Selenium is a naturally occurring element that usually is present in only small amounts in water. It is an essential human nutrient, but it has known toxic effects on animals exposed through grazing. It is an essential human nutrient but only a very small amount is needed.

Selenium occurs naturally in soil and is a by-product of copper mining and smelting.

The EPA has established the MCL for selenium in water at 0.05 mg/L.

The best way to remove selenium is reverse osmosis or with an anion exchanger. Anion exchange reduces selenium by 60 to 95%.

Source: Enting Engineering Handbook.