A model was developed to calculate the mass and heat balances of wastewater and municipal solid waste treatment plants when these plants operate either separately or together with a mutual dependence on mass and energy. Then the energy consumption, life cycle costs (LCCs), greenhouse gas (GHG) emissions and effluent quality were evaluated under various scenarios to identify the most effective co-management and treatment system. The results indicated that co-digestion of kitchen waste and sewage sludge, and their co-combustion reduced LCCs by 30%, energy consumption by 54% and GHG emissions by 41% compared to the base case. However, co-digestion increased the total nitrogen load in the wastewater treatment plant effluent. Even if an advanced wastewater treatment system was applied to improve total nitrogen concentration, the above indicators were affected but still reduced compared to the base case. Therefore, it was confirmed that the integrated system was beneficial for megacities.