Geotechnical properties of a municipal water treatment sludge incorporating a coagulant

Abstract

The geotechnical properties of a municipal water treatment sludge from an upland catchment are presented. The gelatinous sludge comprised flocs of mainly quartz, manganoan calcite, and clay-sized organic solids, and incorporated an alum coagulant and an anionic polyelectrolyte. Standard Proctor compaction yielded low bulk density values of 0.95–1.10 t/m3and dry density values of 0.12–0.36 t/m3(water content is 160%–780%) in line with the low specific gravity of solids value of 1.86. The undrained shear strength and the water content were inversely related on a semi-log plot. The effective stress shear strength parameter values were c' = 0 and ϕ' = 39°. The consolidation properties were studied using the oedometer, consolidometer, and triaxial apparatus. The material was highly compressible with primary compression index (Cc) values of 2.5–3.7, and primary compression ratio (C*c) values of 0.20–0.28. The majority of the strain response occurred due to primary consolidation although the material had a very low permeability (coefficient of permeability values decreasing from 2 × 10−9to 5 × 10−11m/s for an effective vertical stress of σ'v= 3–800 kPa). Secondary compression was minor, with a mean secondary compression index (Cαe) value of 0.15, and Cαe/Cc= 0.04–0.06.