ABSTRACT
Selenium in various forms is present in subsurface tile drainage from agricultural lands in the western San Joaquin Valley of California, usually at concentrations ranging from 100 to 1400 µg/l. Concentrated waste-grown microalgal sludge originating from Richmond, CA sewage and produced during methane fermentation has been demonstrated to be a satisfactory reducing material, since it converts soluble selenate in drainage waters to insoluble forms. The cost of microalgal sludge as a reductant will mainly depend on the cost of ponds and the productivity of the algae. The fact that microalgae readily grow on drainage waters in highrate ponds and tend to harvest themselves by sedimentation, along with their tendency to undergo methane fermentation in a well-designed system, assures a continuous supply of material with reducing capability. Poor removal compared with Richmond algae is believed to be caused by the presence of large amounts of dissolved and possibly insoluble sulfate in settled concentrates.
