Inhibition mechanisms of high salinities on flocculation of marine Algal-Mycelial pellets

Microalgae are widely used in wastewater treatment because they can absorb nitrogen and phosphorus pollutions and reduce CO₂ emissions. However, they are hard to collected due to tiny cell sizes. Flocculation of microalgae by fungi to form the algal-mycelial pellets (AMPs) is one of the efficient collecting methods from wastewater. With the large amount of saline wastewater being discharged, the flocculating effects and mechanisms of AMPs in high saline wastewater are still unknown. Flocculation experiments were performed by Aspergillus niger and Chlorella sp. to study the effects and mechanisms in 0 %-4% salinities. Results showed that the flocculating efficiency (FE) in the 0 %–2% salinities exceeded 95 % at 24 h, whereas the FE reached only 63 % ± 2 % in the 4 % salinity. The flocculating biomass were also decreased with the increasing salinity. Fungi pellets increased in volume and mass at high salinity, resulting in a more compact mycelium structure with less space for microalgae to attach, which was not conducive to flocculation. Furthermore, contents of proteins (PN) and polysaccharides (PS) in the tightly bound EPS (TB-EPS) of AMPs at 4 % salinity decreased by 44 % ± 8 % and 33 % ± 4 % respectively compared to those at 0 % salinity. The decrease in the content of PN and PS led to a weakening of hydrophobicity, a rise in electrostatic repulsion, and an increase in the energy barrier of AMPs, all of which impeded flocculation. This study will provide theoretical bases for the treatment and the recovery of microalgae in high saline wastewater.