Chromium (VI) reduction in activated sludge bacteria exposed to high chromium loading: Brits culture (South Africa)

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. Cr(VI) reduction rate up to 8 times higher than the rate in previous cultures was achieved by the Brits culture under aerobic conditions. Near complete Cr(VI) reduction was observed in batches under initial concentrations up to 200mg Cr(VI)/L after incubation for 65h in aerobic cultures. Under anaerobic conditions up to 150mg Cr(VI)/L was completely removed after incubating for 130-155h. In the previous cultures, complete removal was only achieved in cultures at an initial Cr(VI) concentration lower than 30mg/L after incubation for 96-110h. 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. More biodiversity was observed in anaerobic cultures with the marked appearance of Enterococcus, Arthrobacter, Paenibacillus and Oceanobacillus species. Experiments run on purified individual species did not achieve the same level of Cr(VI) reduction as observed in the original consortium from sludge indicating possible existence of interspecies interactions necessary for optimum Cr(VI) reduction. All Cr(VI) reduced was accounted for as Cr(III) with a small error range (2-6%).