Effective strategy for improving sludge treatment rate and microbial mechanisms during chromium bioleaching of tannery sludge

Abstract By using an enriched microbial community, repeated-batch processes for bioleaching of chromium from tannery sludge was set up. The effects of batch-feeding ratio (FR) and sludge total solid (TS) on sludge retention time and treatment rate were investigated and a two-stage repeated-batch process was developed. The results of flask-scale repeated-batch bioleaching suggested that the sludge TS and FR impacted the bioleaching performance and a maximum sludge treatment rate of 6.82 g/L/d was achieved at a TS of 6% and FR of 0.5. Repeated-batch bioleaching using single continuously stirred tank reactor (CSTR) did not yield a satisfactory treatment rate. However, a two-stage process (two CSTRs in series) successfully reduced the sludge retention time and a maximum sludge treatment rate of 4.60 g/L/d with over 90% chromium removal ratio was achieved. The high relative abundance of Acidithiobacillus was found critical for effective bioleaching; however, other genera including Sulfobacillus, Alicyclobacillus, Mycobacterium , and Acidiphilium might also play important supplementary roles.