Effect of calcium oxide on enzymatic activities in co-composting of sewage sludge and municipal solid waste

• The role of lime in enhancing enzymatic activity in co-composting was investigated. • Amended treatments exhibited higher enzymatic activities compared to controls. • A lime dosage of 2 % was optimal for in co-composting of primary and mixed sludge. • Co-composting of secondary sludge achieved the best performance with lime of 3 %. • Enzymatic activities were correlated with each other and composting parameters. The sustainable management of municipal solid waste and sewage sludge is essential for mitigating environmental risks and promoting resource recovery. Co-composting, an advanced biological treatment method, relies on enzymatic activities as critical indicators of microbial processes and composting efficiency. This study evaluated the co-composting of MSW and SS in an in-vessel system, focusing on the impact of calcium oxide (CaO) on enzymatic activities and organic matter mineralization. Twelve treatments were analyzed across three groups (primary sludge, secondary sludge, and a 50:50 mixture), with CaO amendments (1 %, 2 %, and 3 %) and controls. Enzymatic activities, including dehydrogenase, catalase, alkaline phosphatase, and urease, increased during the bio-oxidation phase and declined during maturation. Adding CaO significantly enhanced these activities and accelerated organic matter mineralization. Secondary sludge and MSW showed limited performance at lower CaO, but exhibited the highest enzymatic activity (dehydrogenase: 27,350 μgTFP/g compost.h; catalase: 48.6 mL KMnO 4 /g dry solid.h; alkaline phosphatase: 1624 µg PNP/g compost.h and urease: 2328 µg NH 4 + -N/g DM.h) at 3 % CaO. Primary and mixed sludge groups achieved superior enzymatic activities (dehydrogenase: 27,872 and 27,642 μgTFP/g compost.h; catalase: 52.6 and 50.3 mL KMnO4/g dry solid.h; alkaline phosphatase: 1786 and 1661 µg PNP/g compost.h and 2547 and 2335 µg NH 4 + -N/g DM.h) at 2 % CaO. These results demonstrate that CaO amendments optimize microbial activity, enhance enzymatic functions, and promote efficient organic matter decomposition. The findings underscore the potential of tailored CaO dosages to improve compost quality and efficiency, advancing co-composting as a sustainable waste management strategy.