A comparative study of the coagulation behaviour of marine microalgae
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Coagulation is an important step in the harvesting of algal biomass. This paper presents experimental results for a variety of prospective marine microalgal species using several inorganic and organic coagulants. Tetraselmis suecica and Chlorococcum sp. are readily coagulated using alum or iron(III) sulphate without any pH adjustment; doses of 3–5 mg L−1 or 0.2 mmol m−2 of Al3+ or Fe3+ yielding cell recoveries above 90 % after only 5-min settling. Nannochloropsis salina, Dunaliella tertiolecta and Isochrysis galbana are harder to coagulate and require at least two times more coagulant to achieve similar recoveries. Several cationic polyacrylamides were investigated but were less effective than Al3+ or Fe3+. Addition of NaOH to control pH improved the coagulation efficiency of N. salina but not of D. tertiolecta. The high coagulant demand of N. salina is due in part to its small size and large surface area, while that of D. tertiolecta may be attributable to its high production of extracellular polymer. The implications of cell surface properties for coagulation efficiency are discussed. At the coagulant doses used herein, settled cells remain viable. Resuspension is a potential problem with some species, arising either from cell motility or from flotation of flocs by oxygen bubbles generated by photosynthesis. These effects can be eliminated by small additions of chlorine or by settling the algae in the dark.
KeywordsMicroalgae Dewatering Harvesting Coagulation Flocculation Biofuels
This work was funded by an ARC linkage grant with financial support from Biofuels Pty Ltd. We thank Tina Hines and Kerrie Browne at Water Studies Centre for DOC and Fe analyses. Al analyses were performed by ALS Laboratory Group, Springvale, Victoria, Australia. The authors also thank Sherrie Caarels for her monitoring work and maintenance of the algal cultures.
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