Sustainable bioreduction of toxic levels of chromate in a denitrifying granular sludge reactor

Abstract

Biological removal of chromate [Cr(VI)] in the presence or absence of nitrate by granular sludge biofilms was investigated in batch experiments and in a sequencing batch reactor (SBR). Denitrifying granular sludge cultivated from activated sludge was able to directly reduce Cr(VI) in the presence of an electron donor. Bioreduction was dependent on the initial Cr(VI) and the granular sludge concentrations. Bioreduction of Cr(VI) was followed by Cr(III) precipitation or entrapment in the granular sludge which was corroborated with decrease in total soluble Cr and increase in inorganic content of biomass. Batch experiments revealed that Cr(VI) addition has no major influence on high-strength nitrate (3000 mg L⁻¹) denitrification, but nitrite denitrification was slowed-down. However, SBR experiment demonstrated successful denitrification as well as Cr(VI) removal due to enrichment of Cr(VI)-tolerant denitrifying bacteria. In fact, stable SBR performance in terms of complete and sustained removal of 0.05, 0.1, 0.2, 0.3, 0.5 and 0.75 mM Cr(VI) and denitrification of 3000 mg L⁻¹ was observed during 2 months of operation. Active biomass and electron donor-dependent Cr(VI) removal, detection of Cr(III) in the biomass and recovery of ~ 92% of the Cr from the granular sludge biofilms confirms bioreduction followed by precipitation or entrapment of Cr(III) as the principal chromate removal mechanism. Metagenomic bacterial community analysis showed enrichment of Halomonas sp. in denitrifying granular sludge performing either denitrification or simultaneous reduction of nitrate and chromate.