## 3 Analysis of Kaunas WWTP Case
Thus, 1817 MWh or 19.4% of the total electricity consumed by WWTP is produced in WWTP from renewable resources. Kaunas WWTP has three cogeneration units with a total capacity of 900 kW\({}_{\text{el}}\). The total operating time of all cogenerators in 2023 was 11,368 h, or 43.3% of the total possible operating time, and a significant amount of the time the cogenerators were operating at partial capacity. Installing an electrolyzer in the WWTP and utilizing the waste heat of the electrolyzer would make the use of biogas in cogenerators solely for electricity production economically inexpedient. Kaunas WWTP currently has an installed 200-kW solar power plant, and a project has been prepared to expand the solar power plant by another 200 kW. Based on the estimates presented in articles [20; 21; 22], the total solar electricity potential of Kaunas WWTP could reach 3639 kW, and the wind electricity potential--1.7 MW. The limited free territory of WWTP (7.8 ha) and location (city area, vicinity of living areas) are strongly limiting factors for expanding green electricity production at WWTP, especially for wind electricity. A common limiting feature of solar and wind power plants is the unstable electricity production during the day and year. In Lithuania, the intensity of sunlight depends significantly on the time of year. According to Kaunas WWTP data, the average monthly solar electricity generation during the light period (May-September) is on average 3.6 times higher than during the dark period of the year (October-April). If the electrolyzer operates in continuous mode, to ensure an uninterrupted supply of green electricity, it is necessary to build electricity storage facilities or ensure the supply of green electricity from other sources, such as a biogas cogenerators. The second option to secure renewable electricity for the production of green hydrogen is to use electricity from solar and wind parks located outside the WWTP and connected to the WWTP by direct cable lines. This option is difficult in the case of Kaunas WWTP since the WWTP is located in an urbanized area. The third option for supplying renewable electricity for green hydrogen production is to use renewable electricity from the grid, if the average CO\({}_{2}\) emissions from electricity production in the country or region do not exceed 18 g CO\({}_{2\text{eq}}\)/kWh of electricity. At present, this option is not available to WWTPs in Lithuania, as the average CO\({}_{2}\) emissions from electricity production in Lithuania in 2023 were 124 g CO\({}_{2\text{eq}}\)/kWh of electricity [90]. Kaunas WWTP has a small hydropower potential. According to the information provided in the study [25], the total hydropower potential, considering wastewater turbining before and after the treatment plant, is 654 MWh, which would be about 7% of the current electricity demand of the WWTP. **Natural gas network. ** Kaunas WWTP has direct access to the medium-pressure (3 bars) natural gas distribution network in its territory. The capacity of biomethane injection into the gas network exceeds 5 million m\({}^{3}\)/year, which fully satisfies the current and prospective biomethane production capabilities of Kaunas WWTP. **District heating network. ** The nearest Kaunas city district heating subnetwork is located 1000 m from the WWTP plant. Winter/summer heat demand in this subnetwork varies from 2.5 MW to 0.5 MW. Limiting factor--a new heating pipeline should be installed through the territory of the botanical reserve. Mitigating circumstance--biogas and natural gas pipelines are already laid through the botanical reserve. The heat supply route could be laid next to biogas and natural gas pipelines, using trenchless technologies. The Kaunas city district heating system produces 90% of heat from renewable sources [130].