DESORPTION OF ORGANIC MACROMOLECULES FROM ACTIVATED-SLUDGE - EFFECT OF IONIC COMPOSITION

Structural properties of activated sludge flocs were found to be sensi tive 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. Di lution of the activated sludge with deionized water resulted in a rele ase of calcium and organics to bulk water as well. In both types of ex periments 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 calciu m desorption were possibly more important for the observed flee disint egration than were the effects of changes in the ionic strength. The k inetics of dissolution of calcium were shown to be controlled by Ficki an diffusion. A model of a typical activated sludge floc. taking into account the colloidal stability, is suggested, where a ''cloud'' of or ganic macromolecules and single bacteria are attached with very weak F orces to a more rigid backbone consisting of fibers, filamentous bacte ria and bacterial colonies. The observed changes in stability of the f locs leading to increased turbidity have major practical implications in the solid-liquid separation, and effects on effluent quality and de watering are discussed. (C) 1997 Elsevier Science Ltd.