## 3 Analysis of Kaunas WWTP Case
During the heating season, when the outdoor temperature is above 0 \({}^{\circ}\)C, and during the entire non-heating season, the temperature of the water supplied to the district heating system is 65 \({}^{\circ}\)C. Excess waste heat from the electrolyzer, which is not consumed by the wastewater treatment plant itself, could be supplied to the district heating network from the WWTP. When the electrolyzer is not operating, heat from the district heating network, after additional warming up to 85 \({}^{\circ}\)C in the WWTP boiler room, could be used for sludge drying, and without additional warming up--for all other processes. **Power network. ** Kaunas WWTP has its own 110/10 kV power network substation located near the WWTP territory. The available power input from this substation is 20 MW. This fully satisfies the power needs of the planned electrolyzer. **Transport fuel filling infrastructure. ** Kaunas WWTP is in a good location in this regard. The distance from Kaunas WWTP to the transport fuel station, located near the Via Baltica traffic corridor connecting Poland, Lithuania, Latvia, Estonia, and Finland, is only 300 m. This location guarantees constant transport flows, making it convenient for installing biomethane and hydrogen filling stations. **Energy storage possibilities. ** Kaunas WWTP currently has a 1200 m\({}^{3}\) biogas storage capacity. 7.8 ha of free WWTP area can be used for all other types of energy storage. **Use of biomethane and hydrogen in transport. ** Kaunas WWTP generates 31,367 t of dewatered sludge per year, with a moisture content of 77%. Approximately half of the dewatered sludge (15,445 t) is dried. The dried sludge (3926 t) is used for energy production in the cement industry and is transported over a distance of more than 230 km. Part of the dewatered, non-dried sludge (14,244 t) is transported and used in agriculture--for fertilization and composting. The remaining portion of the dewatered, non-dried sludge (1678 t), together with waste from the grades (about 532 t), is transported to the Kaunas city municipal waste incineration plant for energy production. 355 MWh of diesel is consumed annually for sludge transportation, which could be replaced with biomethane or green hydrogen produced at the wastewater treatment plant. The transport fuel consumption for other WWTP needs amounts to 100 MWh per year. **Use of biomethane (biogas) and hydrogen for electricity and heat production. ** Kaunas WWTP has three cogenerators with a capacity of 300 kW\({}_{\text{el}}\) each, with a total electricity generation capacity of 900 kW\({}_{\text{el}}\). After installing an electrolyzer at Kaunas WWTP, these generators could only be used for electricity production from biogas during peak electricity demand. As we noted earlier, the heat needs of the WWTP can be fully covered by the waste heat of the electrolyzer, therefore, in the absence of additional heat demand, the constant use of cogenerators is economically inexpedient. **Energy potential of dewatered sludge. ** In order to have a complete energy balance of the WWTP, the energy value of wastewater sludge should be assessed. Kaunas WWTP generates 7175 t of dewatered sludge by dry mass per year. Almost half of the generated dewatered sludge (3533 t by dry mass, or 49.2%) is dried and used for energy production in the cement industry. Some of the dewatered, non-dried sludge (1678 t by dry mass, or 23.4% of the total sludge) is also used for energy production in the Kaunas municipal waste incineration plant. The authors of article [129] studied the possibilities of using dried sludge from Kaunas WWTP for energy production by gasification. According to the data presented in this article, the total energy value of sludge generated in Kaunas WWTP is more than 38,315 MWh per year. However, there is currently no clear methodology that allows for the inclusion of sludge energy extracted in energy production facilities outside the WWTP into the energy balance of a WWTP. **Heat recovery from wastewater.