Biological selenium removal from wastewaters

In this thesis, microbial conversion of water-soluble, highly toxic forms of selenium (selenate, selenite) to less bioavailable elemental selenium was investigated. By the exploitation of different groups of microorganisms (selenium-respiring, nitrate-reducing and sulfate-reducing bacteria, methanogenic archaea) the operational window of conventional and new hybrid anaerobic bioreactors treating selenium containing anthropogenic waste streams has been determined. A special focus was given to selenium speciation in solid, liquid and gas phase. Selenium speciation in the solid phase was assessed by direct and non-destructive X-ray absorption fine structure spectroscopy. It was demonstrated that selenium solid phase speciation is more complex than initially expected, as different side products such as metal selenides and organic selenium compounds are (trans)formed. Regarding the liquid/gas phase speciation, selenium forms volatile alkylated species under interaction with reduced sulfur compounds depending on operational parameters applied. Selenium oxyanions are not only toxic to animal and human populations, but are here shown to largely influence anaerobic food webs, due to their effect on acetoclastic and hydrogenotrophic methanogen