Laboratory-scale bioreactors for soluble selenium removal from selenium refinery wastewater using anaerobic sludge

Laboratory-scale bioreactors were constructed to investigate their applicability for recovery of soluble selenium from industrial wastewater. Microbial reduction of soluble selenium into elemental selenium is the key reaction because, as a result of its insoluble characteristics, it can be removed easily from the aqueous phase. Wastewater of a selenium refinery plant showed extremely low pH (<1.0), high salinity (6-7%), and various selenium concentrations (mainly selenite, 13.2-74.0mgl^-1). As preconditioning for microbial reduction, the wastewater pH and salinity were adjusted, respectively, to about 7.5 and 1.1%. Granular sludge of an up-flow anaerobic sludge blanket (UASB) reactor and suspended sludge of a sewage sludge digester were seeded respectively to a UASB-type reactor and a suspended sludge bed reactor. The 2.2-l bioreactors were operated at 30°C and hydraulic retention time of about 24h for preconditioned wastewater supplemented with ethanol as a carbon source. The removal of 1.5-3.5mgl^-1 soluble selenium was only 60% in the swim-bed reactor. The selenium concentration in effluent of the UASB-type reactor dropped to less than the regulated amount of the effluent concentration of selenium of 0.1mgl^-1 by day 15.