Environmental Technology - Manuscript ID TENT-TENT-2013-1182
-----------------------------------Harvesting Chlorella vulgaris by natural increase in pH: effect of medium composition
Thi Dong Phuong NGUYEN(1), Matthieu FRAPPART(1), Pascal JAOUEN(1), Jérémy PRUVOST(1), Patrick BOURSEAU(1,2)* (1)
LUNAM Université, Université de Nantes, CNRS, GEPEA, UMR 6144, CRTT, 37 bd de
l’Université, BP 406, 44602 Saint Nazaire Cedex, France (2)
Univ. Bretagne-Sud, EA 4250, LIMATB, F-56100 Lorient, France
* Corresponding author: Tel.: +33 240 172 668; Email address:
[email protected]
Abstract The freshwater microalgae Chlorella vulgaris is harvested by flocculation resulting from the precipitation of inorganic compounds induced by a slow, natural increase in pH in the absence of CO2 input (natural flocculation). Flocculation is tested in two culture media having respectively ammonium and nitrate ions as nitrogen source. Flocculation is observed in none of the two media as they are but becomes possible in the nitrate based medium if it is complemented in Ca2+ or Mg2+. The minimal concentrations required for flocculation in the two media are firstly estimated by a quick, artificial increase in pH by soda addition, before carrying out natural flocculation attempts. It is found that natural flocculation requires 120 mg.L–1 of Ca2+ (for a concentration of 70 mg.L–1 in phosphate) or 1000 mg.L–1 in Mg2+, which are much higher than the concentrations estimated beforehand by NaOH addition, ca 3.5 times higher for [Ca2+]min and 20 times for [Mg2+]min. EDX and ionic chromatography measurements show that both magnesium phosphates and magnesium hydroxide are involved in the flocculation induced by Mg2+ doping. Cells are more concentrated in aggregates obtained by magnesium compounds than by calcium phosphates precipitation.
Keywords: dewatering; harvesting; Chlorella vulgaris; natural flocculation; pH-induced flocculation; medium composition
