Optimisation of coagulation conditions for the removal of cyanobacteria by dissolved air flotation or sedimentation

Analysis of surface water treatment practice in the Netherlands shows that cyanobacteria, especially Microcystis aeruginosa and Oscillatoria aghardii, are the main problem causing algae. Conventional (sedimentation) and more advanced (dissolved air flotation) treatment of algae-laden (model) water were studied. The agglomeration (coagulation/flocculation) phase was found to be determining the process efficiency and is therefore a prerequisite for process improvement. Both processes were assessed in terms of their removal efficiency of the cyanobacterium M. aeruginosa. Relevant parameters were addressed, including the influence of coagulant (FeCl3) dose, coagulation pH, flocculation time, energy input (G value), single stage vs. tapered flocculation, application of cationic polymer (C-573) as coagulant aid, and surface loading. The process efficiency was assessed in function of the agglomeration and the particle (floc) size distributions obtained. The results were evaluated based on existing theoretical and practical knowledge of process kinetics and modeliing.