OPTIMIZATION 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 (sedimenta tion) and more advanced (dissolved air flotation) treatment of algae-l aden (model) water were studied. The agglomeration (coagulation/floccu lation) phase was found to be determining the process efficiency and i s therefore a prerequisite for process improvement. Both processes wer e assessed in terms of their removal efficiency of the cyanobacterium M. aeruginosa. Relevant parameters were addressed, including the influ ence of coagulant (FeCl3) dose, coagulation pH, flocculation time, ene rgy input (G value), single stage vs. tapered flocculation, applicatio n of cationic polymer (C-573) as coagulant aid, and surface loading. T he process efficiency was assessed in function of the agglomeration an d the particle (floc) size distributions obtained. The results were ev aluated based on existing theoretical and practical knowledge of proce ss kinetics and modeliing.