Desorption of organic macromolecules from activated sludge: Effect of ionic composition

Structural properties of activated sludge flocs were found to be sensitive to small changes in ionic strength and ionic composition. Removal of calcium from the bulk water by an ion exchange resin resulted in a release of calcium ions and organic macromolecules from the flocs. Dilution of the activated sludge with deionized water resulted in a release of calcium and organics to bulk water as well. In both types of experiments the small particles in the bulk water showed an increase in the negative surface charge density, as measured by zeta-potential or by colloidal titration. A change in the repulsive forces due to calcium desorption were possibly more important for the observed floc disintegration than were the effects of changes in the ionic strength. The kinetics of dissolution of calcium were shown to be controlled by Fickian diffusion. A model of a typical activated sludge floc, taking into account the colloidal stability, is suggested, where a `cloud' of organic macromolecules and single bacteria are attached with very weak forces to a more rigid backbone consisting of fibers, filamentous bacteria and bacterial colonies. The observed changes in stability of the flocs leading to increased turbidity have major practical implications in the solid-liquid separation, and effects on effluent quality and dewatering are discussed.