NewConcepts in Sludge Dewatering

ReportNo WSAA 120

May 1997

SYNOPSIS

Theinter-relationship between the addition of chemicals in the clarification ofpotable water and the volume and ease of dewatering of the subsequent sludge ispoorly understood and indeed, poorly quantified. Current operating practiceshave been reviewed and the application of the most recent theories of thecompressional dewatering of sludges has been considered for potable water(nominally alum) sludges. This involved laboratory, pilot plant and full scaletests.

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

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

Thekey conclusions of the study relate to the role of the large doses of alumcharacteristically used in water treatment in controlling the rigidity of watertreatment sludges. The control renders polymeric additives used in secondaryconcentration processes such as thickening and centrifugation ineffective.Recommendations are made as to the optimisation of sludge handling proceduresand equipment.

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