A study on the effects on a wastewater treatment plant when recovering the heat from the wastewater

Abstract: source of thermal energy. Wastewater as a source of energy has gotten more attention in recent times, as the Sustainable Development Goals in combination with local challenges encourage new technical solutions that can replace old and outdated technology. By installing a heat exchanger to recover the heat, it can be used locally to heat buildings as well as urban areas. The technology and the opportunity to recover and use the energy is a way to operate in a circular way. Still, the infrastructure systems of water, wastewater, and thermal energy are intertwined, creating a complex and balanced environment where several actors representing their own interest and service to a community and urban area. Some actors may be the municipality handling the wastewater, real estate, and owner of buildings or the district heating company.  This complexity in the urban area where several actors are involved with their own goal and all using the infrastructure surrounding water and wastewater in different ways, creates challenges. The users want to get rid of the wastewater and possibly recover heat from the wastewater, while the producer is responsible for the treatment plant and treatment, and is dependent on the wastewater reaching the plant with the highest temperature possible, and thereby not disrupting the process by a decrease in temperature created by the upstream heat recovery systems. The purpose of this study is to examine how the wastewater treatment plant react and is affected if the influent wastewater temperature decreases as a consequence of heat recovery from the wastewater, as the removal of nutrients in the treatment plant are chemical and biological processes which are depending on the temperature. By examining operational data from Henriksdals wastewater treatment plant and a model simulating the removal of nutrients in activated sludge, some light can be shed on how the processes of the treatment plant react to colder incoming wastewater. The legal framework and regulations around the wastewater treatment will also be examined due to the complexity of these systems, which is often overlooked.