| Microalgae has found wide applications for food, feed-stock, pharmeutical, chemical because of its high photosynthesis efficiency, fast growth and rich of nutrients. Especially, microalgae biomass has been taken to be one of most potential resource for biofuel. Whatever the microalgae is to be used, how to harvest the microalgae cells economically and efficiently from dilute culture medium is one of bottlenecks of its industrial applications. Obviously conventional harvesting methods such as sedimentation, filtration and centrifuge, etc could not satisfy industrial requirement with continuity, low cost in large scale. Dissolved air flotation (DAF), originally developed from ore flotation in metallurgy, is thought to be a high efficient and low cost solid-liquid separation technology, and has been widely used in waste water treatment. In this work, dissolved air flotation was induced for microalgae Chlorella sp. harvesting. A dissolved air flotation system in lab has been setup and DAF behavior of Chlorella sp. were investigated. The results have shown that:(1) The increase of pressure would increase the amount of gas dissolved in water greatly. Under 0.6MPa, the calculated dissolved air is about 108ml/L, and the released bubble size is estimated about 58μm, which could provide good flotation conditions.(2) The formation of cell-flocs is the precondition for dissolved air flotation. polyaluminium chloride has been found to be a good flocculant to form flocs of Chlorella sp. cells. And polyacry lamide could further improve flocculation by polyaluminium chloride. The optimum flocculation conditions are polyaluminium chloride dosage 160mg/L, polyacry lamide dosage 6mg/L and reaction time 8 minutes, at which,92.43% cells will be flocculated.(3) Fractal dimension was induced to describe morphology of Chlorella sp. flocs. The floc+culation rate was related with fractal dimension of flocs. Small fractal dimension would have higher flocculation rate, but too small fractal dimension flocs means over-loose and fragile structure of floes, causing lower flocculation rate. A suitable range of fractal dimension for good flocculation is 1.37-1.54, corresponding 400-600μm in average size of flocs.(4) Recovery of dissolved air floating of Chlorella sp. flocs by Polyaluminium chloride is increased with the increase of polyaluminium chloride dosage, from 49% to 98%, with concentrated time from 17 to 9.8. Polyacry lamide could enhance the recovery rate of flocs by polyaluminium chloride, but over dosed polyacry lamide would cause over-tight of flocs, difficult for flotation. The increase of the amount of dissolved air water to culture medium helps the recovery rate of flocs. To small or too big flowrate of dissolved air water would decrease the recovery of flocs. The optimized DAF conditions-is:the dosage of PAC is 160mg/L; the dosage of PAM is 6mg/L; the flocculation reaction time is 8min; the dissolved gas-water flow rate is 75L/h; the water/algae liquid volume ratio is 20%.(5) pH adjusting with sodium hydroxide could also produce flocs of Chlorella sp. cells and then realize flotation. The operation factors such as coagulant doses, flow rate of air dissolved water and water-feed volume ratio is also investigated. The optimal conditions are determined as exponential phase 350mg/L, stable phase 600mg/L,dissolved air water flow rate 60L/h, and the ratio between a 25%. For either growth period, the recovery efficiency is not significantly various by sedimentation under the same conditions; however the recovery rate of exponential phase is higher than that of stable phase by DAF.
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