## 3 Analysis of Kaunas WWTP Case  
This section applies the seven key presumptions for the successful integration of green hydrogen and biomethane production processes in the Kaunas City wastewater treatment plant, which was commissioned in 1999. The total amount of wastewater inflow is 25.5 million m\({}^{3}\) per year, or 392,400 PE. **Description of Kaunas WWTP. ** The technological diagram of the Kaunas City WWTP is presented in Figure 1.  
The wastewater from the city flows into the WWTP through three pipes with a diameter of 1.2 m. Large solids are retained in perforated grates with a hole diameter of 6 mm. About 532 tons of various floating waste are generated per year. The solids are drained from the grates by a press and used in the Kaunas city municipal waste incineration plant for energy production. Sand is settled in aerated grit chambers. About 250 tons of sand are generated per year. Sand is drained by a separator and taken to a landfill. Wastewater is mechanically treated in two primary settling tanks with a diameter of \(\O \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Technology Name**} & \multicolumn{2}{c}{**Technology Readiness Level**} & \multicolumn{2}{c}{**Application in WWTP**} \\ \cline{2-6}  & **Laboratory Scale** & **Pilot Scale** & **Industrial Scale** & **Demo Cases** & **Industrial Cases** & **Wide Use** \\ \hline Biogas production & & \(\times\) & & & \(\times\) \\ \hline Biogas upgrading & & \(\times\) & & & \(\times\) \\ \hline Biogas cogeneration & & \(\times\) & & & \(\times\) \\ \hline Biological removal of H\({}_{2}\)S with oxygen aeration & \(\times\) & & & \(\times\) & \\ \hline Oxygen aeration & & & \(\times\) & & \(\times\) \\ \hline Co-digestion & & \(\times\) & \(\times\) & & \(\times\) \\ \hline Electrolysis & & & \(\times\) & & \(\times\) \\ \hline Biological CO\({}_{2}\) methanation in situ & \(\times\) & & & \(\times\) & \\ \hline Biological CO\({}_{2}\) methanation ex situ & & \(\times\) & & & \(\times\) \\ \hline Chemical (catalytic) CO\({}_{2}\) methanation & & & \(\times\) & \(\times\) & \\ \hline Hydrogen cogeneration & & \(\times\) & \(\times\) & \(\times\) & \\ \hline Sludge gasification & \(\times\) & \(\times\) & & \(\times\) & \\ \hline Sludge incineration & & & \(\times\) & & \(\times\) \\ \hline Heat pumps & & & \(\times\) & & \(\times\) \\ \hline Solar power plants & & & \(\times\) & & \(\times\) \\ \hline Wind power plants & & & \(\times\) & & \(\times\) \\ \hline Electricity storage (batteries) & & & \(\times\) & & \\ \hline Hydro power plants & & & \(\times\) & & \(\times\) \\ \hline Biomethane-powered transport & & & \(\times\) & & \(\times\) \\ \hline Hydrogen-powered transport & & & \(\times\) & & \\ \hline Biogas boiler exhaust gas CO\({}_{2}\) methanation & \(\times\) & \(\times\) & & \(\times\) & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Main WWTP processes according to the technological readiness level (structured by the authors). depth of 3 m, and a total volume of 7500 m\({}^{3}\). Biological treatment consists of 4 parallel technological lines, each with a 1674 m\({}^{3}\) bio-P (biological phosphorus removal) reactor and a 15,165 m\({}^{3}\) N/DN (simultaneous nitrification-denitrification) reactor. The total volume of aerotanks is 67,356 m\({}^{3}\). The design concentration of activated sludge is 4.8 g/L (at a daily wastewater flow rate of 81,890 m\({}^{3}\)/d). In the N/DN reactors (aerotanks), organic dissolved pollutants are decomposed and nitrogen is removed (nitrification and denitrification in parallel). In order to ensure simultaneous conditions for nitrification and denitrification, it is necessary to maintain an optimal dissolved oxygen concentration of about 0.15-0.3 g/L. Each aeration tank section is equipped with ammonium nitrogen and nitrate concentration meters, which are used to control the operation of blowers and mixers using intelligent algorithms.