Unraveling the influence of the COD/sulfate ratio on organic matter removal and methane production from the biodigestion of sugarcane vinasse

Throughout the sugarcane harvest, it is common for sulfate to accumulate in the vinasse of sugar and ethanol plants. However, little is known regarding the influence of sulfate on the anaerobic digestion (AD) of vinasse, which may lead to severe performance losses. This study assessed the influence of various COD/sulfate ratios (12.0, 10.0 and 7.5) on both COD removal and methane (CH4) production from sugarcane vinasse AD. Batch assays were conducted in thermophilic conditions. At a COD/sulfate ratio of 7.5, CH4 production was 35% lower compared with a ratio of 12.0, considering a diversion of approximately 13.6% of the electron flow to sulfidogenesis. The diversion of electrons to sulfidogenesis was negligible at COD/sulfate ratios higher than 25, considering the exponential increase in CH4 production. Organic matter degradation was not greatly affected by sulfidogenesis, with COD removal levels higher than 80%, regardless of the initial COD/sulfate ratio.