatment requirements.
Selenium is also present in many foods, which is the main source of selenium intake for most people. The selenium content of foods is mainly dependent on the selenium content of soil for plants, and selenium content of feedstuff for livestock. Selenium is an essential element for human health, but high levels are associated with numerous diseases. The World Health Organisation uses 40 ppb as a guideline for the maximum selenium concentration of drinking water, but notes the majority of selenium intake is from food sources, and as such a limit on drinking water isn’t necessary in a lot of cases.
High selenium concentration in water is also toxic to the aquatic environment. Aquatic organisms are impacted by selenium which effects the aquatic environment’s food-web, leading to bio-accumulation and toxicity in larger species such as fish, amphibians and invertebrates. To address this, some countries have mandated a discharge limit of 5 ppb of selenium to surface water.
Reducing the concentration of selenium in water can be difficult, as commonly used methods such as lime softening cannot remove selenate from a solution. Precipitation is a commonly used treatment method, but requires large chemical input and produces unstable sludge as a waste product. To reach a level of 5ppb, a combination of chemical precipitation and a biological system are usually required.
Our DESALX® technology is a high recovery two stage CIF® process that can remove selenium, sulphate, and calcite, as well as lower the overall TDS of the water. The low volume by‐products from the DeSALX® process are converted to a manageable mix of salts of sulphate, calcium, magnesium and selenium to produce a minimum or zero liquid discharge process.
Clean TeQ Water has previously developed flowsheets and piloted systems for clients based on their feed and product water specifications that comply to selenium discharge limits as low as 5 ppb.
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