Continuous biological removal of selenate in the presence of cadmium and zinc in UASB reactors at psychrophilic and mesophilic conditions

Abstract

This study investigated the continuous biological removal of selenate [Se(VI)] in the presence of cadmium [Cd(II)] and zinc [Zn(II)] by anaerobic granular sludge in two upflow anaerobic sludge blanket (UASB) reactors operating under psychrophilic (17.5 ± 0.4 ℃) and mesophilic (30 ± 0.5 ℃) conditions for 42 and 74 days, respectively. Se(VI) and total selenium (Se) removal efficiencies reached, respectively, 99.6% and 95.0% in the UASB reactor operating at psychrophilic conditions. Cd(II) and Zn(II) removal efficiencies by the mesophilic UASB reactor reached, respectively, 99.2% and 96.2%. In the presence of Cd and/or Zn in the influent, both Se(VI) and total Se had a higher removal efficiency than when solely feeding Se(VI), both at 17.5 ℃ and 30 ℃. The lactate removal rates of the UASB reactors amounted to 1.0–1.2 mM/h. The mass balance showed 64.8%–79.5% of the supplied Se was entrapped in the biomass at the end of the experiment. The polysaccharides and protein concentrations of the extracellular polymeric substances (EPS) increased ∼3 to 5 times upon heavy metal exposure. A dramatic change in the microbial community occurred in the UASB reactors during long term operation. This study showed that UASB reactors have desirable Se(VI), Cd(II) and Zn(II) removal efficiencies both at 17.5 and 30 ℃.