System-level integration of hydrogen-assisted biogas upgrading and oxygen-enriched aeration in wastewater treatment plants

The increasing penetration of variable renewable energy sources has intensified the need for controllable demand-side flexibility capable of utilising surplus electricity without compromising process stability. Wastewater treatment plants (WWTPs) are well-positioned to provide such flexibility due to their continuous operation, energy-intensive process, and biogas production potential. This study develops a plant-wide, dynamic Power-to-X modelling framework by extending the Benchmark Simulation Model No. 2 (BSM2) to integrate hydrogen-assisted biogas upgrading and oxygen-enriched aeration, enabling system-level assessment of demand-side flexibility within a WWTP. Electrolytic hydrogen production driven by surplus renewable electricity was coupled with in-situ hydrogen injection into a pressurised anaerobic digester, considering open-loop and PI-controlled dosing strategies with and without gas storage. In parallel, the electrolyser by-product, oxygen, was routed to the activated sludge process through an oxygen storage and enrichment framework. Multiple scenarios were evaluated under realistic surplus electricity conditions to assess impacts on biogas quality, process stability, aeration performance, and overall energy efficiency. Results show that PI-controlled hydrogen dosing is essential for stable, system-scale biogas upgrading under variable surplus availability. Methane content increased from 62% in the baseline case to 86–89% under closed-loop control, with hydrogen utilisation efficiencies of approximately 97%. Hydrogen storage improved resilience to surplus variability. Oxygen reuse enabled a 24–37% reduction in aeration energy demand without affecting treatment performance. Overall, the integrated system converted 66% of the surplus renewable electricity consumed by the plant into process benefits, demonstrating the potential of WWTPs to operate as flexible, energy-responsive infrastructure assets within integrated, low-carbon energy systems.