Harvesting Chlorella vulgaris by natural increase in pH: effect of medium composition

The freshwater microalga Chlorella vulgaris was harvested by autoflocculation resulting from the precipitation of magnesium or calcium compounds induced by a slow increase in pH in the absence of CO2 input. Autoflocculation was tested in two culture media with, respectively, ammonium (NH (+)(4)) and nitrate (NO (-)(3)) ions as nitrogen source. The culture pH increased because of photosynthesis and CO2 stripping. pH rose to 11 after 8h in the NO (-)(3) medium, but did not exceed 9 in the NH (+)(4) medium. No flocculation took place in any of the media. Autoflocculation tests were repeated in the NO (-)(3) -based culture medium by progressively increasing the concentrations of Ca2+ and Mg2+ until inorganic compounds precipitated and flocculated microalgae. The minimal concentrations for flocculation were found to be 120mg Ca-2 L-1 and 1000mg Mg2+ L-1. These values were, respectively, 3.5 times and 20 times higher than those allowing flocculation by NaOH addition. Energy-dispersive X-ray spectroscopy, zeta potential measurement, and ionic chromatography suggest that the mechanisms involved are different. The rate of cell removal was close to 90% in both cases, but cells were more concentrated in the aggregates obtained by magnesium compound precipitation, with an estimated concentration close to 33g (dry matter) L-1, against 19g L-1 for calcium phosphates.