The paper presents a simulator for optimizing a new type of sector integration between district heating and water supply, where the thermal energy of the district heating return water is utilized in wastewater heating. The aim is to reduce fuel consumption in district heating production and improve the efficiency of biological wastewater treatment, especially nitrogen removal. Through modeling, simulation, and optimization, new information is generated about the energy efficiency and environmental impacts of sector integration, as well as its optimal implementation. Preliminary simulation results show that sector integration enables year-round stabilization of wastewater temperature and is economically viable for all parties.