Reduction of Selenium Oxyanions in Wastewater Using Two Bacterial Strains

The biological reduction of selenium oxyanions is capable of reducing both selenate and selenite to insoluble elemental selenium. In this process, however, bacteria inevitably require expensive chemicals such as yeast extract in almost all cases. Therefore, the reduction of selenium oxyanions with inexpensive alcohol would be more practical. A Pseudomonas sp. strain 4C‐C isolated from a sludge in a wastewater treatment facility was able to reduce selenate to selenite using ethanol as an electron donor for its anaerobic respiration, but could not reduce selenite to elemental selenium. Paracoccus denitrificans JCM‐6892, on the other hand, was observed to be able to reduce selenite to elemental selenium in the presence of ethanol, but not selenate to selenite. Therefore, a mixture containing a suspension of Pseudomonas sp. strain 4C‐C and P. denitrificans JCM‐6892 cells allowed selenate to be reduced to insoluble elemental selenium via selenite in the presence of ethanol and was also capable of reducing nitrate to nitrogen gas. Aiming at simplicity of the recovery process of insoluble elemental selenium, a polymeric gel immobilized mixture of the two bacterial strains was examined using ethanol as an electron donor. The immobilized mixture could therefore reduce not only selenate to elemental selenium, but also nitrate to nitrogen gas in a single step. The gel that immobilized the microbial mixture changed its color during the process to bright red and no red elemental selenium was left in the wastewater. This indicates that the reduced elemental selenium was completely absorbed in the gel. This simple bacterial combination would therefore be effective in the presence of ethanol to reduce selenium oxyanions in various wastewaters containing selenium and the other oxyanions.