This paper presents the results of research on coagulation sludge properties in three waterworks. The investigation assumed: chemical composition, specific surface of coagulation structures, their sedimentation, gravity thickening and rheological parameters. The solid fraction of the examined structures consisted mostly of mineral substances, especially silica, aluminium and iron oxides. This composition is typical for most coagulation sludges formed in surface water treatment facilities. It has been determined, that the main factor determining structures properties is chemical composition of the solid fraction, mainly alum and iron oxides. Their presence in the sludge causes a high specific surface in coagulation structures, and significantly influences the rheological properties. It has been proved that non-Newtonian properties appear first in structures with the highest contents of alum and iron oxides, with limit concentrations of c. 1.5%. Investigations on sedimentation and gravitational thickening showed a high stability of coagulation structures vs. forces. The water amount in the investigated sludges during their compression was no lower than 94-98%, which means that gravitational forces did not destroy these structures. The factors stabilizing coagulation sludges are; fine dispersion of the solid phase and high affinity to water molecules. This is why in the range of hydrations lower than the compression hydration, the coagulation structures are classified as quasihomogenous systems consisting of two interpenetrating quasicontinuous media, which means; solution of liquid dispersed to colloidal and semicolloidal particles of the solid phase. They act on each other by intermolecular forces, which are the volume forces.

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