Coagulation with magnesium was found to be more effective for harvesting microalgae Chlorella zofin-giensis 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. (C) 2016 Published by Elsevier Ltd.