## 3 Analysis of Kaunas WWTP Case
\begin{table}
\begin{tabular}{l c c c c c} \hline
**Parameter Name** & **Value** & **Units** & **Parameter Name** & **Value** & **Units** \\ \hline
**Wastewater treatment plant** & & & & **Power** & & \\ Total territory of WWTP & 19.3 & ha & Total power demand of WWTP & 9350 & MWh/a \\ Free territory of WWTP & 7.8 & ha & Power demand for basic tech. & & \\  & & & processes, MWh/a: & & \\
**Wastewater inflow** & & & -sludge management; & 4115 & MWh/a \\ Number of inhabitants in Kaunas & 302,875 & vrt & -biological treatment of wastewater; & 4382 & MWh/a \\ Wastewater inflow into WWTP & 25,501,000 & m\({}^{3}\)/a & -other processes and administrative premises. & 854 & MWh/a \\ Wastewater amount in popul. & 392,413 & PE & Power demand for basic tech. & & \\ equivalent (PE) & 392,413 & PE & processes, \%: & & \\ Wastewater biological oxygen & & & -sludge management; & 44.0\% & \% \\ demand (BOD) & & & -biological treatment of wastewater; & 46.9\% & \% \\
**Sludge** & & & & -other processes and administrative premises. & 9.1\% & \% \\ Dewatered sludge (dry mass) & 7175 & t DM/a & Power production from biogas & 1627 & MWh/a \\ Dewatered sludge (wet mass) & 31,367 & t/a & -Power from the grid & 7533 & MWh/a \\ Humidity of dewatered sludge & 77.1\% & \% & Power production in a solar & 190 & MWh/a \\ Dried sludge (dry mass) & 3533 & t DM/a & Power plant & & \\ Dried sludge (wet mass) & 3926 & t/a & Total power prod. in WWTP from RES, MWh/a & 1817 & MWh/a \\ Humidity of dried sludge & 10.0\% & \% & Total power production in & & \\ Moisture evaporated during & 11,519 & t/a & WWTP from RES, \% & & \\ sludge drying temperature & 85 & \({}^{\circ}\)C & Installed power of & & \\ Installed power of & 1.95 & MW & Natural gas consumption, nm\({}^{3}\)/a & 37,215 & nm\({}^{3}\)/a \\ the sludge dryer & & & & \\ Heat consumption for & 7320 & MWh/a & Natural gas consumption, & 395 & MWh/a \\ sludge drying & 38,315 & MWh/a & **Biogas** & & \\
**Aeration** & & & & & \\ Power consumption for aeration & 4382 & MWh/a & Biogas production, MWh/a & 27,161 & MWh/a \\ Oxygen amount for aeration & 5,823,558 & m\({}^{3}\)/a & Biogas energy value & 7.12 & kWh/m\({}^{3}\) \\
**Heat** & & & Biogas storage volume & 1200 & m\({}^{3}\) \\ Total heat demand of WWTP & 16,129 & MWh/a & Biogas cons. for heat production in boilerhouse & 2,711,456 & m\({}^{3}\)/a \\ Heat demand for basic tech. & & & Biogas cons. for power prod. in & 924,813 & m\({}^{3}\)/a \\ processes, MWh/a: & & & & \\ -heating of anaerobic digesters; & 4633 & MWh/a & Biogas utilization in the flare & 178,531 & m\({}^{3}\)/a \\ -sludge drying; & 7320 & MWh/a & **Transport** & & \\ -other processes and & & & & \\ administrative premises. & 4176 & MWh/a & Diesel consumption in & & \\ Heat demand for basic tech. & & & WWTP transport & 100 & MWh/a \\ processes, \%: & & & & \\ -heating of anaerobic digesters; & 28.7\% & \% & **Hydro energy** & & \\ -sludge drying; & 45.4\% & \% & Hydro power production & & \\ -other processes and & & & & \\ administrative premises. & 25.9\% & \% & & & \\ Heat production from natural gas & 301 & MWh/a & & \\ Heat production from biogas in & 14,692 & MWh/a & & \\ the boiler house & & & & \\ Heat production from biogas & 1137 & MWh/a & & \\ in cogenerators & & & & \\ Total heat prod. in WWTP from RES, MWh/a & 15,829 & MWh/a & & \\ Total heat production in WWTP & 98.1\% & \% & & \\ \end{tabular}
\end{table} 
Table 3: The main data of Kaunas WWTP (structured by the authors). If oxygen generated during electrolysis was used for aeration, the efficiency of the aerators and the electricity consumed at the same time could be reduced by 81.2%, or 5.5 times [51]. **Heat demand and production. ** The annual heat demand of Kaunas WWTP is 16,129 MWh. Of this, 28.7%, or 4633 MWh, is used for heating anaerobic digesters, and 45.4%, or 7320 MWh, for drying sludge. The remaining 25.9% of thermal energy is used for other technological processes and heating administrative premises. Approximately half of the total sludge is dried.