PosiDAF: Simplifying flotation for algal cell separation

Microalgae separation has to be undertaken in drinking water treatment plants, advanced wastewater treatment plants and in algae harvesting, where algae might be used to produce biofuel or nutraceuticals. Potential separation processes include centrifugation, membrane filtration, electrophoresis and clarification by either sedimentation or dissolved air flotation (DAF) with coagulation-flocculation as a pretreatment. In water treatment, coagulation-flocculation-flotation is a preferred method as it takes advantage of the natural buoyancy of low density algae; however, rapid and dramatic variations in algal populations mean that preceding coagulation-flocculation can be difficult to optimise, leading to ineffective flotation. DAF is also of interest for algal harvesting for the same reasons, but in this instance, the issue is associated with reducing the cost of separation, as this can reach 20-30% of the total cost of biomass production. The PosiDAF process has been developed to improve process robustness and cost effectiveness. In this novel DAF adaptation, the pretreatment stage has been replaced by polymer dosing direct to the saturator to produce positivelycharged bubbles that will attract and attach to the negatively-charged cells: novel polymers are being developed to enhance these interactions. At pilot scale, it was shown that separation effectiveness was comparable to that of optimised conventional DAF with the added benefits of a reduced footprint, reduced chemical demand (leading to lower chemical concentration in the algal-rich float layer) and an increased solid:liquid ratio of the resultant float which would in turn mean less intensive post-treatment was required, for example prior to conversion to biofuel