Nutrient Transformations in LED Tubular Photobioreactors Used for a UASB Effluent System Followed by a Percolator Biological Filter

This study investigated nutrient removal in tubular photobioreactors (PBR) illuminated by Light-Emitting Diodes (LEDs), treating the effluent from an Up-Flow Anaerobic Sludge Blanket (UASB) system followed by a Biological Percolator Filter (BPF). Designed as a tertiary treatment step, the PBRs aimed to minimize eutrophication while promoting microalgal growth through CO2 assimilation released by bacterial activity—resulting in biomass with potential for value-added applications. The objective of this study was to elucidate the mechanisms responsible for nitrogen and phosphorus removal under a controlled environment. A key novelty of this work lies in the comparative evaluation of red and blue LED illumination in the pilot-scale PBRs used for wastewater treatment. All systems achieved >99% ammoniacal nitrogen removal, while soluble phosphorus removal achieved up to 95%. The highest nitrogen removal rate was observed under red light (10.60 mg L−1 d−1), although there was no difference for blue light, while blue light was more effective for phosphorus removal (3.28 mg L−1 d−1). Assimilation was the primary removal mechanism, supported by microalgae–bacteria interactions and artificial CO2 injection. The research highlights distinct advantages of each light illumination: the blue-light photobioreactor was more suitable for tertiary treatment, whereas the red-light system showed potential for microalgal biomass-based co-product generation.