Anaerobic co-digestion of domestic sewage sludge with food waste as a substrate for biogas production and as a mean for waste management was conducted. The food waste was incorporated into the bioreactor as a co-substrate semi-continuously via replacement mode and addition mode of operations in ratios up to 50%. The methane gas yield under the replacement mode of operation ranged from 295 to 1358 ml/gVSadded and from 192 to 462 ml/gVSadded for the replacement mode of operation and the addition mode of operation, respectively. The results indicate that the methane gas yield increases along with the percentage share of food waste in the feed. Anaerobic co-digestion under semi-continuous operation enabled handling large organic loadings compared to batch co-digestion processes.