## 2 Methodology
### District Heating Network  
Access to a district heating network is important from two perspectives. First, it is the efficient use of waste heat released during the production of green hydrogen by electrolysis or recovered from wastewater by heat pumps [11; 29; 46]. From 20% to 40% of the electricity consumed during the electrolysis process is converted into heat, which is often emitted into the atmosphere [29]. Some of this heat can be used in the WWTP itself. However, depending on the power of the electrolyzer, the amount of excess heat can be quite large, and its efficient utilization is relevant to ensure the energy neutrality of the WWTP. Withthe help of modern heat exchangers, 85% of the heat can be recovered from the electrolyzer stack and the outgoing gas (hydrogen and oxygen) flows [29]. Central heating systems in Europe are most developed in Denmark, Sweden, Estonia, Lithuania and some other countries [98]. For example, in Lithuania, 56% of households are connected to district heating networks [98]. And even in the UK, where only 2% of consumers are currently connected to the district heating network, plans are underway to increase this figure to 43% by 2050 [99]. Paper [29] provides data that an 11 MW electrolyzer operating only 5 h a day (when there is a surplus of solar and wind power) can provide heat for 190 households [29]. By fully utilizing the waste heat of the electrolyzer, the overall efficiency of the hydrogen and heat production system can reach 90.0-94.6% [29; 100; 101]. Also, after signing the relevant contracts with the district heating network operator, the district heating network could be used as a heat accumulator if the electrolyzer operates intermittently. The district heating network can also be used as a source of heat from renewable energy sources, thus increasing the GHG neutrality of the WWTP. If the heat in the district heating network is produced from renewable sources, WWTP can abandon the combustion of natural gas for heat production. Lithuania has a favorable situation in this regard. In the district heating system of Lithuania as a whole, 73% of heat was produced from RES in 2024, the target for 2030 is 90% of heat from RES [102]. The available power of the electricity grid at WWTP can be significant for the production of green hydrogen, but in the EU, this is possible only in regions with low CO\({}_{2}\) emissions [9]. However, with the rapid expansion of the share of renewable electricity generation in the overall electricity balance, this opportunity will be available to more and more WWTPs in different countries. This can be indicated by the decreasing trend of EU power sector emissions, which in 2023 dropped by 46% below the peak in 2007 [103]. The available power of the electricity grid may also be important if the WWTP plans to supply balancing electricity to the grid (whether this is possible and logical), produced from biomethane or green hydrogen in cogeneration plants [104]. As mentioned above, one of the results of integrating green hydrogen production into WWTPs could be a significant increase in biomethane production. Currently, Europe's largest share of biomethane (23% in the year 2023) is used in transport [105]. Over the next ten years, as heavy transport decarbonizes, the demand for biomethane in transport is expected to increase fourfold. The development of hydrogen heavy transport is also predicted. Biomethane and hydrogen produced in WWTPs are already being used as transport fuels. For example, in 2021, the 13 hydrogen refuelling stations in the UK were equipped to deliver around 60 kg of hydrogen per day. 60 kg of hydrogen is enough to fill around 10 hydrogen fuel cell cars, and although there are only around 2500 registered in the UK, the government predicts that there could be over 1.5 million hydrogen-powered vehicles on the roads by 2030. Future filling stations are expected to deliver over 1200 kg of hydrogen per day to meet the increased demand for hydrogen for fuel [29].