## 1 Introduction
### Biomethane and Green Hydrogen in WWTPs: Situation Overview
Another method of green energy production found in the literature is hydropower from wastewater flow [23; 24; 25]. This is a more theoretical possibility in WWTPs, located in the lowlands, since the amount of electricity generated is small, and the capital investments are significant. The use of solar and wind electricity as variable energy sources necessitates the assessment and analysis of the use of electricity storage systems in WWTPs. One of the most important challenges is the reduction of the cost of produced green hydrogen [26]. The development of green hydrogen in the EU is currently mainly based on political decisions. However, the EU operates in a global world market. Energy prices have a major impact on the competitiveness of the economy. As the EU competes with producers in the USA, Asia, especially China and India, South America, Russia, and other regions of the world and some of them have large fossil fuel resources or have not set ambitious climate change mitigation targets, it is important to ensure that green energy prices approach economically viable prices in the long term. Another way to protect the EU producers using sustainable energy is to apply regulatory measures, such as a border CO\({}_{2}\) tax. However, the application of such measures is associated with certain problems, in particular the possibility of assessing the energy greenness of third-country producers fairly and independently. In a WWTP, there is an opportunity to directly use the by-products of hydrogen production--oxygen (O\({}_{2}\)) and heat. The effective use of hydrogen production by-products in a WWTP can significantly contribute to reducing the cost of green hydrogen [13; 27]. The most often found cases for integrating green hydrogen facilities in WWTPs include the use of these by-products or the methanation of biogas. The literature most often analyzes the utilization cases of separate electrolysis products. The use of oxygen for aeration purposes is presented in articles [13; 19]. The use of hydrogen for CO\({}_{2}\) methanation is presented in articles [14; 28]. The utilization of waste heat from electrolyzers is presented in articles [14; 29]. However, there are not many articles that analyze the potential for the use of all three electrolysis products in WWTPs together. In a series of articles [30; 31; 32; 33], WWTP is analyzed as a technological system, the main input, output, and internal parameters of which are various quantitative and qualitative indicators of sewage, sludge and treated water--the amount of chemical substances, bacteriological contamination, fertilizer value, purification level, etc. The main goal of these models is to achieve the required quality of wastewater treatment. In other works [13; 34; 35; 36; 37], the main goal of WWTP modeling is to reduce GHG emissions, including primarily carbon dioxide (CO\({}_{2}\)), methane (CH\({}_{4}\)), and nitrous oxide (N\({}_{2}\)O). In most cases, the main amount of electricity is obtained from the grid, although efforts are made to ensure that the largest share is renewable electricity, obtained mainly from solar [18; 19] and wind power [13; 20] plants or combined solar-wind-hydro [21; 38] sources. The integration of green hydrogen and biomethane technologies in WWTP can provide synergy for the implementation of both above-mentioned goals, that is, increasing biomethane production in WWTP and reducing the cost of green hydrogen production. At the same time, this would also contribute to achieving the strategic goal of complete neutrality of WWTPs [5].