Compressibility and consolidation of water treatment residues

Abstract

The index, compression and consolidation properties are presented for alum water treatment residues (WTRs) derived from three different catchments and dewatered using different methods. Oedometer, consolidometer and triaxial consolidation tests were carried out on specimens of different size and consistency over a wide range of applied stresses (3–800 kPa) and under different drainage conditions. The solids fraction mainly comprised organic matter and micro-organisms (loss-in-dry-mass on ignition of between 40 and 60%) and hence the residues had a low specific gravity of solids of 1·83–1·99 and a low dry density of 0·14–0·26 t/m3 measured over the mass water content range of 300–700%. The wet residue was highly compressible (primary compression index of 2·1–3·1) and the lagooned or landfilled material will settle significantly, but over a long period of time, due to its low consolidation and high creep rates (coefficient of consolidation of 0·1–0·8 m2/year and secondary compression index of 0·005–0·010). The WTRs had a very low hydraulic conductivity (coefficient of permeability of 10−9–10−11 m/s) due to the microstructure of the constituent flocs, the high organic content and alum's exceptionally high affinity for water. Although the non-chemically coagulated residue was found to be slightly more compressible, the geoengineering properties were generally independent of seasonal variations in the source waters and the catchment geology.