Co-digestion of sewage sludge and fat, oil, and grease with BioAmp pretreatment under mesophilic conditions

This study investigated the potential benefit of co-digesting sewage sludge (SS) and settled/mixed fat, oil, and grease (FOG) with BioAmp pretreatment, a bio-additive product, for a utility. BioAmp was found to increase the soluble chemical oxygen demand (SCOD)/COD ratio of FOG (from 4.19 to 4.49), consequently enhancing volatile solid (VS) reduction (from 75.9 to 84.8%). The ratio of SS and FOG for co-digestion was based on their actual daily production volume, resulting in FOG content ranging between 15 and 20% VS for co-digestion assays. Co-digestion of SS and mixed FOG waste after BioAmp pretreatment produced the highest cumulative methane yield (CMY) of 400 mL/g VS_added, which is 98% higher than that of mono-digestion of SS (202 mL/g VS_added). Kinetic analysis demonstrated a synergistic effect in co-digestion of FOG and SS with both increased CMY and degradation rate constant. For the utility, co-digestion can increase methane yield by 141%; however, 7.77% more biosolids are produced and 6% more total nitrogen will return to mainstream, a potential burden to nutrient management. Overall, the results of batch test demonstrated the benefits of co-digestion of SS and FOG with BioAmp pretreatment in terms of increased CMY, degradation rate constant, and VS reduction. The analysis of the utility showed that co-digestion can produce more biogas but also increase more biosolids and total nitrogen return.