Flocculation can be induced by several mechanisms (Fig. 12.4). A first mechanism is charge neutralization, or neutralization of the negative surface charge by adsorption of oppositely charged ions such as Mg+2 or Ca+2 to the cell surface. When the surface charge of the cell is neutralized, the electrostatic repulsion between the cells disap­pears. As a result, cells can approach each other and will connect through Van der Waals interactions resulting in the formation of flocs. In charge neutralization, the flocculant dose is a linear function of the surface area that needs to be neutralized. When dosage exceeds the optimal dose for charge neutralization, the surface charge

may shift from negative to positive, and the cell suspension may become stabilized again; a phenomenon that is known as dispersion restabilization. Flocs formed by charge neutralization are often small and unstable (Gregory 2006).

A second flocculation mechanism is bridging by polymers. A polymer carrying positive charges, for example, chitosan, may bind to the negatively charged cell surface with the polymer chain extending from the cell surface. If the polymer connects to another cell, bridges can be formed between cells and flocs are formed. Longer polymers and weakly charged polymers are often more efficient than shorter and highly charged polymers. When a high polymer dose is used, the cell sus­pension may become stabilized again. This dispersion restabilization can be due to an inversion of the surface charge from negative to positive, but it can also be due to steric hindrance caused by the polymer chains that are associated with the surface (steric stabilization). Flocs formed by bridging are often large and stable (Fellows and Doherty 2006; Bolto and Gregory 2007).

A third mechanism of flocculation is the electrostatic patch mechanism. Here, positively charged polymers connect to the negatively charged cell surface and form patches on the cell surface, where the surface charge is inversed from negative to positive. Positively charged patches on one cell can connect to a negatively charged patch on another cell and flocs can be formed. Short and highly charged polymers often cause flocculation by the electrostatic patch mechanism (Bolto and Gregory 2007).

The final mechanism reviewed is sweeping flocculation or flocculation by enmeshment. In this mechanism, the flocculant forms a precipitate that enmeshes the microalgal cells. The cells then will settle or flocculate together with the precipitate. The precipitate often works as a ballast that facilitates the settling of the cells (Yahi et al. 1994). In sweeping flocculation, the flocculant dose is often independent of the concentration of particles that is flocculated. Flocs formed by a sweeping mechanism are often more stable than flocs formed by charge neutralization.

It is often not straightforward to conclude which mechanism is responsible for flocculation. A polymer flocculant may induce flocculation through charge neu­tralization, bridging or electrostatic patch neutralization. A metal salt coagulant may induce flocculation through charge neutralization or a sweeping mechanism (Duan and Gregory 2003). In many studies on flocculation of microalgae, one or a combination of mechanisms is often proposed, but few studies so far have dem­onstrated unequivocally which mechanism is responsible for inducing flocculation.