Diversion of food waste (FW) away from the solid waste stream into the wastewater stream is proved viable through the use of food waste disposers (FWDs). However, this may cause unwanted influences on the wastewater treatment system. In this context, this study has comprehensively evaluated integrated food waste and wastewater management on a city scale for the first time. A plant-wide COD-based transformation model was first established to assess the impacts of the use of FWDs on the networks of biological wastewater treatment plants (WWTPs) in Hong Kong. The biological WWTPs can remove about 78% of solids and 58% of chemical oxygen demand in FW. Moreover, the diversion of FW poses limited impacts on treatment capacity and effluent quality in WWTPs with the FWDs penetration rate up to 30%. The increases in energy consumption and operational cost are highly dependent on the treatment processes and the FWDs penetration rates, while municipal solid waste treatment can benefit from the diversion of FW. This study suggests that upgrading treatment processes (e.g., with less aeration) and optimizing the operation of WWTPs (e.g., reduce sludge retention time) may be required with the use of FWDs to achieve an energy-efficient and cost-effective goal. More importantly, this study not only provides a methodology for effectively evaluating the impacts of diverting FW into wastewater treatment in Hong Kong but also facilitates FW management in similar metropolises.