Due to increasing demands for microalgal biomass and products originating from microalgae, large-scale production systems are necessary. However, current microalgal production technologies are not cost-effective and are hindered by various bottlenecks, one of which is the harvesting of microalgal biomass. Cell separation is difficult because of the low sedimentation velocity of microalgae, their colloidal character with repelling negative surface charges, and low biomass concentrations in culture broths; therefore, large volumes need to be processed in order to concentrate the cells. Flocculation is considered to be one of the most suitable methods for harvesting microalgal biomass. This article provides an overview of flocculation methods suitable for microalgal harvesting, their mechanisms, advantages and drawbacks. Special attention is paid to the role of surface charge in the mechanism of flocculation. The novelty of the review lies in the interconnection between the context of technological applications and physico-chemical surface phenomena.

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