Wastewater from households, commerce and industry contains a good deal of energy. So far, though, very little use has been made of sewage water as a source of energy in Austria. The thermal energy contained in sewage can be recovered in heat exchangers and raised to the temperature level required for heating or cooling by means of heat pumps. Wastewater temperatures are at or above 10°C all year round, so they are very suitable for heat-pump applications. In general sewage water accumulates in substantial quantities where energy is needed, i.e. where there are enough customers for heat and cold. The engineering and economic prerequisites for using energy from sewage water, and the possibility of incorporating this in communal energy supply systems, have been investigated in several of the Austrian Energy Agency’s research projects, in collaboration with Ochsner Wärmepumpen GmbH. On the basis of the results obtained a facility was implemented for STADTwerke Amstetten while the project was still in progress.

Recovering heat from sewage water

The aim of the project “Energy from sewage water” was to explore the technical possibilities of recovering energy from sewage water with a heat pump, assess the potential and the legal framework, and thus create a basis for spreading this way of obtaining energy in Austria. The investigations revealed that there are worthwhile possibilities of utilizing heat from sewage plant wastewater, e.g. to dry sludge and to heat sewage plant buildings and other buildings close to sewage treatment plants and main sewers. From numerous examples in other countries it is clear that when heat from sewage water is put to work downstream from the sewage plant even buildings several kilometres away can be supplied cost-effectively.

Whether recovering heat from sewage water is feasible technically and cost-wise at a given location always needs to be investigated in detail. In the course of research work strategies and planning tools were developed for communal spatial planning for energy purposes, e.g. to decide whether nearby buildings can be linked up via pipelines, or to estimate how the demand for heat and cold will develop in new settlements. The issue of whether/how the demand for heat and cold close to sewage plants can be increased in cases where new facilities with a constant demand for low temperatures all year round (such as greenhouses, gymnasia or indoor swimming-pools) are involved was also analysed.

Technology for utilizing sewage water

To obtain heat or cold, sewage water can be utilized upstream or downstream from the sewage plant, or inside the actual building. Energy is recovered either from the sewer (raw sewage) or down-stream from the sewage plant (treated sewage). Wastewater from large buildings can be utilized directly via a settling basin. The key components in an energy-from-sewage facility are the heat pump and the wastewater heat exchanger. How efficient and how cost-effective the system is depends on how these elements are matched to each other. In a specific application a suitable wastewater heat exchanger, a heat pump with a specially tuned refrigeration circuit for maximum performance and a heating/cooling system matched to the heat pump’s characteristics need to be selected. It is also important to define the right temperature level as early as at the planning stage, since this has a major effect on overall efficiency. Research activities continue within the project „Energy from sewage water“, with focus on the utilization of energy resources of the sewage plant for surrounding heat consumers.