ABSTRACT
The research investigated to improve (algae) coagulation and down-stream dissolved air flotation (DAF) removal, by particle (algae) conditioning. The application of organic and synthetic polyelectrolytes, of cationic, non-ionic and anionic charge nature was investigated. The polyelectrolytes were applied as sole coagulant or as coagulant aid with Fe(III) primary coagulant. Bench-scale experiments on model water (tap water spiked with laboratory cultured algae) confirm that cationic polyelectrolytes inducing charge neutralisation and the adsorption-coagulation mechanism are superior to anionic and non-ionic polyelectrolytes associated with the bridging mechanism. The application of oxidants was investigated for the same purpose. Ozone and ICMnO4 improved the particle (alga) removal. Ozone doses as low as 0.2 mg O3/mg C (0.48 mg O3/L) increased the DAF process efficiency. Similarly, 0.7 mg KMnO4/L resulted already in improved DAF process efficiency, however with lower reproducibility and the drawback of high Mn residual, mainly caused by the colloidal MnO2 with an accompanying turbidity increase. Chemical modifications of the alga cell layer, and the production of in-situ coagulant aid caused by extra-cellular organic matter (EOM) release and in some cases intra-cellular organic matter (I0M) leakage, are considered to be the major process mechanisms responsible for the efficiency improvement. Cationic polyelectrolytes had a positive impact on DAF process efficiency in a combination of ozone or ICMnO4 with inorganic coagulant, as well as in combination of ozone or KMnO4 with the polyelectrolyte as sole coagulant. The dual coagulant system lowered the overall coagulant demand and reduced the coagulant and Mn residual concentrations. The polyelectrolyte coagulant alone proved to be more sensitive to the ozone dose, but avoided any metal coagulant residue.
