Valorization of waste activated sludge through enzymatic pretreatment and co-digestion with food waste: impacts on methane production and process stability

In this study, the integration of enzymatic pretreatment with anaerobic co-digestion (ACoD) was evaluated as a strategy to enhance methane production from waste activated sludge (WAS) and food waste (FW). Hydrolytic enzymes were produced via solid-state fermentation (SSF) using Aspergillus niger and babassu cake as substrate, resulting in a crude enzyme extract with high xylanase and protease activities, indicating strong hydrolytic potential for sludge pretreatment, along with lower activities of amylase and endoglucanase. Enzymatic pretreatment of WAS at 0.2 U/mg total suspended solids promoted up to 95% organic matter solubilization. ACoD was evaluated under three conditions: (I) FW with untreated WAS, (II) FW with enzymatically pretreated WAS, and (III) FW with WAS and direct enzyme addition. Enzymatic pretreatment increased methane yield by approximately 30%, from 210 to 270 NmL CH₄/gVS, while direct enzyme addition led to more moderate improvements. Kinetic modeling using modified Gompertz and logistic models confirmed enhanced methane production but indicated longer lag phases in enzyme-treated systems, attributed to intermediate product accumulation. Under these conditions, elevated volatile acidity/total alkalinity ratios and increased ammonia concentrations suggested process stress. Overall, SSF-derived enzymes combined with co-digestion represent a promising strategy to boost methane production, while requiring careful control to avoid acidification.