The performance of microflotation, dispersed air flotation with microbubble clouds with bubble size about 50 µm, for algae separation using fluidic oscillation for microbubble generation is investigated. This fluidic oscillator converts continuous air supply into oscillatory flow with a regular frequency to generate bubbles of the scale of the exit pore. Bubble characterization results showed that average bubble size generated under oscillatory air flow state was 86 µm, approximately twice the size of the diffuser pore size of 38 µm. In contrast, continuous air flow at the same rate through the same diffusers yielded an average bubble size of 1,059 µm, 28 times larger than the pore size. Following microbubble generation, the separation of algal cells under fluidic oscillator generated microbubbles was investigated by varying metallic coagulant types, concentration and pH. Best performances were recorded at the highest coagulant dose (150 mg/L) applied under acidic conditions (pH 5). Amongst the three metallic coagulants studied, ferric chloride yielded the overall best result of 99.2% under the optimum conditions followed closely by ferric sulfate (98.1%) and aluminum sulfate with 95.2%. This compares well with conventional dissolved air flotation (DAF) benchmarks, but has a highly turbulent flow, whereas microflotation is laminar with several orders of magnitude lower energy density. Biotechnol. Bioeng. 2012; 109:1663–1673. © 2012 Wiley Periodicals, Inc.

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In this study, the effect of microfluidic microbubbles on overall gas-liquid mass transfer (CO 2 dissolution and O 2 removal) was investigated under five different flow rates. The effect of different liquid substrate on CO 2 mass transfer properties was also tested. The results showed that the K L a can be enhanced by either increasing the dosing flowrate or reducing the bubble size; however, increasing the flow rate to achieve a higher K L a would ultimately lower the CO 2 capture efficiency. In order to achieve both higher CO 2 mass transfer rate and capture efficiency, reducing bubble size (e.g. using microbubbles) has been proved more promising than increasing flow rate. Microbubble dosing with 5% CO 2 gas showed improved K L a by 30% -100% across different flow rates, compared to fine-bubble dosing. In the real algal culture medium, there appears to be two distinct stages in terms of K L a, divided by the pH of 8.4.

One of the most important stages in biofuel production from microalgae is the harvesting and dewatering of the microalgal biomass. The lack of cost-effective methodologies has been the major hurdle for the economic production of algal biofuel production. The variability in the properties of microalgal species such as cell size, robustness, surface charge, culture medium constituent [1] and desired end product [2] pose challenges to generic engineering implementation across the range of biomass feedstock and biorefinery processing. These properties influence algal recovery processes and therefore play a key role in the selection of suitable recovery methods. Furthermore, given the typical dilute nature of algal media (although more concentrated than freshwater supplies processed by dissolved air flotation), recycling of medium after cell recovery must be considered to avoid disposal challenges and ultimately to save costs.In order to overcome these challenges, highly robust, effective and energy-efficient harvesting techniques must be sought to meet the high volumetric throughput of large-scale production. A number of recovery methods exist and have been investigated for various microalgal species. Filtration, for instance, is a relatively cost-effective technique suitable for large filamentous algae such as Spirulina [2], but the shape and size (2-50 µm) [1] of lipid-rich unicellular microalgae such as Dunaliella salina or Chlorella make effective filtration difficult [2]. Similarly, sedimentation is a very slow recovery technique and often requires significant space for settling tanks.Unlike filtration and sedimentation, the use of centrifugal force for algal separation from culture medium is a rapid and relatively more effective recovery technique. In spite of the high recovery with this approach, the major disadvantage of centrifugation is the high capital and operational costs associated with sophisticated machinery and their high energetic consumption [1,3]. In addition, the high shear and gravitational force resulting from centrifugation could be detrimental to fragile microalgal cells, affecting the quality of the end products. Background:Harvesting algal biomass is an important unit operation in the production of biofuel from algae. However, many of the known techniques available for harvesting and dewatering microalgal biomass are energy intensive and in some cases intrusive and inefficient. Here we show that microflotation mediated by fluidic oscillation is an approach that differs from dissolved air flotation by the nonintrusive laminar flow approach, as well as low energy consumptions, and is also different from dispersed air flotation by the method of bubble generation. Results and discussion: Using three metallic coagulant types, recovery efficiencies of 99.2, 98.1 and 95.2% were obtained for ferric chloride, ferric sulfate and aluminum sulfate, respectively. The benchmarks in the literature for dissolved air flotation are 40-98%. Conclusion: Biofuel production from microalgae can be facilitated ...

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