• decolorization;
  • azo dyes;
  • anaerobic baffled reactors;
  • sulfate;
  • anaerobic conditions

This study investigates the effects of sulfate on biological azo dye decolorization in anaerobic baffled reactors (ABRs) using Acid Red 18 as a model azo dye. The ABRs supplemented with sulfate at chemical oxygen demand (COD):sulfate ratios of 2.5 and 0.6 had the decolorization efficiencies of (89.4 ± 3.62)% and (89.9 ± 5.50)%, respectively. These were significantly greater than the decolorization efficiency of the ABR without sulfate supplements that had the decolorization efficiency of (76.7 ± 5.14)%. The primary decolorization mechanisms in the ABR with no sulfate added and the ABR with the COD:sulfate ratio of 2.5 were biological and required the presence of cells. By contrast, the azo dye decolorization in the ABR with the COD:sulfate ratio of 0.6 can occur to a great extent in the absence of cells, suggesting the role of extracellular mechanisms on azo dye decolorization under this condition. © 2012 American Institute of Chemical Engineers Environ Prog, 32: 1045–1054, 2013