Double-edged sword effects of mechanical aeration on granulation of Spirulina platensis for mariculture wastewater treatment

Abstract

Microalgae biogranulation holds immense promise for the efficient treatment of high-salinity mariculture wastewater. However, a comprehensive understanding of the underlying mechanisms and critical operational parameters driving microalgae granule formation is crucial for successful practical implementation. In this study, granulation of the salt-tolerant microalgal strain Spirulina platensis was attempted under different aeration intensities to treat synthetic mariculture wastewater. Although all the four reactors at upflow air velocity (UAV) of 0.45, 0.9, 1.35, and 2.70 cm/s, respectively achieved high removals of total nitrogen (85 %), total phosphorus (90 %), and dissolved organic carbon (80 %), S. platensis granules were only formed under UAV of 1.35 cm/s or higher. The granules at UAV 2.70 cm/s doubled alginate-like exopolymers recovery to 113.60 mg/g-MLVSS and P content to 11.86 mg/g-MLSS, whereas the proportion of S. platensis in the microbial community decreased significantly to 4.91 % and energy consumption increased. Mechanism analysis suggested S. platensis granulation commenced with tryptophan- and aromatic protein-like substances, followed by Marinobacter-mediated flocculation and augmented extracellular polymeric substance excretion, and final shaping through shear force. To conclude, this study highlights the dual nature impact of mechanical aeration on S. platensis granulation in mariculture wastewater treatment, and the need for diverse microalgae granulation strategies tailored to specific objectives in practical applications is emphasized.