Algal cells and coagulation, flocculation and sedimentation processes

Flocculation generally removes two classes of suspended particles during adsorption-coagulation, namely colloids (inorganic in nature) and bacterial and algal cells, colonies, and filaments (organic in nature). Different interaction mechanisms, i.e. hydrodynamic, electrical and electromagnetic, play important roles in the removal of algal and colloidal entities. Algal entities, however, show morphological characteristics (such as elongated shapes, arranged in cells, colonies and filaments, containing spines or able to change shape or to move with flagella) not shared by colloids, that will affect the flocculation of the algal cells. If algal cells are globally in equilibrium with themselves, it is possible that the negative surface charge of algal cells will be restored after charge neutralisation. In addition to Van der Waals forces, affecting the coagulation of colloidal and algal entities, the flocculation of algal entities may also be affected by gravitation forces (because of larger sized algal entities) and forces created in the immediate vicinity of the cells by metabolic processes such as photosynthesis and respiration.