## 5 Conclusions  
1. The task of energy neutrality of the WWTP should be divided into two stages. In the first stage, it should be achieved that the energy balance of the WWTP becomes positive--that is, the amount of all types of useful energy produced in the WWTP exceeds the consumption of all kinds of energy in the WWTP. In the second stage, it should be sought that all renewable electricity used for green hydrogen production is generated at WWTP as is required by the EU Regulations [9]. 2. The analyzed case of Kaunas WWTP showed that installing an electrolyzer in a WWTP can significantly improve the energy balance of the WWTP. 3. To effectively utilize all electrolysis products and achieve the highest energy efficiency of the WWTP, the electrolyzer selection should be conducted by assessing the possibilities of using oxygen, hydrogen, heat and the availability of green electricity. For this, detailed technical and economic modeling of the WWTP should be carried out. This is the goal of the following research by the authors of this article. In this article, the electrolyzer was selected for analysis based on the oxygen demand. 4. The main problem in ensuring that all hydrogen produced by WWTP is green usually is the unavailability of renewable electricity sources installed onsite of WWTP. The potential for solar and wind electricity production in the WWTP territory is often limited due to the size of the free WWTP territory and the immediate vicinity of the living areas. Therefore, in the case of Kaunas WWTP, the most realistic source of green electricity could be the construction of remote solar and wind parks, once the appropriate economic or political conditions are in place. * 5. In the analyzed case of Kaunas WWTP, there is good access to all types of infrastructure--natural gas, electricity, heating networks, and transport refueling. In this way, the injection of the produced biomethane into the natural gas network, the mixing of hydrogen into biomethane, the use of waste electrolysis excess heat, the direct use of biomethane and hydrogen for filling transport fuel tanks and the supply of electricity to the electrolyzer can be ensured. * 6. By completely replacing the air in the aeration with oxygen generated during electrolysis, the performance, and thus electricity consumption of blowers in Kaunas WWTP could be reduced by 5.5 times, from 4382 MWh to 797 MWh. The electricity consumption for wastewater treatment processes in the WWTP would be reduced by 38.3%. * 7. An essential factor is the availability of electricity to the electrolyzer. Although replacing air with oxygen significantly reduces electricity demand for wastewater treatment processes in Kaunas WWTP, but due to the installation of an electrolyzer, the total electricity demand of the WWTP can increase by up to 7 times if the electrolyzer power is selected according to the oxygen demand. Conceptualization, R.L. and S.R. ; methodology, R.L. ; resources, R.L. ; writing--original draft preparation, R.L. ; writing--review and editing, S.R. and R.P. ; visualization, R.L. ; supervision, S.R. All authors have read and agreed to the published version of the manuscript. This research received no external funding. Not applicable. Restrictions apply to the availability of these data. Data was obtained from the Kaunas city water management company UAB \"Kauno vandenys\" (Lithuania) and are available from the authors with the permission of UAB \"Kauno vandenys\". We would like to thank the Kaunas city water management company UAB \"Kauno vandenys\" (Lithuania) for providing data on technologies used in Kaunas WWTP and consultations. The authors declare no conflicts of interest. The following abbreviations are used in this manuscript: