New Concepts in Sludge Dewatering

This report was produced for the Urban Water Research Association of Australia, a now discontinued research program.

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Report no. UWRAA 120

May 1997

Synopsis

The inter-relationship between the addition of chemicals in the clarification of potable water and the volume and ease of dewatering of the subsequent sludge is poorly understood and indeed, poorly quantified. Current operating practice shave been reviewed and the application of the most recent theories of the compressional dewatering of sludges has been considered for potable water(nominally alum) sludges. This involved laboratory, pilot plant and full scale tests.

Quantitative laboratory characterisation methods for the compressibility (extent of dewatering) of sludges have been available for a number of years along with other methods for characterising the permeability of sludges (rate of dewatering). Sludges such as those typical of alum flocculated potable water sludges are usually of low volume fraction solids and even after clarification and centrifugation, the increase in solids of these sludges is not extensive, despite a residual cake that has no apparent free water. It is the inter-relationship between the solids content of the cake, the energy required to achieve this condition with typical dewatering equipment and the type and quantity of chemicals present that was of interest to this study.

The compressibility of real and model alum coagulated sludges was examined using a laboratory based centrifugation technique in the presence of a range of polymeric additives to gauge the absolute dewaterability of the sludges. This proved useful in scaling the effectiveness of equipment currently utilised in the industry. The role of alum in controlling the dewatering process was examined in detail. The role of dispersant style chemical additives on reducing the effect of the alum was also examined. Tests were performed at a laboratory, pilot and full scale.

The key conclusions of the study relate to the role of the large doses of alum characteristically used in water treatment in controlling the rigidity of water treatment sludges. The control renders polymeric additives used in secondary concentration processes such as thickening and centrifugation ineffective. Recommendations are made as to the optimisation of sludge handling procedures and equipment.

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