Harvesting freshwater microalgae with natural polymer flocculants

Abstract

This study revealed the differences in harvesting performance and potential mechanisms of four natural flocculants - Chitosan, Tanfloc, Cationic starch, Moringa oleifera - on microalgae cells and explored the potential of four flocculants for harvesting microalgae. Influence of dosage, culture pH, cell density and algal organic matter (AOM) on harvesting using four flocculants were compared. The effects of the flocculants on cell viability and the reusability of separated medium were also investigated. The results demonstrated that the optimal dosages of Chitosan/Tanfloc/Cationic starch for harvesting Chlorella vulgaris and Scenedesmus obliquus were much lower than Moringa oleifera. Chitosan/Tanfloc/Cationic starch and Moringa oleifera improved microalgae harvesting through electrostatic binding and bridging, respectively. The harvesting of other flocculants were affected by culture pH except for Moringa oleifera. Nevertheless, as biomass concentration increased from ~0.3 g L−1 to ~1.5 g L−1, the optimal dosage of Moringa oleifera increased significantly, which was about 20–100 times than other flocculants. AOM could interfere all harvesting processes to varying degrees. Moreover, the flocs formed from Chitosan/Tanfloc/Cationic starch were smaller but more compacted compared to Moringa oleifera, thus possessing a better dewatering functionality. The harvesting process of four flocculants did not affect cell viability or lead to the loss of cell extractions including lipid, carbohydrates and protein. In addition to Chitosan, the medium separated from the other flocculants after harvesting could be recycled for next cultivation, reducing costs of microalgae cultivation. Amongst four natural flocculants, Tanfloc displayed >98% high harvesting efficiency with low dosages (30 mg L−1 for Chlorella vulgaris and 20 mg L−1 for Scenedesmus obliquus). It could effectively harvest microalgae in a wide pH range (pH 4.0–9.0) and showed a good dewatering potential. More importantly, the lowest harvesting cost of the four flocculants facilitates its application for large-scale harvesting. We therefore recommend Tanfloc for microalgae harvesting of the four flocculants.