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Globally, kitchen waste (KW) and sewage sludge (SS) are two important forms of organic wastes that need immense attention during handling and management. As the characteristics and components of KW vary significantly with respect to location and season, it is often difficult to understand the behavior of KW during the (co-)digestion process. Therefore, this study aimed to investigate the synergistic influence of KW over SS and vice versa during anaerobic (co-)digestion. Anaerobic mono-digestion and co-digestion of SS and KW, considering three different mixing ratios SS:KW; (SK1-25:75, SK2-50:50, SK3-75:25), was conducted to investigate their specific methane yields (SMY) and biodegradability (BD). The modified Gompertz equation was used to estimate the theoretical methane potential and used as a reference for experimental data assessment. The results demonstrated that SMYs were 212 and 368 L kg −1 VS added for SS, and KW, respectively. Under the category of co-digestion, sample SK2 (SS:KW; 50:50) had the highest BD of 91% as compared to digestion of SS (61%) and KW (74.9%) separately. A synergistic effect on methane production was witnessed with all ratios and the maximum was noticed with SK2, ca. 40%, which can be ascribed to the enhanced hydrolysis because of the addition of KW, which is readily degradable by nature. Further, this study could be used as guidelines to the design and optimization of the co-digestion process.