## 2 Methodology
### Presumptions for the Successful Integration of Green Hydrogen and Biomethane Production Processes
#### 2.3.1 Utilization Possibility of All Products from Electrolysis in WWTP
This means the supply of 7.54 m\({}^{3}\) air/kWh to get the required O\({}_{2}\) amount (with 20,95% volume O\({}_{2}\) in the air) [13]. In the study [51], it was investigated the impact of using pure O\({}_{2}\) instead of air, showing far higher (between 5 and 6 times) oxygen transfer rate. As noted in this study, the 5.5 times increase in O\({}_{2}\) transfer rate was achieved due to complex effects when switching from air to pure O\({}_{2}\) (higher O\({}_{2}\) solubility, 20% increase in mass transfer coefficient for small bubbles, 5 times higher O\({}_{2}\) driving force). Thus, oxygen supply efficiency of 12.5 kg O\({}_{2}\)/kWh can be achieved [13]. The total oxygen demand for aeration can be determined using Equation (1) [52]:  
\[ \begin{array}{c}OD=0.75\times Q\times(BOD_{i}-BOD_{e})+2\times V_{A}\times \frac{MLSS}{1000}+4.3\times Q\times(Amm_{i}-Amm_{e})\\ -2.83\times Q\times[(Amm_{i}-Amm_{e})-N_{e}]\end{array}  \tag{1}\]  
where \(OD\) is the oxygen demand for aeration, (g/h); \(Q\) is the wastewater inflow, (m\({}^{3}\)/h); \(V_{A}\) is aeration tank volume, (m\({}^{3}\)); \(MLSS\) is the mixed liquor suspended solids in the aeration tank (g/L); \(BOD_{i}\) and \(BOD_{e}\) are the inflow and outflow concentrations of biological oxygen demand (BOD) (g/m\({}^{3}\)); \(Amm_{i}\) and \(Amm_{e}\) are the inflow and outflow concentrations of ammoniacal nitrogen (g/m\({}^{3}\)), and \(N_{e}\) is the outflow of nitrate nitrogen (g/m\({}^{3}\)). Authors of the article [13] present an evaluation of a 426,400 PE wastewater treatment plant stating that 27,344 kg O\({}_{2}\) would be required per day, which equates to ca. 6,994,000 m\({}^{3}\) O\({}_{2}\)/a generated by the electrolyzer. Using pure O\({}_{2}\) aeration instead of air aeration may reduce aeration electricity demand by 82% (from 4440 MWh/a to 799 MWh/a). Considering that aeration is typically 49% of the power demand, the overall WWTP electricity demand may be reduced by 40% through O\({}_{2}\) use. Another option for using oxygen in a wastewater treatment plant is the removal of sulfur from biogas. Sulphur compounds, including both hydrogen sulphide (H\({}_{2}\)S) and other sulphur-based compounds, are one of the most hazardous impurities found in raw biogas [53]. The concentration of H\({}_{2}\)S in municipal wastewater-derived biogas can reach 400-4000 ppm [53; 54]. In other applications, H\({}_{2}\)S levels could be as high as 4500 ppm in dairy manure-derived biogas [55] and 12,000 ppm in distillery anaerobic digestion biogas of the ethanol industry [56]. H\({}_{2}\)S tolerance levels for the currently most often used conversion technologies for biogas [57] are: 50-200 ppm for internal combustion engines [55; 58], 4 ppm for biomethane injection to natural gas grid [55], 16 ppm for biomethane as transportation fuel [55]. For innovative use in solid oxide fuel cells for electricity production, this level should be 1-4 ppm [53] or even <1 ppm [59; 60]. Over the past decade, biogas biodegradation technologies have shown a growing interest in research and applications due to their environmental, economic and energy advantages [61; 62; 63]. When cleaning sulfur using biological methods, oxygen is required for the nutrition of H\({}_{2}\)S-decomposing bacteria. Until now, in most cases, air has been blown into the anaerobic digestion (AD) reactor under the dome or into the desulphurization unit due to its simplicity and cheapness. However, along with the air, nitrogen is mixed into the biogas in this way, which makes up about 70% of the air volume. If biogas is used for heat and electricity production and is burned in cogeneration plants or biogas boilers, the amount of nitrogen added does not affect it. However, if biogas is upgraded and biomethane is produced, the amount of nitrogen added can significantly impact the quality of biogas upgrading.