**1. Introduction**

The energy and the water sector are both essential and vital elements of modern life. Water has to be provided and distributed to citizens, as well as discharged and treated with a substantial use of energy. On the other side, water is used to generate power and deliver or recover energy for human purposes. Due to these facts, the challenges are globally to provide these important services. Furthermore, water and energy systems are complex and strongly linked but are mostly operated independently. Facing climate change forces this water-energy nexus into a more environmentally sustainable system for a rapidly growing population on the globe. Therefore, energy e fficiency, energy savings and energy recovery have become development goals all over the world. Nevertheless, every country has, on the basis of its power generation composition, its own unique chances, opportunities and obstacles to handle [1–4].

In regard to meeting the targets of the Paris Agreement of 2015 [5] or the United Nation Sustainable Development Goals [4], a system change is needed now, and the world has to act immediately. Although some countries are proactive and take a leading part in promoting renewable energy sources (RES), the actual e fforts are, by far, not su fficient. Nevertheless, there are promising developments for the year 2018. The global investments in the new generating power capacity of RES exceeded that in fossil and nuclear power, with a share of 65%. In several countries, the power sector is transforming rapidly, with growth rates of over 10%, and proportions of more than 20% RES in energy production have been reached, like in Uruguay, Germany and the United Kingdom [6]. The gradual extension of RES and

the expedited abandonment of fossil and nuclear energy production results in new problems but also new opportunities for power supplies. There is a shift from demand-oriented power generation to a production-driven generation of electrical energy. In an electricity system with a high share of RES, flexibility options are needed to counterbalance fluctuating wind and solar-based power production to maintain the high standards in its supply [7]. For now, there are just a few hours of surplus energy but with proportions of more than 60% RES; times in which supply exceeds demand are increasing significantly. On this basis, there will be a high need of short-term flexibility in the near future to stabilize power grids and further integrate RES into the energy grids [8]. Short-term storage options are classified in the range of seconds up to 24 h (daily storage). These energy surpluses and deficits have to be balanced by flexible energy generators and consumers. Furthermore, long-term storage capacities are needed to provide enough energy in times of deficits on a larger scale. This is caused by longer periods of low amounts of available wind and sun. To compensate these fluctuations and store or generate energy, based on its availability, fundamentally di fferent applications compared to short-term flexibility are required [9].

Energy has always been stored, but the focus and the technologies used have changed. Storage concepts like pumped storage plants, battery or compressed air systems are not suitable for long-term storage—too expensive or cannot provide enough capacities for an extensive use in every country [10]. Additionally, ecological issues and resource scarcity have to be considered. Unlike solar and wind-based energy production, biogas is the only RES that can be directly stored and flexibly adjusted to the production of electrical energy or heat purposes. This is possible due to a flexible power and gas production, which can be realized by biogas plants or wastewater treatment plants (WWTPs) with anaerobic sludge digestion [11–13]. The required storage technologies are available but must be re-evaluated, utilized or combined appropriately to the new challenges and obstacles, especially regarding economic feasibility during the transformation process from a fossil-based energy system to a renewable energy system [9].

Part of a solution in facing climate change could be a comprehensive use of widely spread existing infrastructure, like in the water sector. For short-term flexibility, it has been shown that WWTPs are capable of performing on energy markets without endangering system functionality to stabilize energy grids with its existing aggregates and ensure the further integration of RES into energy grids [14–16]. The capability of providing ancillary services and taking the fluctuation of the electrical energy consumption/generation pattern into account without endangering the WWTPs' systems' functionalities is shown in [16]. Such a contribution applies as well for long-term storage concepts, theoretically shown, e.g., in [15]. In the following, the focus is on practical implementations of long-term storage concepts according to sector coupling, with special focus on its interaction with WWTPs.

Sector coupling describes the interconnection between the sector's heat, gas, mobility, nonenergetic use of fossil resources (e.g., chemistry/industry) and electrical energy under the use of RES due to appropriate technologies [9]. Especially the gas sector provides with its natural gas infrastructure (NGI) a nearly infinite storage option for RES [17,18]. Similar to the NGI, large WWTPs are commonly distributed close to municipal infrastructures with significant synergies in nearly all of the targeted fields of sector coupling, which makes them preferable locations for a transposition [15,19,20].

The available literature is widely scattered across the di fferent addressed topics, and a comprehensive compilation is missing—in particular, with special focus on the water sector and WWTPs. Therefore, the present work provides a systematic review on several fields of sector coupling and their interactions with WWTPs based on the literature. Starting from describing available and reasonable technologies, followed by analyzing the current situation, evaluating the e ffects of an implementation on the plant and giving examples for actual worldwide, practical applications in WWTPs, the state of the art is summarized. The objective is not only to collect and present knowledge but also provide a basis for decisions to future plant concepts due to evaluating realized projects. This paper refers to synergies, opportunities and downsides that are given to WWTPs as a local energy center in the role of sector coupling and long-term energy storage.

#### **2. Sector Coupling with WWTPs**

Sector coupling is realized by the interconnection between RE surplus and the conversion into storable energy forms and is widely called power-to-X (PtX) based on the targeted sector (e.g., power-to-heat, power-to-gas, etc.). There are a lot of Power-to-X projects on the international level, whilst countries with high shares of RES are more endeavoring, simply because they are more a ffected. Unsurprisingly, countries like Germany take a leading part in development and realized/planned PtX projects [21]. For the German water sector, potential analysis shows that a significant contribution can be provided in terms of ancillary services, as well as gas flow rates, for long-term storage [16,22], which applies for other countries as well.

WWTPs are ideally suited in a special way for handling di fferent energy forms. They are not only flexible (electrical) energy consumers, with their purification aggregates (cf. [16]), but also flexible producers, with their anaerobic sludge digestion and the subsequent needs-oriented production of electrical energy (cf. [12,23]). They are also able to function as a heat sink (e.g., heating of the digestion tanks) and are capable to provide usable heat sources (e.g., waste heat from combined heat and power plant (CHP) units). Furthermore, a wide range of gases can be utilized: Despite a direct use of electrical energy generated by CHP units, the produced digestion gas supplies, with its 35% carbon dioxide (CO2) content, a valuable and sustainable green CO2 source for further Power-to-Gas (PtG) applications, like methanization, which enables a direct feed-in of methane (CH4) into the local gas grid. Moreover, the produced hydrogen (H2) can be used on the plant as well (e.g., biological methanization) instead of a mere production and feed-in into the natural gas grid. The operation of an electrolyzer for PtG can be integrated in holistic plant solutions considering di fferent gaseous energy forms and heat. The usually unwanted by-product O2 can also be used for aeration in the biological treatment or further treatment processes such as ozonization for removing micropollutants from the wastewater. This enables synergies and opportunities in both sectors: WWTPs are able to contribute to the energy sector and face their own new challenges in wastewater treatment at the same time (e.g., micropollutant removal).

Figure 1 gives an overview of the multiple utilization paths and the complex interactions with di fferent sectors and the WWTP. This demonstrates that WWTPs are reasonable energy centers, which are located at nearly every urban area. In the following, the focus is on new findings and realized projects in the field of sector coupling explicitly concerning and interacting with WWTPs for the di fferent sectors.

**Figure 1.** Interaction of wastewater treatment plants (WWTPs) with different technologies and sectors. Reproduced from [16], Technische Universität Kaiserslautern: 2019.
