## 1 Introduction
### Biomethane and Green Hydrogen in WWTPs: Situation Overview  
The task of energy neutrality of a wastewater treatment plant is twofold: (1) increasing the energy efficiency of technological processes and reducing energy consumption; (2) increasing energy production in the WWTP itself and recovering energy from wastewater [5]. The specific electricity consumption depends on the size of WWTP, ranging from 37 to 96 kWhel/PE/a, with an average of 72 kWhel/PE/a [11]. The largest share of electricity consumed in WWTPs is for these processes--aeration (51%), centrifuges (10%), anaerobic digestion (AD) (9%), and pumping (8%) [12]. One of the most important tasks to increase energy production in the WWTP itself is to increase the production of biogas or biomethane. Biogas, which usually consists of 50-70% CH\({}_{4}\) and 50-30% carbon dioxide (CO\({}_{2}\)) [13], can be used directly in the wastewater treatment plant for heat and electricity generation. In this case, biogenic CO\({}_{2}\) formed during biogas production, and CO\({}_{2}\) formed during biogas combustion in cogeneration or water heating boilers, are emitted into the atmosphere. Since biogenic CO\({}_{2}\) constitutes about 30-50% [13] of the biogas volume, this means that large amounts of CO\({}_{2}\) are emitted into the atmosphere unused. A more efficient way to use biogas can be biogas upgrading, during which CO\({}_{2}\) is separated from biogas and biomethane is produced. Biomethane can be used in the WWTP itself as a transport fuel or transferred to external users. This is most often done by injecting biomethane into the natural gas network. In this way, biomethane can be efficiently consumed in various processes--as a transport fuel, as a raw material for the chemical industry, or as an energy carrier for producing electricity, heat or cold. Also, the energy value of the biomethane produced in the WWTP and supplied to external customers can partially compensate, for example, the energy value of electricity purchased from the network, thus increasing the energy neutrality of the WWTP. However, biogas upgrading does not solve the issue of biogenic CO\({}_{2}\) utilization. Although biogenic CO\({}_{2}\) separated in the biogas upgrading process can be used, for example, in liquefied form in the food or other industry, such a use is not widespread, at least at present due to the relatively high price of biogenic CO\({}_{2}\). Another way of using CO\({}_{2}\) in WWTPs is CO\({}_{2}\) methanation, in order to increase the production of biomethane in the WWTP [13; 14]. The methanation process requires green hydrogen, which can be produced in the WWTP. Hydrogen production by electrolysis and CO\({}_{2}\) methanation refer to the rapidly developing group of so-called Power-to-X technologies. Power-to-X processes can potentially transform hard-to-decarbonize sectors by converting renewable electricity into value-added products such as hydrogen, methane, ethylene, and others, and thereby substantially reduce carbon emissions [15]. Power-to-Gas processes, namely Power-to-Hydrogen and Power-to-Methane, represent some of the only options for seasonal electricity storage at the necessary capacities for a country that relies on renewable energy technologies [16]. CO\({}_{2}\) methanation belongs to CO\({}_{2}\)-consuming technologies, taking into account the need for carbon capture and utilization in future GHG neutral energy systems and carbon neutrality [17], as well as the inevitable need to reduce emissions of the most important greenhouse gas, CO\({}_{2}\), to achieve climate goals. The next important task, closely related to the production of green hydrogen in WWTPs, is the increase in the production of green electricity in WWTPs. Most often, the literature examines the possibilities of solar [18; 19] and wind [13; 20] electricity production and their impact on energy and greenhouse gas balance. The area and location of the WWTP itself almost always limit this production [20; 21; 22].