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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 In this novel DAF adaptation, the pretreatment stage has been replaced by polymer dosing direct to the saturator to produce positivelycharged bubbles that will attract and attach to the negatively-charged cells: novel polymers are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

The role of algal organic matter in the separation of algae and cyanobacteria using the novel “Posi” - Dissolved air flotation process

Algae and cyanobacteria frequently require separation from liquid media in both water treatment and algae culturing for biotechnology applications. The effectiveness of cell separation using a novel dissolved air flotation process that incorporates positively charged bubbles (PosiDAF) has recently been of interest but has been shown to be dependent on the algae or cyanobacteria species tested. Previously, it was hypothesised that algal organic matter (AOM) could be impacting the separation efficiency. Hence, this study investigates the influence of AOM on cell separation using PosiDAF, in which bubbles are modified using a commercially available cationic polyelectrolyte poly(N, N-diallyl-N,N-dimethylammonium chloride) (PDADMAC). The separation of Chlorella vulgaris CS-42/7, Mychonastes homosphaera CS-556/01 and two strains of Microcystis aeruginosa (CS-564/01 and CS-555/1), all of which have similar cell morphology but different AOM character, was investigated. By testing the cell separation in the presence and absence of AOM, it was determined that AOM enhanced cell separation for all the strains but to different extents depending on the quantity and composition of carbohydrates and proteins in the AOM. By extracting AOM from the strain for which optimal separation was observed and adding it to the others, cell separation improved from 90%. This was attributed to elevated levels of acidic carbohydrates as well as glycoprotein-carbohydrate conjugations, which in turn were related to the nature and quantity of proteins and carbohydrates present in the AOM. Therefore, it was concluded that process optimisation requires an in-depth understanding of the AOM and its components. If culturing algae for biotechnology applications, this indicates that strain selection is not only important with respect to high value product content, but also for cell separation