Many of the potential attractive wastewater streams for anaerobic treatment contain sulfate or sulfite, in addition to high concentrations of unacidified organic matter. Typical wastewaters are those originating from food production and pulp and paper manufacturing. Under anaerobic conditions, dissimilatory sulphate reducing bacteria produce sulfide, which is toxic for methanogenic bacteria. Moreover, acidification can result in a sub-optimal pH for the methanogenic conversions. A method to avoid sulfide inhibition is the separation of the sulfide production step from the methanogenic step in a two-phase system. This configuration also improves the overall treatment efficiency for unacidified wastewaters. Thus, sulfate reduction occurs together with acidification in the acidification stage. Wastewater treatment plants using this two-phase configuration, like the one of CERESTAR/CARGILL, add NaOH to the acidification reactor to avoid excessive lowering of the pH, keeping it at 6. The role of pH in the acidification and sulfate reduction pathways and conversion efficiencies is not clear. If operation at lower pH (5 or 4) would be feasible or even beneficial, companies could lower the NaOH addition, therefore making their anaerobic wastewater treatment much cheaper.