Algae laden water treatment by dissolved air flotation (DAF)

Based on previous bench scale experimental research with model water (tap or reservoir water spiked with laboratory cultured Microcystis aeruginosa), dissolved air flotation (DAF)-filtration pilot plant research was conducted using eutrophied reservoir water with a naturally occurring bloom of the same algae. Two basic aspects were addressed : DAF optimisation for the removal of the algae and comparison of the DAF-filtration process scheme with a full scale sedimentation-filtration process scheme. Tested and optimised process parameters include coagulant dose, coagulation pH, flocculation time and energy input, flocculation mode (one, two and three step flocculation), the application of organic and synthetic polymers as coagulant aids, DAF recirculation ratio, etc. The application of ozone and KMnO4 was tested in the context of algae pre-conditioning and improved coagulation/flocculation and down-stream DAF-filtration. In addition to commonly applied analytical techniques e.g. turbidity, residual coagulant, dissolved organic carbon (DOG), electrophoretic mobility (EPM), etc., assessment of the process efficiency and kinetics was based on particle count and computer image analysis. Most important from the point of view of ground water infiltration - the final use of the treated water, the MFI (Membrane Filtration Index) was measured and elaborated. Results indicate the possibility of producing water of the desired quality, most problematic remaining the achievement of an appropriate MFI value (<5 s/l(2)). Suggestions are given and elaborated for the appropriate approach to achieve it, including further optimisation of the agglomeration (coagulation/flocculation) process (combinations of higher coagulant doses and cationic polymer coagulant aids), application of pre-conditioner oxidants like ozone or KMnO, and filtration optimisation.