The influence of extracellular polymeric substances on the coagulation process of cyanobacteria

The hydrophobicity and flocculation properties of cyanobacterial cells are closely related to their extracellular polymeric substances (EPS). During the treatment of drinking water, the coagulation and removal of EPS-wrapped cyanobacterial particles from natural water sources is very difficult. In this work, a series of surface characteristics of cyanobacterial cells with different EPS fractions were analyzed to evaluate their influences on the coagulation process. With the removal of EPS, the coagulation efficiency of cyanobacteria was gradually improved. The intracellular microcystin release showed that the cyanobacterial cells in each EPS removal phase were almost intact with few broken cells. The surface of cyanobacterial cells had higher hydrophobicity and lower zeta potentials with each step of the EPS extraction, which improved the ratio of particles that were in an unstable state. Furthermore, the deeper the EPS extraction phase, the larger the decreased in size of cyanobacterial particles, thus increasing their specific surface area for adsorption with coagulant. It was concluded that the coagulation mechanism of EPS-wrapped cyanobacterial particles was: the cyanobacterial cells were first peeled off through attraction by opposite charges from the coagulant, and then they were adsorbed before settling down. This study provides a scientific basis for the removal of cyanobacteria by enhancing coagulation.