ABSTRACT
Based on previous bench-scale dissolved air flotation (DAF) studies with model water (tap or reservoir water spiked with laboratory cultured Microcystis aeruginosa), DAF + (post) filtration pilot plant research was conducted using eutrophied reservoir water with a naturally occurring bloom of the same alga. The DAF removal of the algae was optimised, and the DAF + filtration process was compared with a full-scale sedimentation + filtration process scheme. Process variables included coagulant dose, coagulation pH, flocculation time and energy input, flocculation mode (one-, two- and three-stage flocculation), the application of organic and synthetic polyelectrolytes as coagulant aids, DAF recirculation ratio, etc. The application of ozone and KMnO, was tested in the context of algae conditioning and improved coagulation/flocculation and down-stream DAF + filtration effectiveness. The assessment of the process efficiency and kinetics was based on particle count and computer image analysis, in addition to commonly applied analytical techniques such as turbidity, residual coagulant, dissolved organic carbon (DOC), and electrophoretic mobility (EM). Similarly, the MFI (Modified Fouling Index) was measured. Results indicate the possibility of producing water of high quality after filtration, though the achievement of an appropriate MFI value (<5 s/L2) remains problematic. Suggestions are given for the appropriate approach to achieve it, including further optimisation of the agglomeration (coagulation/flocculation) process, application of conditioning oxidants like ozone or KMn04, and (post)filtration optimisation.
