A sequencing batch reactor (SBR) supplied with Bacillus subtilis (treatment group) was employed to treat the sludge from a re-circulating aquaculture system (RAS). The crude protein content of bio-flocs from the treatment group increased from 21.52% +/- 1.5% to 29.65% +/- 13.34%, which was 23.97% +/- 11.62% greater than that of the SBRs without B. subtilis (control group). The removal rate of dissolved inorganic nitrogen (RRDIN) for the treatment group was 0.41 +/- 0.079 mg L-1 d(-1), which was 1.17 times greater than that of the control group. The utility rate of total organic nitrogen (URTON) for the treatment group was 1.42 +/- 0.33 mg L-1 d(-1), which was 1.71 times greater than the control. The removal rate of dissolved organic carbon (RRDOC) for the treatment group was 138.39 +/- 7.77 mg L-1 d(-1), which was 1.95 times greater than the control. The extra-cellular polymer substance (EPS) was primarily composed of polysaccharides. The flocs volume after 5 min (FV-5 min) reached 22.67% +/- 2.08% at 19 days. (C) 2012 Elsevier Ltd. All rights reserved.