Wastewater Treatment Plants as Local Thermal Power Stations—Modifying Internal Heat Supply for Covering External Heat Demand

To counteract climate change, the application of renewable energy sources and their efficient use are of crucial importance. In this context, wastewater has also gained increased attention in recent years. For decades, wastewater treatment plants have applied the heat from digester gas combustion to supply internal demands. However, in the context of efficient energy use the question arises: can using high temperature heat for supplying low temperature demand still be considered the best option? This article presents an innovative approach to covering wastewater treatment plant (WWTP) internal demand with low temperature wastewater heat recovery, making thermal energy from digester gas combustion available for feed-in to a local high temperature district heating network. The presented feasibility study was carried out in an Austrian municipality and investigates the heat balance, the economic risk, climatic benefits and the social aspects of the suggested approach. The practical implementation of the novel approach was planned in two steps. First, the WWTP should be connected to the district heating network to enable the feed-in of excess heat. Second, the WWTP internal heat supply should be modified and based on wastewater heat recovery from the effluent. Due to the promising results of the feasibility study, the first step was realized in summer 2020. The second and final step was initiated in 2021.