Influence of magnesium concentration, biomass concentration and pH on flocculation of Chlorella vulgarisAlgal Research, vol 3, 24-29. Archived versionAuthor manuscript: the content is identical to the content of the published paper, but without the final typesetting by the publisher Autoflocculation is a promising low-cost method for harvesting microalgae for bulk 21 biomass production or wastewater treatment. Autoflocculation can be caused by 22 precipitation of calcium or magnesium at high pH. In this study, we investigated the 23 interactive effects of pH, magnesium concentration and microalgal biomass 24 concentration on flocculation of Chlorella vulgaris by magnesium hydroxide. The 25 minimum pH for inducing flocculation was lower when magnesium concentration in 26 the medium is higher. A higher pH and/or higher magnesium concentration are 27 required for flocculation when microalgal biomass concentration is increased. The 28 sludge volume formed during flocculation is highly variable and is influenced mainly 29 by the amount of magnesium hydroxide that precipitates during flocculation. The 30 sludge volume increases with pH and with magnesium concentration in the medium. 31There is an optimal pH where flocculation efficiency is maximized (> 95%) and 32 sludge volume is minimal (1 -2% of culture volume). Increasing the pH slightly 33 above this optimum results either in an increase in sludge volume and/or a decrease in 34 flocculation efficiency. We propose that autoflocculation by magnesium hydroxide 35 can be more easily controlled by the dosage of base rather than by targeting a specific 36 pH level.