The coagulation/flocculation process is important for particle separation in water treatment. However, difficulties arise when coagulation is not optimised for the dominant particle. This paper investigates the surface characteristics and floc properties of three common systems– natural organic matter (NOM), algae and clay – in order to aid understanding of the coagulation/flocculation process. It was demonstrated that charge density and specific surface area are important parameters with respect to coagulant demand for charge neutralisation for all systems. However, extracellular organic matter (EOM) affected the coagulant demand of algae to the extent that it appears that the presence of EOM could dominate the coagulation process. Controlling the zeta potential of the systems prompted improved particle aggregation and hence removal efficiency in all cases. Floc growth profiles revealed that algal flocs required five times the flocculation period to reach a steady-state floc size compared to NOM and clay and on exposure to increased shear were much weaker. Despite similarities between algae and NOM in terms of organic content and coagulant demand, the fact that algae is a dynamic, biological system as opposed to an inert system creates numerous problems for the coagulation/flocculation process.

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