## 4 Results and Discussion
### Energy Balance After Integration of the Electrolyzer  
Two system cases are compared. In the first case (V1), only the installation of an electrolyzer in WWTP was evaluated. In the second case (V2), the potential of additional \begin{table}
\begin{tabular}{l c c c} \hline \hline
**Energy Form** & **Existing Situation** & **Case V1 \({}^{*}\)** & **Case V2 \({}^{*}\)** \\ \hline
**Energy consumption** & **MWh/a** & **MWh/a** & **MWh/a** \\ \hline Heat consumption (produced in electrolyzer) & & 16,129 & 16,129 \\ \hline Natural gas consumption for heat production & 395 & & \\ \hline Hydrogen consumption & & 17,046 & 21,738 \\ \hline Consumption of diesel as a transport fuel & 455 & & \\ \hline Consumption of biomethane as a transport fuel & & 455 & 455 \\ \hline
**Total net energy consumption:** & **26,029** & **97,697** & **102,389** \\ \hline
**Energy production** & **MWh/a** & **MWh/a** & **MWh/a** \\ \hline Heat production from biogas & 15,829 & & \\ \hline Power production from biogas & 1627 & & \\ \hline Power production in the existing solar power plant & 190 & 190 & 190 \\ \hline Biomethane production & & 39,674 & 48,022 \\ \hline Hydrogen production & & 34,941 & 34,941 \\ \hline Useful heat production in the electrolyzer & & 19,823 & 19,823 \\ \hline Solar power production, additional potential & & 3268 \\ \hline Hydropower production & & 654 \\ \hline
**Total useful energy production:** & **17 645** & **94,628** & **106,898** \\ \hline
**Energy balance** & & & \\ \hline Energy balance, MWh/a & \(-\)8383 & \(-\)3069 & 4508 \\ \hline Energy balance, \% & \(-\)32.2\% & \(-\)3.1\% & 4.4\% \\ \hline \hline \end{tabular}  
* Existing situation—without electrolyzer. Case V1—with electrolyzer only. Case V2—electrolyte + food waste + PV + hydro.  
\end{table} 
Table 5: _Cont._raw materials (sorted food waste), solar electricity and hydroelectricity was assessed in addition to installing an electrolyzer. In both cases, the power of the evaluated electrolyzer is selected based on oxygen demand and is 8.3 MW. It is assumed that only the waste heat of the electrolyzer will be used for the needs of the wastewater treatment plant. The excess waste heat of the electrolyzer will be supplied to the Kaunas city district heating network. The green hydrogen produced in the electrolyzer will be used for biogas methanation. The excess green hydrogen produced will be mixed into the natural gas network or used for transport. After biogas methanation, the biomethane produced will be injected into the natural gas network or used in the WWTP's own transport and for treated sludge transportation. Biogas will not be used for heat or electricity production. The total annual energy consumption of Kaunas WWTP is 97,697 MWh (case V1) or 102,389 MWh (case V2). In both cases, the greatest portion is electricity--64,067 MWh (65.6% and 62.6% of the total energy, respectively). Heat consumption is 16,129 MWh (16.52% and 15.87% of the total energy, respectively). Hydrogen consumption for methanation is 17,046 MWh and 21,738 MWh (17.4% and 21.2% of the total energy, respectively). Biomethane consumption in transport is 455 MWh, which in both scenarios is less than 0.5% of the total energy. The total annual energy production in Kaunas WWTP is 94,628 MWh (case V1) or 106,898 MWh (case V2). The main types of energy produced are biomethane--39,674 MWh and 48,022 MWh (or 41.9% and 44.9%, respectively), hydrogen--34,941 MWh in both cases (36.9% and 32.7%, respectively) and heat production in the electrolyzer--25,226 MWh in both cases (23.0% and 20.8%, respectively). A small amount of electricity is also produced: in the first case 190 MWh of solar electricity (0.2% of the total amount of energy produced), in the second case 3458 MWh of solar electricity and 654 MW of hydroelectricity (3.8% of the total energy produced). In the first case, the total energy balance remains slightly negative (\(-\)3.1%) or (\(-\)3.069 MWh). In the second case, the energy balance is already somewhat positive (+4.4%) or 4508 MW. Energy balance data are presented in Table 5.