## 2 Methodology
### Presumptions for the Successful Integration of Green Hydrogen and Biomethane Production Processes
#### 2.3.1 Utilization Possibility of All Products from Electrolysis in WWTP
This means that a 6.25 MW solar power plant could be installed, which would produce about 6413 MWh of electricity per year in Lithuanian climate conditions [81]. If electricity storage of suitable capacity were installed, a solar power plant of this size could supply electricity to an electrolyzer of about 0.92 MW, operating at 80% capacity [13]. The installation of wind turbines is primarily limited by the distance to the nearest buildings. According to the current Lithuanian standards, this distance must be at least 4 times greater than the height of the wind turbine mast [82]. Municipal WWTPs are usually installed in urban areas or near cities. The development of wind energy in such places is impossible due to protests from surrounding residents, even if the area available to WWTP was sufficient to build wind turbines. It is estimated that the ground area required for a wind power plant is 3400 m\({}^{2}\)/MW [20]. Bearing in mind that in a wastewater treatment plant with a capacity of 120,000 m\({}^{3}\)/d, the free area for a wind power plant can be about 10,000 m\({}^{2}\)[20], it would be possible to install an almost 3 MW wind power plant, which would produce about 4700 MWh of electricity per year in Lithuanian climate conditions [83]. If an electricity storage of suitable capacity were installed, a wind farm of this size could supply electricity to an electrolyzer of approximately 0.67 MW operating at 80% capacity [13]. Paper [20] analyzes the economic feasibility of acquiring additional land for solar and wind power plants to supply electricity for green hydrogen production, in order to address the land shortage within the WWTP area itself. However, other conditions, such as the availability of free land and the location of the WWTP, must be met in order to take advantage of this opportunity. The literature describes cases where wastewater hydropower is used to produce green energy. The main advantage of hydropower systems utilizing artificial water sources compared to natural water sources is their predictability. In WWTPs, usually, wastewater flow to the plant is constant, thus ensuring the stable hydro energy generation, in contrast to the hydropower plants using natural water flow in rivers, which is variable in time [25,84,85]. However, the potential for hydropower generation is limited by the amount of wastewater entering the WWTP and the height of the head. In 2020, the amount of municipal wastewater produced in Europe was 55 km\({}^{3}\)[86]. The potential for electricity production from sewage in this region, with some assumptions, is estimated to be equal to 664.24 TWh. However, this value is indicative only and would require separate scientific research [24]. Authors of the study [24] investigated the hydropower capacity of WWTPs by utilizing the kinetic and potential energy from treated wastewater to generate power. WWTP in Torun, Poland, was analyzed as a case study, treating wastewater 42,740 m\({}^{3}\) daily. The installed capacity of the Francis turbine is 55 kW, with a head of 7.45 m. It utilizes the top water from the secondary sedimentation tanks, which is discharged into a system of top and bottom water basins. Research showed that the average and the highest turbine power reached 20.89 kW and 24.82 kW, respectively, resulting in a total production of 150.29 MWh/a. At Al Samra WWTP, treating 364,000 m\({}^{3}\) per day, or 70% of the wastewater in Jordan, electrical energy is generated through hydraulic turbines at the inlet and exit of the plant. Two Pelton turbines are installed at the inlet of the WWTP with a net head of 80 m,producing a net power output of 1.76 MW. At the exit of the plant, there are 3 Francis turbines that receive the exit flow from a head of 42.8 m, resulting in a net power generation output of 2.13 MW [38]. Article [23] presents a methodology, developed and applied to a case study, for evaluation of hydropower potential in a group of WWTPs at the regional or country level.