Techno-economic assessment of PEM electrolysis for O2 supply in activated sludge systems – a simulation study based on the BSM2 wastewater treatment plant

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 renewable energy sector. Wastewater treatment plants (WWTP) could make use of oxygen (O2) produced alongside H2 in biological treatment steps, however, production costs of electrolysis O2 should be competitive with respect to 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 Nr. 2 (BSM2) were used to simulate electrolysis O2 supply to an activated sludge (AS) system and estimate net costs of production (NCP) via a techno-economic assessment (TEA). Assuming that produced H2 is sold to a nearby industry, NCPs for O2 were calculated for two scenarios regarding PEME plant dimensions, which correspond to average (scenario 1) and optimal (scenario 2) electricity availability, i.e. medium and high number of full load hours per year. For each scenario, estimations were made for four alternatives for electricity supply and costs, namely conventional sources, photovoltaic (PV), and on/off shore wind energy, and using sets of optimistic, neutral and pessimistic data regarding system performance and market conditions. The analyses were done for 2020 as reference year and again for 2030 based on forecasts of relevant data. The results of the dimensioning of the PEME plant in scenario 1 show that a 6.4 MW PEME operated for 4,073 full load hours per year is able to cover the O2 demand of the AS system during more than 99% of the simulated period. The same is true for a 4.8 MW PEME operated for 6,259 full load hours per year in scenario 2. The TEA shows that investment costs for the PEME stacks and the operational costs for electricity make up most of the NCP of electrolysis O2. Although NCP for electrolysis O2 are always higher than those of conventional O2 sources for the year 2020, in 2030 estimated NCPs for the smaller PEME plant of scenario 2 become competitive for PV and wind on shore electricity supply under optimistic market conditions. Potential minimum selling prices were also calculated for the produced O2, however, they were above those of conventional O2 sources both in 2020 and 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 to conventional AS, and under different assumptions regarding economic conditions.