*Discussion* **The Possible Roles of Wastewater Treatment Plants in Sector Coupling**

#### **Michael Schäfer 1,\*, Oliver Gretzschel 1 and Heidrun Steinmetz 2**


Received: 23 March 2020; Accepted: 8 April 2020; Published: 22 April 2020

**Abstract:** The development of a power system based on high shares of renewable energy sources puts high demands on power grids and the remaining controllable power generation plants, load managemen<sup>t</sup> and the storage of energy. To reach climate protection goals and a significant reduction of CO2, surplus energies from fluctuating renewables have to be used to defossilize not only the power production sector but the mobility, heat and industry sectors as well, which is called sector coupling. In this article, the role of wastewater treatment plants by means of sector coupling is pictured, discussed and evaluated. The results show significant synergies—for example, using electrical surplus energy to produce hydrogen and oxygen with an electrolyzer to use them for long-term storage and enhancing purification processes on the wastewater treatment plant (WWTP). Furthermore, biofuels and storable methane gas can be produced or integrate the WWTP into a local heating network. An interconnection in many fields of di fferent research sectors are given and show that a practical utilization is possible and reasonable for WWTPs to contribute with sustainable energy concepts to defossilization.

**Keywords:** decarbonization; energy concepts; long-term energy storage; power-to-gas; power-to-X
