In this study, the collaborative effects between wastewater treatment plant and municipal solid waste (MSW) incinerator, including the introduction of waste disposers, energy recovery through mixed methane fermentation, and co-incineration of sewage sludge and MSW, were projected to estimate their future impact on greenhouse gas emissions in Satsumasendai City, as a medium-sized city in Japan. In addition to these collaborations, when an increase in wastewater treatment diffusion rates, an emphasis on MSW reduction, particularly in plastic materials and the introduction of waste heat transport to pulp and paper plant were assumed, GHG emissions in 2050 were reduced by 72% from the 2020 level, assuming no progress toward carbon neutralization (CN) of electricity and heat. The factors contributing to this reduction were energy recovery and waste plastic reduction. However, if CN of electricity and heat progresses, GHG emissions are reduced by only 66% because GHG reductions through energy recovery are no longer expected, suggesting that upstream measures such as waste plastic reduction and the introduction of biomass plastics will become even more important.