Acute effects of hexavalent chromium on the performance and microbial community of activated sludge in aerobiotic reactors

This study investigated the acute effects of hexavalent chromium (Cr(VI)) shock load at 2.5, 6 and 25 mg/L on the performance and bacterial community structures in aerobiotic activated sludge reactors. The results showed that eight-day Cr(VI) toxicity made the removal rates of COD and NH3-N to obviously decrease in all reactors. Furthermore, the higher the Cr(VI) concentration was, the more severe the influence on the whole system would be. The effect of Cr(VI) on NH3-N removal was more serious than that on COD and the function of nitrification was harder to recover. The specific oxygen uptake rate (SOUR) values dropped by 73%, 68% and 31% at 2.5, 6 and 25 mg Cr(VI)/L, respectively. The data of SOUR showed that though the concentration of Cr(VI) was low, the whole respiratory activity of bacteria was much affected. The relative abundance change of genus between the initial stage and the last showed that the bacterial community structure changed significantly. Comparing with the initial stage, the phyla of Proteobacteria, Acidobacteria and Planctomycetes were markedly reduced at the end stage. The genera of Ferruginibacter, Coxiella and Rhodanobacter were also markedly reduced. Although the performance of activated sludge can be restored at the end, the whole respiratory activity of bacteria was still at a low level according to the data of SOUR.