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A novel hydrodynamic separation technology for algae dewatering
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This paper describes a novel hydrodynamic separation technology (HDS) for algae dewatering. HDS uses customized fluid flow patterns in a curved channel to focus suspended particles into a concentrated band and achieves separation by placing a splitter near the channel exit. Two categories of algae based on size differences were dewatered with two versions of HDS devices. Spirulina, often used as a food supplement, has a cork-screw shape with 30-50?m diameter and 200-300?m length, separates well with an HDS “particle separator” device. Starting from a concentration of 0.8 g/l, we achieved a 6-fold increase to 4.6 g/l with over 97% harvesting efficiency, using an HDS device that has an 80:20 split ratio. An increase in concentration can be obtained with a second HDS connected in series to further reduce the total concentrate volume to less than 5%. When the size of the algae is smaller than the cutoff of separation for HDS (15?m), polymer is used to promote flocculation. S. dimorphus, an algae species that is small (<10?m), was separated by adding a flocculent (chitosan) to promote growth of larger aggregates. The resulting large flocs were separable with a low pressure (1.2psi) HDS “floc separator”, which also preserved the integrity of the loose aggregates. We also present a case where algae used for organic matters degradation in a wastewater pond was successfully dewatered and concentrated 10-fold in a single HDS separation. The focusing and separation of particles by HDS is primarily due to a combination of hydrodynamic forces (drag, virtual mass, and shear), which causes a size rather than density dependent separation. This enables concentration of particles with densities equal or close to that of their surrounding fluid without using significantly amount of energy, such as a centrifuge, or chemicals, lowering the cost of algae dewatering in biofuel production.
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