Wastewater heat recovery via heat exchangers and heat pumps constitutes an environmentally friendly, approved and economically competitive, but often underestimated technology. By introducing the spatial dimension in feasibility studies, the results of calculations change considerably. This paper presents a methodology to estimate thermal energy resource potentials of wastewater treatment plants taking spatial contexts into account. In close proximity to settlement areas, wastewater energy can ideally be applied for heating in mixed-function areas, which very likely have a continuous heat demand and allow for an increased amount of full-load hours compared to most single-use areas. For the Austrian case, it is demonstrated that the proposed methodology leads to feasible results and that the suggested technology might reduce up to 17% of the Austrian global warming potential of room heating. The method is transferrable to other countries as the input data and calculation formula are made available. A broad application of wastewater energy with regard to spatial structures and spatial development potentials can lead to (1) increasing OPEN ACCESSSustainability 2015, 7 12989 energy efficiency by using a maximum of waste heat and (2) a significant reduction of (fossil) energy consumption which results in a considerable reduction of the global warming potential of the heat supply (GWP) if electricity from renewables is used for the operation of heat pumps.

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Although the main function of a wastewater treatment plant (WWTP) is to remove various constituents from wastewater it can also serve as a source of energy and other materials. The generated resources can be used either on-site at the WWTP or elsewhere at an adjacent infrastructure. In the course of a current national research project, the possibilities and potentials regarding the integration of WWTPs into local energy supply concepts are being investigated in Austria. Preliminary results show that in particular the amount of thermal energy available exceeds by far the on-site demands of WWTPs. Even on-site electrical energy demands could be selfaddressed under certain conditions. This paper describes the estimation of total energy consumption and generation and the related degree of energetic self-sufficiency at certain Austrian WWTPs.Preliminary results regarding the development of a tool for evaluating and optimising on-site and externally supplied use of energy are presented. Finally, the possibilities of energy supply for neighbouring spatial structures are discussed briefly and conclusions drawn about the potential to develop WWTPs as regional energy cells.

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Climate change is one of the great challenges of our times. In the search for renewable energy sources, wastewater has received increasing attention in recent years. This is due to the fact that it can serve as a source of electricity and heat supply, as well as a substitute for natural gas. The current literature frequently addresses more technical aspects from a mostly sanitary engineering-orientated perspective. Social aspects related to the energetic use of wastewater still appear to be underrepresented. To support the closure of existing knowledge gaps and to contribute to the development in this field, this article addresses two issues: it defines and characterises key stakeholder groups required to catalyse broader energetic usage of wastewater, and it proposes a participatory approach to support successfully establishing wastewater as a commonly accepted source of renewable energy to best support the ongoing energy transition.

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