Consolidation properties of a dewatered municipal sewage sludge

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Abstract
The consolidation properties of a dewatered municipal sewage sludge were studied using the oedometer, hydraulic consolidation cell, and triaxial apparatus. Bioactive and stabilized test specimens of dried-compacted material and slurry material at different states of biodegradation (LOI = 55–70%, where LOI is the loss on ignition) were consolidated under applied stresses of 3–400 kPa. The rate of biogas production and the resulting pore pressure response of the unsaturated material were also studied for different specimen drainage conditions. The sludge material largely consisted of organic clay-sized particles, and although highly compressible, the material was practically impermeable (k = 10–9–10–11 m/s for slurry material, where k is the coefficient of permeability). Primary consolidation generally constituted only a small part of the overall deformation response for moderately degraded material (LOI [Formula: see text] 70%). Secondary compression was dominant, and for the bioactive material, included a significant contribution as a result of ongoing degradation of the organic solids. The contribution of primary consolidation to the overall deformation response increased for higher levels of treatment (LOI [Formula: see text] 55%) and the coefficient of secondary compression (Csec) values reduced; for example, Csec = 0.10–0.24 (σa = 35–150 kPa, where σa is the applied stress) for moderately degraded material and Csec [Formula: see text] 0.09 (σa = 300 kPa) for strongly degraded material. Biogas evolved from the material at a steady rate of up to 0.33 m3 gas/day/tonne for moderately degraded slurry, which caused the pore pressure to steadily increase to up to 40 kPa/day when the biogas remained trapped within the specimens.Key words: sewage sludge, consolidation, biodegrade, biogas.

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