A thesis submitted in partial fulfillment of the requirements for the degree of Master of Engineering in Environmental Engineering and Management

Thesis (M. Eng.) - Asian Institute of Technology, 2026

Digestate-derived biochar was used to examine the effectiveness of semi-continuous anaerobic co-digestion of food waste and sewage sludge under mesophilic conditions. Even though anaerobic digestion is a common method for producing biogas, instability brought on by acid buildup, ammonia inhibition, and varying organic loading rates (OLR) frequently limits its effectiveness. In order to overcome these obstacles, biochar made from anaerobic digestate was used to improve methane production and system stability. At 35 ± 1°C, two identical lab-scal Continuous Stirred Tank Reactors with working volumes of 6 and 8 L were used. Reactor R1 received biochar (0.8, 1, 2, and 5 g/L), and reactor R2 was used as a control. To evaluate performance under both stable and stressful situations, reactors were run with increasing OLR. TS, VS, pH, alkalinity, ammonia nitrogen, VFAs, biogas generation, and methane concentration were among the important variables that were tracked. SPSS was used for statistical analysis. Reactor stability and performance were greatly enhanced by the addition of biochar; R1 maintained a more constant pH (6.98{u2013}7.81) than R2 (6.71{u2013}7.35) and demonstrated less ammonia accumulation (95.19{u2013}203.03 vs. 129.28{u2013}319.46 mg NH₄-N/L). Lower FOS/TAC ratios (0.27{u2013}0.36) and lower VFA levels (476{u2013}792 mg/L) in R1 compared to R2 (up to 0.55 and 1148 mg/L) demonstrate improved process stability. Additionally, R1 produced more biogas, with methane content reaching around 53% compared to about 42% in R2, especially at ideal biochar dosages of 1-2 g/L. Better reactor performance was also demonstrated by improved substrate degradation (TS up to 3.92). Biochar's high surface area and porous structure, which encourage microbial activity and DIET, are responsible for these improvements; 500°C biochar exhibits the best overall performance. Overall, digestate-derived biochar is an effective additive for improving anaerobic co-digestion stability and methane production. Future research should focus on optimizing biochar dosage, long-term performance, and microbial mechanisms under varying operating conditions.