Wastewater treatment plants (WWTP) are among the largest energy consumers in municipalities and cause high operating costs. At the same time, many WWTPs produce biogas and have immense untapped potential for the integration of heat pumps (HP). District heating operators are looking for new possibilities to diversify their heat production portfolio and to provide cheap and clean heat to their customers. In our work, we investigate the case study of the WWTP Gleisdorf (Austria) and propose a combination of biogas utilization and heat pump integration to deliver heat for all internal thermal processes as well as to a 1,000 m heat connection line (HCL) toward the district heating network. The net annual costs of different scenarios were calculated for economic comparison. Negative net annual costs mean net annual savings. The reference scenario (biogas combined heat and power, no HCL, no HP; net annual costs of −51,000 €/year) is compared with three different heat pump integration options (HP-IO). The HP-IOs are considering different hydraulic connections, flow temperatures, and heat exchanger placement. The HP-IO-1 focuses on the low-temperature internal demands, but proves to be too limited to balance out the high cost of the HCL. HP-IO-2 operates at higher temperatures (75°C) leading to the lowest efficiency, but ultimately achieving the lowest net annual costs (−57,700 €/years with a 750 kWth HP). HP-IO-3 uses a serial heating concept trying to take advantage of lower flow temperatures while also delivering heat to the district heating network. At 300–400 kWth this leads to net annual costs of −50,100 €/years. The price ratio of 0.5 (40 €/MWh selling price of heat to 80 €/MWh purchasing price of electricity) are varied to analyze the sensitivity of the results. HPs already play an increasing role in the district heating sector, using sewage water as a heat source. The combined analysis of biogas utilization, HP integration options and the thermal as well as electrical demands of WWTP and district heating networks allow the determination of the most viable option.

There is a high degree of freedom and flexibility in the way to integrate renewable process heat in industrial processes.  Nearly in every industrial or commercial application various heat sinks can be found, which are suitable to be supplied by renewable heat, e.g. from solar thermal, heat pumps, biomass or others. But in contrast to conventional fossil fuel powered heating systems, most renewable heating technologies are more sensitive to the requirements defined by the specific demand of the industrial company. Fossil fuel-based systems benefit from their indifference to process temperatures in terms of energy efficiency, their flexibility with respect to part-load as well as on-off operation, and the fuel as a (unlimited) chemical storage. In contrast, the required temperature and the temporal course of the heat demand over the year determine whether a certain regenerative heat generator is technically feasible at all or at least significantly influence parameters like efficiency or coverage rate.

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