reactors (Reis et al., 1995). In general, acidogenesis proceeds well without being adversely affected by SRB activity or H2S toxicity. The critical fact in the two-phase configuration is to assure complete reduction of sulfate in the first stage. Results obtained so far have shown that 80 to 95% of the sulfate is converted to sulfide in the acidification stage, but complete sulfate reduction does not occur (Rinzema et al., 1986; Reis et al., 1995). Complete sulfate removal will only be possible if a sufficient amount of hydrogen is generated, which implies relatively high COD/sulfate ratios in the wastewater.