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 management and the storage of energy. To reach climate protection goals and a significant reduction of CO 2 , 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 different research sectors are given and show that a practical utilization is possible and reasonable for WWTPs to contribute with sustainable energy concepts to defossilization.Energies 2020, 13, 2088 2 of 20 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 different 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 p...

To achieve the Paris climate protection goals there is an urgent need for action in the energy sector. Innovative concepts in the fields of short-term flexibility, long-term energy storage and energy conversion are required to defossilize all sectors by 2040. Water management is already involved in this field with biogas production and power generation and partly with using flexibility options. However, further steps are possible. Additionally, from a water management perspective, the elimination of organic micropollutants (OMP) is increasingly important. In this feasibility study a concept is presented, reacting to energy surplus and deficits from the energy grid and thus providing the needed long-term storage in combination with the elimination of OMP in municipal wastewater treatment plants (WWTPs). The concept is based on the operation of an electrolyzer, driven by local power production on the plant (photovoltaic (PV), combined heat and power plant (CHP)-units) as well as renewable energy from the grid (to offer system service: automatic frequency restoration reserve (aFRR)), to produce hydrogen and oxygen. Hydrogen is fed into the local gas grid and oxygen used for micropollutant removal via upgrading it to ozone. The feasibility of such a concept was examined for the WWTP in Mainz (Germany). It has been shown that despite partially unfavorable boundary conditions concerning renewable surplus energy in the grid, implementing electrolysis operated with regenerative energy in combination with micropollutant removal using ozonation and activated carbon filter is a reasonable and sustainable option for both, the climate and water protection.

As a consequence of a worldwide increase of energy costs, the efficient use of sewage sludge as a renewable energy resource must be considered, even for smaller wastewater treatment plants (WWTPs) with design capacities between 10,000 and 50,000 population equivalent (PE). To find the lower limit for an economical conversion of an aerobic stabilisation plant into an anaerobic stabilisation plant, we derived cost functions for specific capital costs and operating cost savings. With these tools, it is possible to evaluate if it would be promising to further investigate refitting aerobic plants into plants that produce biogas. By comparing capital costs with operation cost savings, a break-even point for process conversion could be determined. The break-even point varies depending on project specific constraints and assumptions related to future energy and operation costs and variable interest rates. A 5% increase of energy and operation costs leads to a cost efficient conversion for plants above 7,500 PE. A conversion of WWTPs results in different positive effects on energy generation and plant operations: increased efficiency, energy savings, and on-site renewable power generation by digester gas which can be used in the plant. Also, the optimisation of energy efficiency results in a reduction of primary energy consumption.

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