The influence of physical floc properties on the separation of marine microalgae via alkaline flocculation followed by dissolved air flotation

Abstract

Flocculation achieved by raising the pH, termed alkaline flocculation is a sustainable way of inducing flocculation in marine microalgae. Flocs formed post-alkaline flocculation have the potential to be harvested via dissolved air flotation (DAF) or sedimentation. While DAF results in faster separation and biomass with a higher solids content compared to sedimentation, it has not been tested in saline environments, particularly when combined with alkaline flocculation. DAF processes, like most separation techniques, are heavily dependent on physical floc properties. In this study, the impact of alkaline floc properties on the performance of DAF versus sedimentation was evaluated to harvest marine Nannochloropsis oculata, benchmarking against the well-studied ferric chloride flocculation. This was followed by the use of the DAF white-water model to illustrate how alkaline and ferric floc properties impact bubble-particle attachment and DAF separation efficiencies. Alkaline flocs were smaller (peakmax < 300 μm; majority flocs 130–470 μm), stronger (~70 % strength factor) and more compact (scattering exponent > 2.30) compared to ferric flocs which were larger (peakmax ~ 1700 μm; >65 % of flocs >1000 μm), relatively weaker (<40 % strength factor), less compact (scattering exponent 1.49–2.30). However, as both flocs were hydrophilic, sedimentation yielded ~15 % greater efficiency than DAF. Stoke's Law suggested that sedimentation benefited due to alkaline floc compactness and large sizes of ferric flocs. Nonetheless, maximum separation efficiencies of ~80–85 % were still obtained via the DAF process. From the white-water model, bubble-floc attachment efficiency of alkaline flocs (0.01–0.001 %) was observed to be at least an order of magnitude greater than ferric flocs (0.001–0.0001 %) despite both methods resulting in comparable DAF separation efficiencies. For alkaline flocs, elevated bubble-floc attachment efficiency compensated low hydrophilicity; for ferric flocs, low bubble-floc attachment was compensated by large floc sizes. Overall, it is suggested that the determination of floc properties post-coagulation-flocculation could be used to optimise separation processes.