In recent years, in addition to establishing a decarbonized society worldwide, a depopulation has become a serious problem especially in smaller municipalities in Japan. Wastewater and municipal solid waste (MSW) treatment are responsibilities of municipalities; both are separately treated in Japan. Wastewater treatment plants (WWTPs) and MSW incinerators (MSWIs) must become more cost-efficient and environmentally responsible. This study focuses on the collaboration between a WWTP and a MSWI in smaller municipalities in Japan. A mass and elemental balance, operating costs and greenhouse gas (GHG) emissions calculation model was developed to evaluate MSW collection and transportation, the WWTP, and the MSWI in model cities with populations of 50,000 (small) and 100,000 (medium). Various treatment scenarios for MSW and sewage sludge were evaluated, including collection of 45 % of all kitchen waste by disposers to the WWTP (Case 1); co-combustion of MSW and dewatered sludge in the MSWI (Case 2); co-digestion of thickened sludge, all kitchen waste, and 60 % of all paper waste in MSW (Case 3); and no cooperation (Case 0, the base case), in terms of operating costs and GHG emissions to identify the most effective plan. The combination of Case 1 and 2 optimally reduced operating costs by 14 % compared to Case 0 in both cities. From the perspective of GHG emissions, the combination of Case 2 and 3 provided the lowest emissions from both small and medium cities: the reductions were 29 % and 33.2 %. Disposers and co-digestion minimized operating costs and GHG emissions, in addition to co-combustion which also contributed to both operating costs and GHG emissions.