This research work proposes an innovative water resource recovery facility (WRRF) for the recovery of energy, nutrients and reclaimed water from sewage, which represents a promising approach towards enhanced circular economy scenarios. To this aim, anaerobic technology, microalgae cultivation, and membrane technology were combined in a dedicated platform. The proposed platform produces a high-quality solid- and coliform-free effluent that can be directly discharged to receiving water bodies identified as sensitive areas. Specifically, the content of organic matter, nitrogen and phosphorus in the effluent was 45 mg COD·L⁻¹, 14.9 mg N·L⁻¹ and 0.5 mg P·L⁻¹, respectively. Harvested solar energy and carbon dioxide biofixation in the form of microalgae biomass allowed achieving remarkable methane yields (399 STP L CH₄·kg⁻¹ COD_inf) equivalent to theoretical electricity productions of around 0.52 kWh per m³ of wastewater entering the WRRF. Furthermore, 26.6% of total nitrogen influent load was recovered as ammonium sulphate, while nitrogen and phosphorus were recovered in the biosolids produced (650 ± 77 mg N·L⁻¹ and 121.0 ± 7.2 mg P·L⁻¹).

Recycling carbon-rich wastes into high-value platform chemicals through biological processes provides a sustainable alternative to petrochemicals. Cupriavidus necator, known for converting carbon dioxide (CO₂) into polyhydroxyalkanoates (PHA) was studied for the first time using biogas streams as the sole carbon source. The bacterium efficiently consumed biogenic CO₂ from raw biogas with methane at high concentrations (50%) proving non-toxic. Continuous addition of H₂ and O₂ enabled growth trends comparable to glucose-based heterotrophic growth. Transcriptomic analysis revealed CO₂-adaptated cultures exhibited upregulation of hydrogenases and Calvin cycle enzymes, as well as genes related to electron transport, nutrient uptake, and glyoxylate cycle. Non-adapted samples displayed activation of stress response mechanisms, suggesting potential lags in large-scale processes. These findings showcase the setting of growth parameters for a pioneering biological biogas upgrading strategy, emphasizing the importance of inoculum adaptation for autotrophic growth and providing potential targets for genetic engineering to push PHA yields in future applications.