Alum-induced flocculation of preconcentrated Nannochloropsis salina: Residual aluminium in the biomass, FAMEs and its effects on microalgae growth upon media recycling

Flocculation with polyaluminium complexes has been regarded as an unsafe method of harvesting microalgae due to the potentially toxic effects of aluminium. Varying concentrations of Nannochloropsis salina were flocculated with different concentrations of aluminium nitrate sulphate. The level of aluminium in the microalgal biomass, lipids and fatty acid methyl esters (FAMEs) was then determined by differential pulse adsorptive cathodic stripping voltammetry. These flocculation experiments were carried at both the laboratory and pilot scale, demonstrating efficiencies of between 79% and 99%. The highest efficiencies were observed when the concentrations of N. salina suspensions were either 15 or 20 g L−1. Despite the application of different doses of flocculant, the equilibrium content of aluminium in the filtrate averaged between 0.47 and 0.64 mg L−1 for all tested N. salina biomass concentrations. With increasing microalgae biomass concentration, an exponential decrease in aluminium content per weight of microalgal biomass was observed. After recycling the filtrate, the residual aluminium did not affect growth or photosynthetic performance of N. salina. Moreover, following extraction and conversion of lipids and FAMEs respectively, the residual aluminium decreased by 2–3 orders of magnitude. Aluminium was not detected in FAMEs from microalgae flocculated at a starting concentration of 20 g L−1N. salina biomass.

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► Flocculation of Nannochloropsis salina with alum is an economical biomass production method.
► After preconcentration, content of aluminium in lipids and FAMEs decreased.
► Aluminium in extracted lipids and fatty acids decreased by 2–3 orders of magnitude.
► Aluminium in recycled media did not affect photosynthesis and growth of N. salina.