Modeling the anaerobic digestion of wastewater sludge under sulfate-rich conditions

Anaerobic digestion (AD) is a biological treatment process to stabilize organic solids and produce biogas. If present, sulfate is reduced to sulfide by anaerobic sulfate-reducing bacteria and the sulfide can be toxic to anaerobic microorganisms. Here, the effect of high initial sulfate concentration on AD of wastewater sludge was investigated using lab-scale batch experiments. Additionally, a systematic mathematical modeling approach was applied for insight into the experimental results. Cumulative biogas and methane production decreased with increasing initial sulfate doses (0-3.300 mg S L^-1 ). The correlation between the sulfate dose and methane production was consistent with theoretical predictions and model results, indicating no toxic effect of sulfide on methane production. The carbon dioxide content in the biogas decreased linearly with the increasing sulfate dose, which is consistent with the model-predicted behavior of the bicarbonate and hydrogen sulfide buffering system. The examined high sulfate concentrations resulted in no clear negative effects on the COD removal or VSS destruction of the wastewater sludge, indicating negligible inhibition by sulfide toxicity. Even considering the possibility of ferrous sulfide precipitation and the low model estimates of residual sulfide concentration the residual sulfide concentration was higher than reported concentrations that trigger process inhibition. PRACTITIONER POINTS: The effect of sulfate loading on anaerobic digestion of waste activated sludge was characterized. The stoichiometry of sulfate reduction allows accurate prediction of CH₄  loss. High sulfate levels (up to 3300 mg/L as S) did not affect COD/VSS removal. Sulfide formation increases effluent COD; often misinterpreted as sulfide toxicity. Correcting COD for sulfide's contributions is crucial for results interpretation.