Comparing sedimentation and dissolved air flotation for microalgae harvesting: Influence of ionic strength on floc size and separation efficiency
Tyagi, S.; Rao, N.R.H.; Thielemans, W.; Muylaert, K. (2025). Comparing sedimentation and dissolved air flotation for microalgae harvesting: Influence of ionic strength on floc size and separation efficiency. J. Appl. Phycol. 37(1): 221-231. https://dx.doi.org/10.1007/s10811-024-03385-w
In: Journal of Applied Phycology. Springer: Dordrecht. ISSN 0921-8971; e-ISSN 1573-5176, more
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| Keywords |
Flotation
Harvesting
Separation > Chemical precipitation > Flocculation
Marine/Coastal |
| Author keywords |
Marine microalgae · Nanocellulose · Floc visualisation |
| Authors | | Top |
- Tyagi, S.
- Rao, N.R.H.
- Thielemans, W., more
- Muylaert, K., more
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| Abstract |
Flocculation followed by gravity sedimentation is a low-cost method for microalgae harvesting. Cationic cellulose nanocrystals (CNCs) are novel flocculants composed of cellulose, a bio-based and abundantly available material. While CNCs yield good separation efficiencies for freshwater microalgae, they are less effective for marine microalgae because of the formation of small flocs with a poor settling rate. In this study, we evaluate whether dissolved air flotation (DAF) is a more effective separation method than gravity sedimentation for marine microalgae. The freshwater microalga Chlorella vulgaris formed bigger flocs (390 µm) than the marine microalga Nannochloropsis oculata (60 µm) and the smaller floc size resulted in a slower settling rate of the flocs and a lower maximum separation efficiency for N. oculata (71 %) compared to C. vulgaris (99 %). Transfer of C. vulgaris to a marine medium and N. oculata to a freshwater medium resulted in a decrease and increase in floc size respectively, as well as an increase and decrease in separation efficiency respectively. This demonstrated that differences in floc size between the marine and freshwater species are due to the differences in ionic strength of the culture medium and not the inherent properties of the two model species. When DAF was used to separate the flocs from the culture medium, the separation efficiency of C. vulgaris remained as high as when sedimentation was used, while the separation efficiency for N. oculata increased from 71 to 90 %. Simultaneously, the minimum dose of cationic CNCs required to induce flocculation in N. oculata decreased from 16 mg L-1 to 3.2 mg L-1. These results suggest that, for harvesting marine microalgae, flocculation is better combined with DAF rather than with gravity sedimentation. The better performance of DAF compared to sedimentation is ascribed to the fact that DAF is highly efficient in removing small flocs from suspension. |
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