Harvesting microalgal biomass using magnesium coagulation-dissolved air flotation

Coagulation with magnesium was found to be more effective for harvesting microalgae Chlorella zofingiensis with dissolved air flotation (DAF) than the use of Fe³⁺, Al³⁺ or chitosan, and the required coagulant dosage was in the order Mg²⁺ < chitosan < Al³⁺ < Fe³⁺. The Mg²⁺ dosage required depended on the growth phases and culture medium characteristics. An early exponential culture required the highest Mg²⁺ dosage (226 mg g⁻¹), while a late stationary culture required the lowest dosage (36 mg g⁻¹). HPO₄²⁻ and CO₃²⁻ in the culture medium competed with the microalgal cells for Mg²⁺ and increased the Mg²⁺ dosage necessary. No Mg²⁺ addition was required to harvest the freshwater microalgae Scenedesmus dimorphus grown in a pond with tap water with a high Mg²⁺ concentration or the marine microalgae Nannochloropsis sp. The critical coagulation pH ranged between 10.8 and 11.8, with a lower pH requirement at a higher Mg²⁺ concentration. Magnesium hydroxide precipitated with the harvested biomass; however, over 99.5% of the precipitated Mg²⁺ was recovered by washing the biomass with 0.1 M HCl. Microalgal harvesting with Mg²⁺ did not introduce extrinsic coagulant; thus, neither the biomass nor the medium was contaminated.