Dissolved air flotation of Chlorella sp. using chitosan: influence of algal organic matter and growth phase on coagulant dose

Abstract

This study investigates the efficient separation of Chlorella sp. microalgae using dissolved air flotation with chitosan as a coagulant. Improving the harvesting process, which significantly contributes to total costs and carbon footprint, could lead to competitive microalgae products and commodities. The exponential phase required 0.20 mg·mg⁻¹ chitosan, which is almost two times more than the chitosan dose of 0.12 mg·mg⁻¹ required for the stationary phase of growth, although the algal biomass concentration had increased from 0.26 ± 0.06 to 0.53 ± 0.01 mg·L⁻¹ and the concentration of algal organic matter from 8 to 32 mg C·L⁻¹ in dissolved organic carbon, respectively. It is also shown, via Microscope Force Spectroscopy, that the cell measured directly from the culture had bound algal organic matter to their surface at pH 8.5, increasing their softness (Young's Modulus = 8.1 ± 10 kPa), roughness (4.2 ± 2.4 nm) and interaction with bubbles (2.5 ± 2.1 nN, adhesion 64.4 %), compared to the washed cells in PBS buffer at pH 7.4 (1474 ± 1053 kPa; 1.3 ± 0.4 nm; 0.8 ± 0.5 nN, adhesion 9.1 %, respectively). Polysaccharides, mainly containing arabinose (29.2 ± 6.9 % and 33.0 ± 1.7 % dw polysaccharide) and galactose (34.2 ± 8.1 % and 17.43 ± 1.3 % dw), while acidic residues of the polysaccharides (6.7 ± 2.1 % and 13.1 ± 5.8 % Area) were also present along with protein (9.1 ± 2.2 % and 0.9 ± 0.2 % dw AOM) in exponential and stationary growth phase, respectively. Nevertheless, the relationship between AOM composition and its underlying structure and functionality is not yet fully understood.