The conversion of renewable energy into hydrogen (H₂) by power-to-gas technologies involving electrolysis is seen today as a key element in the transition to a sustainable energy sector. Wastewater treatment plants (WWTP) could be integrated into future green H₂ networks as users of oxygen (O₂) produced alongside H₂ in water electrolysis. In WWTPs, O₂ is required for biological treatment steps, e.g., in activated sludge (AS) systems. However, the production costs of electrolysis O₂ should be competitive with those of conventional O₂ production processes. In this study, mathematical models of a polymer electrolyte membrane electrolyser (PEME) plant and the WWTP of the Benchmark Simulation Model No. 2 (BSM2) were used to simulate electrolysis O₂ supply to an AS system and estimate net costs of production (NCP) for produced O₂ via a techno-economic assessment (TEA). Assuming that produced H₂ is sold to a nearby industry, NCPs for O₂ were calculated for two different PEME plant dimensions, four alternatives regarding electricity supply and costs, and three sets of assumptions regarding system performance and market conditions. The analyses were performed for 2020 as a reference year and 2030 based on forecasts of relevant data. Results of the dimensioning of the PEME show the O₂ demand of a municipal WWTP with an installed capacity of 80,000 population equivalents (PE), such as the one of the BSM2, can be covered for more than 99% of the simulated period by either a 6.4 MW PEME operated for 4073 full load hours or a 4.8 MW PEME operated for 6259 full load hours. Investment costs for the PEME stacks and the operational costs for electricity make up most of the NCP of electrolysis O₂. The projected decrease in PEME stack costs and renewable energy prices in favourable market conditions can result in a competitive NCP for electrolysis O₂ in 2030. The approach described in this study can be applied to analyse O₂ supply to biological wastewater treatment in WWTPs with different characteristics, in processes different from AS, and under different assumptions regarding economic conditions.

Power-to-gas-to-power technologies incorporating electrolysis, methanation, SNG-fired Allam cycles and subsurface storages allow for a confined and circular use of CO ₂ /CH ₄ and thus an emission-free seasonal storage of intermittent renewable energy.

Nowadays, soda (Na ₂ CO ₃ ) is predominantly produced via the conventional Solvay process, generating CO ₂ emissions contributing to climate change and saline CaCl ₂ waste streams that pose local environmental concerns. Electrochemical alternatives, such as chlor‐alkali electrolysis (CAE), have been explored for decades but still remain unused for large‐scale production of soda. Bipolar electrodialysis promises lower energy consumption than CAE while generating the valuable by‐product hydrochloric acid (HCl). This work provides mass and energy balances for electrochemical alternatives to the Solvay process. The results indicate a lower total energy requirement for the electrochemically based processes, whereas energy‐related costs remain comparable to the Solvay process.