The effect of sterilization of grease trap sludge on the performance of their joint anaerobic digestion with sewage sludge was investigated. The process was carried out in two continuously stirred-tank glass reactors at mesophilic conditions. The hydraulic retention time was 10 days, and the experiment was divided into three stages. In two stages, anaerobic digestion of sewage sludge was carried out. In the first stage, the batch assay was applied to promote the development of an anaerobic community. In the second stage, the reactors were fed with sewage sludge in a semi-continuous regime. Meanwhile, in the next step, grease trap sludge was co-digested with sewage sludge. For the first period of the 3rd stage, grease trap sludge without sterilization was dosed into feedstock. In the last period, waste prior to introduction to the feedstock, was sterilized at 133 degrees C for 20 min at a steam pressure equal to 3 bars. The results of the statistical analyses revealed that the addition of co-substrates to the feedstock had a significant influence on biogas yield, as well as volatile solids removal. However, the conditioning of the grease trap sludge did not have a significant statistical effect on the production of biogas (enhanced by 3%) and volatile solids removal. Moreover, there were no statistically significant changes in the composition of biogas. (C) 2020 Elsevier Ltd. All rights reserved.