DUAL MEDIA SYSTEM FOR TREATING WASTEWATER CONTAINING SELENIUM

摘要

A bioreactor has a biofilm supporting bed 20 with at least two types of media. An upper media is relatively porous, preferably porous enough to admit particles of a lower media. In use, wastewater 18 flows downwards through the bed. Soluble nitrogen is reduced in the upper media and converted into nitrogen gas. Nitrogen gas bubbles rise through the upper media and escape from the bed. Selenium is reduced in the lower media and converted into elemental selenium. The elemental selenium is released periodically by backwashing the bed, which may cause fluidization or other expansion of the lower media into the upper media.