Fig. 7 Effect of micro-bubbles flotation on Microcystis sp. harvesting under different conditions(A) and density(B)
The results of microbubble air flotation treatment of Microcystiswith different density under the condition of condition 5 are shown in Fig. 7(B). It can be seen from the diagram that with the increase of Chla concentration from 85 mg/m3 to 844 mg/m3, the recovery rate of microbubble air flotation to Microcystis also showed an increasing trend. After flotation, the Chla concentration of density 1 decreased to 54 mg/m3, and the Chla concentration of density 5 decreased to 126 mg/m3. The lowest algae collection rate of microbubble air flotation for density 1 was 35.6%, while the algae collection rate for density 5 was as high as 63.5%. The analysis shows that under low concentration conditions, the number ofMicrocystis colonies is small, and the probability of adhesion by microbubbles is small; when the algae concentration increased to 844 mg/m3, the number of Microcystis colonies in the water increased, the probability of microbubbles colliding with them increased, and the adhesion probability also increased. Microbubble air flotation has a better harvesting effect on higher Microcystisdensity.
4.Conclusion
Microbubble air flotation is an effective way for microalgae harvesting. During the process of microbubble generation, the proportion of microbubbles with particle size less than 50 μm is optimal with the pressure of 0.55 MPa, the gas-liquid ratio of 5 % and the length of the release pipe of 10 cm, and the length of the release pipe is the main influencing factor, followed by the gas-liquid ratio. No matter a single microbubble or a combination of two microbubbles, they all undergo variable acceleration motion during the floating process. In the process of adhesion, when the size of Microcystis was 0.6~1.5 times of the size of microbubbles, microbubbles could adhere to 63% ofMicrocystis . When the size of Microcystis was equivalent to the size of microbubbles (ratio of 1.03 to 0.9), the adhesion probability of the two was the largest (both more than 50%). That can also explain that the best the harvesting rate of Microcystis (63.5%) was obtained under the working condition 5 (pressure 0.45 MPa, gas-liquid ratio 5% and length of the release pipe 100 cm) due to the similar size distribution of the microbubbles and the Microcystis in this study. The harvesting performance was also affected by theMicrocystis density and the water flow environment. More investigations would need to be undertaken for application in large scale with varied concentrations and other dissolved organic matters.
Acknowledgement
This work was financially supported by the Key Technologies Research and Development Program (2017YFE0135500); the National Natural Science Foundation of China (51908355); the Natural Science Foundation of Shanghai (19ZR1443700); and the Scientific and Innovative Action Plan of Shanghai (19DZ1204500).
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