The Adsorption Behaviour of Cationic Polyelectrolytes in Dissolved Air Flotation

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Abstract
This study examines the adsorption behaviour of polymers of various molecular weights (MW) in dissolved air flotation (DAF) systems. Batch and full scale DAF tests were done using activated sludge. Residual polymer was measured by a titri-metric method, and Zeta potential (ZP) measurements of the effluent were also made. Two highly charged cationic polyelectrolytes of different MW were used. In batch tests, the two polymers yielded similar performance in terms of effluent and float solids concentrations, but slightly less high MW polymer was needed to attain low effluent SS. In both cases, polymer residual was detectable only when polymer doses beyond that needed for lowest effluent SS were applied. Significant increases in float solids concentration also occurred only at the higher doses. Polymer adsorption followed the Langmuir isotherm behaviour. Higher MW polymer adsorbed in larger quantity, indicating that the polymer “monolayer” was not flat on the surface of the sludge particles. ZP reversal (to a positive value) occurred only when polymer residual was detected, not at the polymer dose required for lowest effluent SS. Results were confirmed with full scale tests. A model for polymer-induced flocculation is presented, suggesting that polymer attachment occurs by attraction of opposite charges, but that bridging of colloids occurs to cause incorporation in the float. It is indicated that the cationic condition in the float at excess polymer dose causes exclusion of water in favour of air bubbles, and hence a more concentrated float.

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