(93a) Treatment of Selenium Oxyanions in Wastewater by Using Artificial Bacterial Combinations

Desulfurized wastewater in coal-fired thermal power stations contains selenium oxyanions in the form of selenate (SeO₄^2-) and selenite (SeO₃^2-). As soluble selenium oxyanions are deadly chemicals toward living things, industrial wastewater containing selenium oxyanions is strictly regulated. Selenite can be removed by chemical treatment, but selenate is difficult to remove. The chemical method needs plenty of chemicals and a large number of sludge is produced. On the other hand, both selenate and selenite can be reduced to insoluble elemental selenium by biological reduction of selenium oxyanions. In this process, however, expensive chemicals like yeast extract are inevitably required for the bacterial reduction of selenium oxyanions. Therefore, treatment of selenium oxyanions with inexpensive alcohol is indispensable to practical application. We have proposed and investigated a novel biological treatment of selenium oxyanions in wastewater with inexpensive alcohol by using artificial bacterial combinations.

An isolated selenate-reducing bacterium termed 4C-C, which was identified as a Pseudomonas sp., was capable of reducing selenate to elemental selenium via selenite formation by using alcohol as an electron donor; however, its potential to reduce selenite was lesser than its ability to reduce selenate. As a result, selenite reduced from selenate was accumulated temporarily. To overcome this problem, an improvement of process from selenite to insoluble selenium was investigated with hydrogen sulfide by sulfate-reducing bacterium. A Desulfovibrio sp. strain HT-1 that was capable of reducing sulfate to hydrogen sulfide by using alcohol as an electron donor was newly isolated. An artificial combination of selenate-reducing and sulfate-reducing bacteria (Pseudomonas sp. strain 4C-C and Desulfovibrio sp. strain HT-1) was examined in suspension for the treatment of artificial wastewater containing selenate and sulfate by using alcohol. This bacterial combination enabled the treatment of selenate to insoluble selenium without selenite accumulation. The process from selenite to insoluble selenium was improved using the isolated sulfate-reducing bacteria.

In addition, the artificial wastewater containing nitrate in addition to selenate and sulfate, which simulated the desulfurized wastewater in coal-fired thermal power stations, was treated by using the artificial combination of Pseudomonas sp. strain 4C-C and Desulfovibrio sp. strain HT-1 with inexpensive alcohol. In this case, selenite and nitrite reduced from selenate and nitrate were accumulated temporarily. To prevent the inhibition due to an existence of nitrate in the wastewater, the denitrifier Paracoccus denitrificans JCM 6892 was added to the bacterial combination. A reformed artificial combination of selenate-reducing, sulfate-reducing and nitrate-reducing bacteria (Pseudomonas sp. strain 4C-C, Desulfovibrio sp. strain HT-1 and Paracoccus denitrificans JCM 6892) was examined in suspension for the treatment of artificial wastewater containing selenate, sulfate and nitrate by using alcohol. The bacterial combination enabled the treatment of selenate to insoluble selenium without selenite and nitrite accumulation. A reformed method for the treatment of selenium oxyanions was proposed by using selenate-reducing, selenite-reducing and nitrate-reducing bacteria with alcohol as an electron donor.

Furthermore, a polymeric gel immobilized three bacterial strains could also treat the selenate to insoluble selenium using alcohol. The immobilized cells in the polymeric gel were pigmented thin pink, and there was no elemental selenium in the artificial wastewater containing selenate, sulfate and nitrate. This indicated that the insoluble selenium was completely captured in the polymeric gel. The reformed novel artificial combination of selenate-reducing, sulfate-reducing and nitrate-reducing bacteria was able to treat selenium oxyanions to insoluble selenium rapidly and to simplify the recovery of insoluble selenium. This simple artificial bacterial combination would be effective in the presence of inexpensive alcohol to treat selenium oxyanions in various wastewaters containing selenium and the other oxyanions such as nitrate, nitrite and sulfate.