**6. Conclusions**

The evaluation in this paper illustrates the wide spectrum of interactions and possibilities for WWTPs. It is shown that the water sector is capable of taking part in sector coupling in many ways and generates multiple synergies and advantages towards a stand-alone solution of the di fferent applications. The scale of an implementation in WWTPs is mostly much lower than on an industrial level but with a transferability to a high number of suitable locations and individual solutions. For many cases, a quantitative statement from literature/reports is not transferable due to the laboratory scale. Mostly, just the opportunity and feasibility could be evaluated, but in the end, it has to be tested individually on the considered plant. First, practical implementations are realized by small, local networks taking environmental protection as a top priority for their own local policy development, even without funding.

Across all the literature, obstacles and challenges resemble one another. Technological feasibility is mostly given, but the most relevant obstacle is the lack of economic viability driven by a regulatory framework that does not fit to a fast-changing energy environment. That relates, e.g., to Germany, from not fitting the taxation treatment of energy storage systems and power purchases to inconsequential funding mechanisms and other regulatory backgrounds, which lag behind the technological progress for a fast energy transition [6]. There is a need for establishing a framework for sector coupling regarding the special, manifold and complex interactions of the many a ffected sectors with each other. This includes a further development and coordination of norms and standards across the national level as well.

Furthermore, WWTP operators have to be supported in day-to-day businesses to handle additional tasks caused by sector coupling. Key objectives of the plants will always be treating wastewater as the main priority. Especially, smaller WWTPs are often not able to provide the needed financial and human resources for additional activities in such a variety of tasks outside the usual disciplines. New business models for operating innovative technologies have to be found and to make use of as much of the synergies as possible. WWTPs would switch from locations of just treating wastewater to local energy and resource facilities, with all their benefits and drawbacks. In conclusion, the water sector is a notable and reasonable player in the field of sector coupling, especially in times of changes to a renewable energy system.

#### **7. Outlook and Further Approach**

To implement such innovative technologies in WWTPs, a holistic analysis of the available resources on the plant is necessary, e.g., energy production, flexibility options and gas flow rates. These have to match with the surrounding infrastructure (industry, public transportation system and energy grids) and the energetic environment (local heat demands, available energy surplus, etc.). For the Mainz WWTP (400,000 PE120; Germany), a wide-range sector-coupling analysis was carried out, which includes an overall analysis followed by a detailed feasibility study for the most promising results [65,66]. In the following, based on these conductions, the findings that are necessary to prepare and realize such an implantation are presented:


Assembling a responsible interdisciplinary team is therefore a prerequisite for working on such a diversified project. This includes at least one of the following experts: general planner for large construction measures, plant operator, energy supplier, grid operator, scientific support and various engineering expertise depending on the targeted sectors. This team should develop a practicable and technical-economic feasible concept based on the local situation. After selecting and specifying a preferred variant to be implemented, the funding situation needs to be clarified before finally deciding on the implementation.

In the meantime, at the Mainz WWTP, the political and technical decision-maker has given their consent, and the positive notification of federal funding has arrived. The project will start in 2020 with further regulatory preparations, assessments and the invitation of tenders. The start of implementation is planned for 2021.

**Author Contributions:** Conceptualization, M.S.; methodology, M.S.; formal analysis, M.S. and O.G.; investigation, M.S. and O.G.; writing—original draft preparation, M.S.; writing—review and editing, M.S., O.G. and H.S.; supervision, H.S.; All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.
