The aim of this study was to evaluate the effect of light intensity and temperature on nutrient removal and biomass productivity in a microalgae–bacteria culture and their effects on the microalgae–bacteria competition. Three experiments were carried out at constant temperature and various light intensities: 40, 85 and 125 µE m⁻² s⁻¹. Other two experiments were carried out at variable temperatures: 23 ± 2°C and 28 ± 2°C at light intensity of 85 and 125 µE m⁻² s⁻¹, respectively. The photobioreactor was fed by the effluent from an anaerobic membrane bioreactor. High nitrogen and phosphorus removal efficiencies (about 99%) were achieved under the following operating conditions: 85–125 µE m⁻² s⁻¹ and 22 ± 1°C. In the microalgae–bacteria culture studied, increasing light intensity favoured microalgae growth and limited the nitrification process. However, a non-graduated temperature increase (up to 32°C) under the light intensities studied caused the proliferation of nitrifying bacteria and the nitrite and nitrate accumulation. Hence, light intensity and temperature are key parameters in the control of the microalgae–bacteria competition. Biomass productivity significantly increased with light intensity, reaching 50.5 ± 9.6, 80.3 ± 6.5 and 94.3 ± 7.9 mgVSS L⁻¹ d⁻¹ for a light intensity of 40, 85 and 125 µE m⁻² s⁻¹, respectively.

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).