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

Abstract

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 8 h 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 120 mg Ca² L⁻¹ and 1000 mg 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 33 g (dry matter) L⁻¹, against 19 g L⁻¹ for calcium phosphates.

Keywords:

• dewatering
• harvesting
• Chlorella vulgaris
• autoflocculation
• pH-induced flocculation
• medium composition

Acknowledgements

The authors thank Guillaume Roelens (assistant engineer, GEPEA, Université de Nantes) and Jérémie Castello (PhD research engineer, TIMR 4297, Université de Technologie de Compiègne) for their contribution to scanning electron microscopy.

Funding

This work was supported by the French National Research Agency and is part of the ‘DIESALG’ project [ANR-12-BIME-000] on biodiesel fuel production from microalgae (http://www.biosolis.org/). The Vietnamese government is also acknowledged for funding the PhD of T.D.P. Nguyen.