Shear strength properties of water treatment residues

This paper presents the physical, standard Proctor compaction and shear strength properties of alum water treatment residues derived from the production of potable water at municipal works. In particular, the effects of catchment geology, chemical treatments at the municipal works, hardening phenomena and shearing rate on the constitutive and shear strength responses of these high-plasticity organic clays were studied. Slurry residues have low bulk and dry densities (0·96–1·13 and 0·21–0·36 t/m³ respectively) and low specific gravity of solids (1·83–1·99), and are highly compressible, although the consolidation rate is low. The dewatered residues have high values of effective angle of shearing resistance of 39–44°. Low concentrations of polyelectrolyte added to the residues altered the constitutive response (more elastic perfectly plastic, with shear failure occurring between 2% and 10% compressive strain): the undrained shear strength was enhanced by 10–20% and the effective angle of shearing resistance increased by 2°. Alum residues derived from peaty catchments were found to have marginally higher undrained shear strengths than those from a limestone-bedrock catchment. Recommendations are made regarding adequate dewatering of the slurry residue and the efficient landfill disposal of the pressed residue cake.