349455 Reversible and Recyclable Flocculants for the Harvesting of Microalgae

Monday, November 4, 2013
Grand Ballroom B (Hilton)
Lucjan Zolnierowski, Chemical Engineering, Yale Univeristy, New Haven, CT

Microalgae-derived liquid biofuels have potential advantages over other renewable, crop-based resources; however, large-scale production is not currently economical due to challenges in each processing step including harvesting.  Technoeconomic analyses indicate that for an algal biofuels process to meet economic targets, microalgal suspensions cultured as low as 0.02 wt% must be pre-concentrated during harvesting to a 1 wt% slurry.  Flocculation provides an attractive route for the pre-concentration and dewatering of microalgae.  Commercial flocculants, while efficient at aggregating and concentrating microalgae, have a high materials cost and have been shown to inhibit lipid extraction, foul downstream processes, and contaminate waste streams.  We present a novel approach for the dewatering and harvesting of microalgae from dilute culture suspensions using copolymer flocculants that can be recovered and recycled.  The model recyclable flocculants consist of polyelectrolytes with overall molecular charges dependent upon the system pH, thereby providing reversible electrostatic interactions with the primarily negatively-charged algal cells.  These pH-dependent properties allow the flocculants to efficiently desorb from concentrated biomass and, unlike many commercial flocculants that have permanently charged functionalities, to be recovered and recycled for further dewatering processes.  Tailored copolymer flocculants with mixed-charge character can also be applied to other charged cellular and biological systems of interest (i.e., yeast).  

Here we will present bench-scale experiments that demonstrate the entire process of flocculant reversibility, recovery with >50% yields, and recycling for the dewatering of Chlorella vulgaris (UTEX 395), Nannochloropsis gaditana (CCMP 526) and a model system consisting of a charge-stabilized colloidal suspension of silica microparticles. Furthermore, the recyclable flocculants are shown to be competitive with conventional flocculants in terms of flocculation efficiency, optimal dosage, settling time, and achieving biomass concentrations greater than 10 g/L.  Additionally the copolymer flocculants have been proven to provide enhanced flocculation efficiencies, up to 1.5 times greater than comparable homopolymer flocculants, with their ability to adsorb to the diverse range of charge character in cellular suspensions and to provide flocculation over an extended range of pH values.  The flocculation approach and recyclable materials presented have the potential to reduce materials costs associated with cellular separation processes and to furthermore enhance downstream processes, for example in microalgal harvesting, by improving oil recovery from the concentrated biomass and reducing equipment fouling.

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