Optimization of Auxenochlorella pyrenoidosa Photobioreactor Parameters for Low Carbon-to-Nitrogen Ratio Wastewater Treatment: Effects of Inoculum Density, Aeration, Light Intensity, and Photoperiod

Treating wastewater with a low carbon-to-nitrogen (C/N) ratio remains a major challenge for conventional biological processes because insufficient organic carbon limits heterotrophic denitrification. To address this issue, microalgae-based photobioreactors offer a sustainable alternative that couples nutrient removal with biomass valorization. This study systematically evaluated the effects of four key operational parameters—initial inoculum density, aeration rate, light intensity, and photoperiod—on nutrient removal, biomass productivity, and metabolite accumulation of Auxenochlorella pyrenoidosa (A. pyrenoidosa) treating synthetic low C/N wastewater. Optimal operating conditions were identified as an initial OD680 of 0.1, aeration rate of 2 L air min−1, light intensity of 112 μmol m−2 s−1, and a 16L:8D photoperiod. Under these conditions, the photobioreactor achieved 86.35% total nitrogen and 98.43% total phosphorus removal within 11 days while producing biomass rich in proteins, polysaccharides, and lipids. Metagenomic analysis revealed a metabolic transition from denitrification-driven pathways during early operation to assimilation-dominated nitrogen metabolism under optimized conditions, emphasizing the synergistic interactions within algal–bacterial consortia. These findings demonstrate that optimized A. pyrenoidosa-based photobioreactors can effectively recover nutrients and produce valuable biomass, offering a viable and sustainable solution for the treatment of low C/N wastewater.