Heterotrophic bio-reduction process of hexavalent chromium: Toxic effects, concentration-adaptation and sustainable sludge-based bio-augmentation strategy

Abstract

For high hexavalent chromium (Cr(VI)) industrial wastewater, heterotrophic bio-reduction is a promising process. Nevertheless, a high concentration of Cr(VI) (e.g., more than 100 mg L−1) may still devastate the sustainability of the bioprocess. In this paper, the Cr(VI) toxic effects and resistance on concentration-adaptation and a sludge-based bio-augmentation were studied. The results suggested: 1) The Cr(VI) exposure took 6–24 h to cause significantly the toxic inhibition and lethal effects on the sludge communities. 2) Although adaptation to 50 mg L−1 influent Cr(VI) improved Cr(VI) reduction by 2 times, the stepwise adaptation was ineffective for 100 mg L−1 Cr(VI). Lowering the Cr(VI) loading could not recover the dysfunctional sludge. 3) A bio-augmentation strategy by exchanging Cr(VI) stressed sludge with Cr(VI) free sludge at intervals (e.g., 0.8 gMLVSS L−1 every 3.5 d) was proposed and treated successfully 100 mg L−1 Cr(VI). The mechanism behind might be that exchanging sludge at intervals maintained a relatively high Cr(VI) reduction bacteria activity and a stable live/dead bacteria ratio. 4) The variation of Cr(VI) reductase distribution, morphology and microbial community during the concentration-adaption and bio-augmentation implied the sludge might use multiple toxic resistance mechanisms such as sludge aggregation, the enrichment of Cr(VI) reduction bacteria. Therefore, through comprehensively testing, this work not only revealed the draws of concentration-adaptation for Cr(VI), but also developed a sustainable, convenient and robust sludge-based bio-augmentation strategy to overcome the highly toxic Cr(VI) wastewater.