Kinetic modeling of anaerobic co-digestion with glycerol: Implications for process stability and organic overloads

The energy crisis, depletion of fossil fuels, and global waste issue highlight the need for sustainable and ecofriendly energy processes. In this study, the anaerobic co-digestion of sewage sludge (DS), milk sludge (MS), and food waste (FW) with glycerol (GL) from biodiesel production was investigated. Binary co-digestion of MSGL, DS-GL, and FW-GL, as well as ternary co-digestion of MS-FW-GL and DS-FW-GL, were examined to determine the optimal combinations for biogas production and process stability. Adding 5% GL (v/v) increased methane yields by 88.78%, 405.82%, and 40.03% for binary mixtures and 55.57% and 298.53% for ternary mixtures, respectively. In addition, the ternary mixtures resulted in the accumulation of volatile fatty acids at 2136.96 mg/ L and 1843.62 mg/L, respectively, causing a reduction in methane yield compared to that in binary mixtures. GL added to binary and ternary mixtures delayed optimal methanogenic activities, with higher hydrolysis rates and shorter lag times observed in single substrate-contained and binary mixture reactors. Ternary mixtures showed lower hydrolysis rates and longer lag times, indicating that methanogens required more time to adjust to the increased organic content. Co-digestion procedures using different substrate ratios should consider the risk of organic overloads, which can lead to system instability or failure.

Automated summary

This study investigated the anaerobic co-digestion of sewage sludge, milk sludge, and food waste with glycerol from biodiesel production. The results showed that adding 5% glycerol significantly increased methane yields in both binary and ternary mixtures. However, ternary mixtures had a lower hydrolysis rate and longer lag time compared to binary mixtures, indicating a need for more time to adjust to the increased organic content. Co-digestion procedures using different substrate ratios should consider the risk of organic overloads to avoid system instability or failure.