**3. Gas-Sector**

## *3.1. Power-to-Gas in WWTPs*

A capable long-term storage concept for renewable surplus energy is the transformation into gaseous energy carriers, like H2 and CH4, called power-to-gas (PtG). With that technology, energy can be stored and, later on, used again for power production to compensate energetic deficits on a large scale. In a first step, H2 is produced by the utilization of surplus energy via an electrolyzer, which splits water (H2O) into H2 and oxygen (O2). In a further step, the produced H2 can be upgraded with CO2 to CH4 (Sabatier process). Thus, PtG is divided in two classes: PtG-H2 and PtG-CH4. A general comparison of the two concepts is given in Table 1. The core process thereby is the electrolysis, with three different suitable processes: alkaline water electrolysis, polymer-electrolyte membrane electrolysis (PEM) and high-temperature steam electrolysis. Further information and more detailed descriptions on different electrolyzer types can be taken from the report [24]. Whereby the PEM electrolysis seems to be the best-suited option in this framework due to technical advantages regarding fast load changes, good partial-load operations and a good scalability, especially in small-scale implementations (usually below 1 megawatt connected load), like in WWTPs [15,25].


**Table 1.** General comparison of Power-to-Gas (PtG)-H2 and PtG-CH4 concepts (adapted and extended from [9,18,26]).

\* Efficiency may vary due to the type of storage, the pressure and the used technology (further information is given in [9]). NGI: natural gas infrastructure.

Nevertheless, the produced gas needs to be stored. The NGI is a huge, easily accessible and well-distributed storage system for long-term storage and providing natural gas to customers. Furthermore, it is one of the best-developed and accessible infrastructures in most countries. A part of the produced H2 can be directly stored in the NGI (e.g., 2◦–10% volume in Germany), and for the CH4, the storage capacities are nearly infinite due to almost identical chemical attributes of the gases [10,18]. With this background, the surplus energy can be stored, distributed and is accessible for months up to years to compensate seasonal fluctuations in the energy production without building one's own distribution/storage system [9,26].

Therefore, PtG plants have to be located close to the gas grid. As biogas plants are mostly located in rural areas, large WWTPs are present in nearly every urban location, mostly close to the NGI. This makes WWTPs favorable as a PtG location. Another big advantage of WWTPs with anaerobic digestion as a possible location for a PtG plant is the already existing gas infrastructure on-site and the existing know-how on handling gases. On plants with anaerobic sludge digestion, a suitable infrastructure is available in terms of digestion tanks, gas storages and combined heat and power plants (CHP). Furthermore, one of the biggest challenges for PtG(-CH4) is the lack of an appropriate CO2 source ("green carbon") [9]. This is a major benefit of WWTPs due to an easily accessible and sustainable biogas with a high CO2 content of nearly 35% volume [27]. To use the CO2, it is possible to extract it from the biogas (biogas treatment) or use the raw biogas directly to upgrade the CH4 content to feed-in quality for the NGI. A techno-economical study of state-of-the-art of such methanization concepts for PtG was conducted by [28] and showed that, especially, biological methanization has a grea<sup>t</sup> potential for further development. For WWTPs, a utilization is possible by directly inserting H2 into the digestion tank or upgrade the biogas to a feed-in quality with a special external reactor [28–31].

Basic plant concepts for PtG implementations in WWTPs and the theoretical feasibility are shown in [15], characteristic values and a feasibility study for upgrading biogas with H2 are given in [31]. A large-scale practical demonstration for a two-year operation is located at the Avedøre WWTP, Denmark [32] and for a biogas plant in Pirmasens, Germany [30]. Both implementations could demonstrate a successful upscaling from a laboratory scale to a nearly full scale. A continuous feed-in of the produced CH4 with a high level of flexibility of the biological processes could be established and is running in a stable operation. Nevertheless, an implementation of PtG requires a high control of real-time data and complex coordination of the different parameters, like the CO2 supply, power supply, operation of the electrolyzer, storage of intermediate gases and utilization of oxygen and heat [32].

Although PtG in WWTPs is technologically possible, much remains to be done. Especially regarding optimization in the coordination of the different material flows and embedding the system in the whole WWTP operation business to gain the full benefits of the synergies. Due to the complexity of process engineering of big WWTPs itself, implementing PtG concepts are quite challenging. However, the greatest problems in implementing are not technology driven but the legal and regulatory framework, which makes the systems so far not feasible without funding (e.g., for Germany: cf. [15,31]). Better conditions would lead to dropping prices, a needed market penetration and faster development for full-scale application.

It can be stated that WWTPs are able to implement PtG systems and can ensure an efficient methanization due to a constant and viable CO2 source. In addition, it provides a supplement flexibility option for energy grids as a flexible RES. Furthermore, a sustainable and beneficial utilization path for oxygen is given, which leads to unique location advantages for WWTPs.
