Flocculation is based on the addition of materials (flocculants) or changing the medium in such a way the cells are attracted to each other and therefore more rapidly settle to the bottom of a holding tank. This is really the result of coagulation and then flocculation. Coagulants destabilize the charges and surface properties of the cells in suspension, so they do not resist agglomeration. Flocculants focus on stimulating the formation of larger aggregates from the destabilized algal cells.

Autoflocculation, chemical flocculation, bioflocculation, and electroflocculation are all examples of processes that rely on flocculation to aggregate and speed the removal of algal cells. At scale, all methods for flocculation require large amounts of space and are expensive due to costs of coagulants and/or flocculants and operators (Bosma et al. 2003) or other chemical additions (e. g., pH adjustment in autoflocculation).