ORIGINAL ARTICLE Methane production from anaerobic mono- and co-digestion of kitchen waste and sewage sludge: synergy study on cumulative methane production and biodegradability S. S. V. Varsha 1 & Abdul F. Soomro 2 & Zenab T. Baig 2 & Arun K. Vuppaladadiyam 3 & S. Murugavelh 4 & Elsa Antunes 5 Received: 10 May 2020 /Revised: 23 June 2020 /Accepted: 8 July 2020 # Springer-Verlag GmbH Germany, part of Springer Nature 2020 Abstract 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 demon- strated 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. Keywords Methane production . Kitchen waste . Sewage sludge . Mono-digestion . Co-digestion 1 Introduction Kitchen waste (KW) is often defined as food and other wastes generated from sources such as families, hotels, and catering services [1]. Kitchen waste contains a wide range of energy containing components, in the form of volatile fatty acids (VFAs), proteins, carbohydrate poly- mers, and lipids, which can be further processed into re- newable energy in the form of hydrogen, ethanol, biodie- sel, and methane [2]. Consistent with the UN Food and Agriculture Organization, it is estimated that ca. 1.3 billion tons of food are lost in the supply chain in the form of fresh vegetables, meat, fruits, dairy products, and baked goods. It is reported that KW is one among the major waste streams and is responsible for about 30% of the total MSW [ 3]. The anaerobic digestion (AD), is considered as an environmentally friendly way of treating and converting organic kitchen waste into re- newable energy, mainly in the form of methane [ 4]. However, the major limitation of just using organic mate- rial, which is readily degradable, in the AD process is that digestion efficiency would be significantly reduced [5, 6]. * S. Murugavelh murugavelh.s@vit.ac.in * Elsa Antunes elsa.antunes1@jcu.edu.au 1 Department of Civil Engineering, AVN institute of Engineering & Technology, Hyderabad, Telengana, India 2 School of Environment, Tsinghua University, Beijing 100084, China 3 School of Civil Engineering, Shenzhen University, Shenzhen 518000, China 4 CO2 Research and Green Technologies Centre, VIT, Vellore, Tamil Nadu 632014, India 5 College of Science & Engineering, James Cook University, Townsville, Queensland 4811, Australia Biomass Conversion and Biorefinery https://doi.org/10.1007/s13399-020-00884-x