Optimizing pre-treatment conditions for anaerobic co-digestion of food waste and sewage sludge

In this study, the optimum conditions of thermal-alkali pre-treatment and the performance of ammonia stripping were investigated for improving solubilization efficiency and methane yield in the anaerobic co-digestion of food waste (FW) and sewage sludge (SS). The reaction temperature, NaOH concentration and reaction time for the thermal-alkali pre-treatment were investigated to determine optimum pre-treatment conditions. Solubilization rate, volatile suspended solids (VSS) reduction rate and total volatile fatty acid (VFAs) yields were improved with increasing reaction temperature, NaOH concentration and reaction time. In addition, by applying the optimum pre-treatment conditions (140 °C, 60 meq/L and 60 min), the experimental methane yield of thermal-alkali pre-treatment of a mixture of FW and SS was 483.0 ± 15.7 mL CH₄/g VS_added, which was about 84% higher than that of the untreated one. However, after thermal-alkali pre-treatment, the NH₄⁺ concentration of the thermal-alkali pre-treatment liquid showed a concentration that could inhibit anaerobic digestion, so ammonia stripping was performed to remove NH₄⁺. As a result, the experimental methane yield was increased by about 7% compared to when ammonia stripping was not performed.