Chromium (VI) reduction in activated sludge bacteria exposed to high chromium loading

A mixed-culture of bacteria collected from a wastewater treatment plant in Brits, North-West Province (South Africa) biocatalytically reduced Cr(VI) at much higher concentrations than previously observed in cultures isolated in North America. Complete Cr(VI) reduction in aerobic cultures was achieved at a high concentration of 200 mg/L after incubation for only 65 hours. Under anaerobic conditions up to 150 mg, Cr(VI)/L was completely removed after incubating for 130 to 155 hours, still higher than the Cr(VI) reduction achieved with previous cultures where complete removal was only observed in cultures with the added Cr(VI) concentration not greater than 30 mg/L. Cr(VI) reduction capability of the cultures was verified in purified cultures. Consortium cultures were characterised using 16S rRNA partial sequence analysis. Results showed that the gram-positive Bacillus genera predominated under aerobic conditions with a small composition of the gram-negative Microbacterium sp. There was more biodiversity observed in the anaerobic cultures with the marked appearance of Enterococcus, Arthrobacter, Paenibacillus and Oceanobacillus species. The results showed that Cr(VI) reduction rate in the new culture was up to eight times higher than that previously observed in other Cr(VI) reducing cultures isolated from Cr(VI) contaminated soil environments in Newark (New Jersey) and other sites in North America.