## 3 Analysis of Kaunas WWTP Case
If the entire sludge were dried, the heat demand would be significantly higher (almost one and a half times). A temperature of 85-92 \({}^{\circ}\)C is required for drying sludge. The temperature of the hot water supplied in the Kaunas city district heating system is 65 \({}^{\circ}\)C. Therefore, it is impossible to use heat from the district heating network to dry sludge without additional warming up. The installed power of the sludge dry is 1.95 MW. Currently, the greatest portion--98.1% of the thermal energy consumed by Kaunas WWTP--is produced from biogas in the boiler house (91.1%) and in cogeneration units (7.0%). The remaining small portion of 1.9% of thermal energy is produced in the boiler house from natural gas. By installing a 8.3 MW electrolyzer, based on oxygen demand, the amount of waste heat from the electrolyzer would fully cover the heat needs of the WWTP (see Table 4). Therefore, the entire amount of biogas could be used for biomethane production. **Hydrogen demand. ** The amount of biogas produced at the Kaunas WWTP is 3.815 million m\({}^{3}\) per year. The average methane concentration in biogas is 62.8%. Thus, during the biogas production process, 1.420 million m\({}^{3}\) of biogenic CO\({}_{2}\) is generated per year. The methanation of this amount of CO\({}_{2}\) would require 5.682 million m\({}^{3}\) of green hydrogen per year and an additional 1.420 million m\({}^{3}\) of biomethane could be produced, or 59.3% more. If sorted food waste from Kaunas city were used for biogas production, another 0.803 million m\({}^{3}\) of biogas and 0.391 million m\({}^{3}\) of biogenic CO\({}_{2}\) would be produced per year. The methanation of this amount of CO\({}_{2}\) would require another 1.564 million m\({}^{3}\) of green hydrogen per year and an additional 0.391 million m\({}^{3}\) of biomethane could be produced. The total demand for green hydrogen for CO\({}_{2}\) methanation could be 7.246 million m\({}^{3}\), and biomethane production could be increased by 2.223 million m\({}^{3}\), or 92.8%. **Electricity demand, production and availability of green electricity. ** The annual electricity demand of Kaunas WWTP is 9350 MWh. Of this, 44.0%, or 4115 MWh, is used for sludge management, and 46.9%, or 4382 MWh, for biological wastewater treatment. The remaining 9.1% of electricity is used for other technological processes and administrative premises. \begin{table}
\begin{tabular}{l c c c c c c} \hline \hline \multirow{2}{*}{**Levels of Electrolyzer Capacity**} & \multirow{2}{*}{\begin{tabular}{c} **Based on O\({}_{2}\)** \\ **Demand** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} **Based on H\({}_{2}\)** \\ **Demand** \\ \end{tabular} } & \multirow{2}{*}{\
\begin{tabular}{c} **Based on Heat** \\ **Demand** \\ \end{tabular} } & \multirow{2}{*}{
\begin{tabular}{c} **Based on Electricity** \\ **Availability** \\ \end{tabular} } \\ \hline
**Case \({}^{*}\)** & **V1 and V2** & **V1** & **V2** & **V1 and V2** & **V1** & **V2** \\ \hline Electrical power of electrolyzer, MW & 8.3 & 4.1 & 5.2 & 6.8 & 1.4 & 2.3 \\ \hline Electricity consumption, MWh/a & 58,302 & 28,442 & 36,271 & 47,440 & 9991 & 15,976 \\ \hline Hydrogen production, min m\({}^{3}\)/a & 11.6 & 5.7 & 7.2 & 9.5 & 2.0 & 3.2 \\ \hline Oxygen production, m\({}^{3}\)/a & 5.8 & 2.8 & 3.6 & 4.7 & 1.0 & 1.6 \\ \hline Heat production, MWh/a & 19,823 & 9670 & 12,332 & 16,129 & 3397 & 5432 \\ \hline \hline \end{tabular}  
* V1—current situation (without additional waste, without solar power plant expansion and hydroelectricity). V2—after assessing the collection of sorted food waste, the potential of solar power and hydroelectricity.  
\end{table} 
Table 4: Electrolyzer power selection options (structured by the authors). Currently, the greatest portion--80.6% of the electricity consumed by Kaunas WWTP--is obtained from the electricity grid. The remaining electricity is produced from biogas in cogeneration units (1627 MWh or 17.4%) and a solar power plant (190 MWh or 2.0%).