Co-digestion of sewage sludge and sterilized solid slaughterhouse waste: Methane production efficiency and process limitations

► Sterilized mass (SM) is an attractive substrate for biogas production. ► High TS, fat and proteins content of SM give methane yield of 590.5 m3CH4 t−l. ► SM addition of 5% (w/w) to sewage sludge increased methane production 5.7 times. ► Free ammonia inhibition threshold value was determined at 596.5 ± 68.6 gNH3 L−1. ► SM addition of 10% (w/w) was detrimental to AD process due to intensive foaming.