Low pH (6, 5 and 4) sulfate reduction during the acidification of sucrose under thermophilic (55 °C) conditions

The effect of a low pH (6, 5 and 4) and different COD/SO42− ratios (9 and 3.5) on thermophilic (55 °C) sulfate reduction and acidification of sucrose was investigated using three upflow anaerobic sludge bed reactors fed with sucrose at an organic loading rate of 3.5 gCOD (lreactor d)−1. The three reactors showed nearly 100% acidification of sucrose for all pH values and COD/SO42− ratios investigated. Sulfate reduction was complete at pH 6 and a COD/SO42− ratio of 9. At pH 5, sulfate reduction efficiencies were 80–95% for both COD/SO42− ratios (9 and 3.5). At pH 4, sulfate reduction efficiencies further dropped to 55–65% at a COD/SO42− ratio of 9 and 30–40% at a COD/SO42− ratio of 3.5. The pH decrease from 6 to 5 or 4 caused a shift in the acidification products from mainly acetate to butyrate, as well as a higher production of ethanol, especially at pH 4. At pH 4, propionate and methane were not formed and hydrogen concentrations in the biogas reached 50%, equivalent to a hydrogen yield of 1.3 mol H2 (mol glucose)−1. This study shows that sulfate reduction is possible in the acidification phase of anaerobic wastewater treatment at pH values as low as 6 till 4 and that the pH strongly affects both the acidification pathways and the sulfate reduction efficiencies.