COAGULATION AS A STRUCTURE-FORMING SEPARATION PROCESS IN WATER AND WASTE-WATER TREATMENT

This paper presents the results of research on coagulation sludge prop erties in three waterworks. The investigation assumed: chemical compos ition, specific surface of coagulation structures: their sedimentation , gravity thickening and rheological parameters. The solid fraction of the examined structures consisted mostly of mineral substances, espec ially silica, aluminium and iron oxides. This composition is typical f or most coagulation sludges formed in surface water treatment faciliti es. It has been determined, that the main factor determining structure s properties is chemical composition of the solid fraction, mainly alu m and iron oxides. Their presence in the sludge causes a high specific surface in coagulation structures, and significantly influences the r heological properties. It has been proved that non-Newtonian propertie s appear first in structures with the highest contents of alum and iro n oxides, with limit concentrations of c. 1.5%. Investigations on sedi mentation and gravitational thickening showed a high stability of coag ulation structures vs. forces. The water amount in the investigated sl udges during their compression was no lower than 94-98%, which means t hat gravitational forces did not destroy these structures. The factors stabilizing coagulation sludges are; fine dispersion of the solid pha se and high affinity to water molecules, This is why in the range of h ydrations lower than the compression hydration, the coagulation struct ures are classified as quasihomogenous systems consisting of two inter penetrating quasicontinuous media, which means; solution of liquid dis persed to colloidal and semicolloidal particles of the solid phase. Th ey act on each other by intermolecular forces, which are the volume fo rces. (C) 1997 IAWQ. Published by Elsevier Science Ltd.