o IWA Publishing 2004 Journal of Water Supply: Research and Technology—AQUA | S3.1 | 2004 Fundamental dewateríng characteristics of potable water treatment sludges Peter J. Harbour, Nevil J. Anderson, Ainul A. A. Aziz, David R. Dixon, Peter Hiilis, Peter J. Scales, Anthony D. Stici<iancl and Martin Tillotson ABSTRACT Capillary suction time (CST) and specific resistance to filtration (SRF) measurements are commonly used in the characterisation of sludges and for the prediction of efficient operation of dewatering processes such as centrifugation and filtration. Whilst occasionally useful in predicting trends, they do not assist in the design and optimisation of dewatering devices from first principles. Recent work in our laboratories has led to the development of a technique for the fast measurement of the permeability and compressibility of sludge. The use of a single volume-fraction dependent parameter, namely the solids diffusivity, D((p), calculated from permeability and compressibility, enables full characterisation of the dewaterability of sludge. This allows different sludges to be compared in an unequivocal fashion. Data is presented for a range of sludges from different sources showing vastly different dewatering properties. The dewaterability of the different sludges is easily compared and the true role of flocculants in dewatering is highlighted. Key words | dewaterability, potable water treatment sludges, pressure filtration INTRODUCTION Until now, characterisation of water treatment plant (WTP) sludges has commonly been performed using tech- niques such as capillary suction time (CST) and specific resistance to filtration (SRF). It is largely accepted that CST is dependent on a number of non-fundamental parameters (Vesilind 1988; Dentel 1997; Novak et al. 1999), which render it unsuitable for the purposes of comparing different sludge types or sludges from different plants (Vesilind 1988). This technique is also highly dependent on initial solids concentration. Efforts to avoid these prob- lems, by taking measurements at different sludge solids or by calculating 'filterability', result in characterisations based on empirical or semi-empirical factors which cannot be universally applied. SRF may give useful infor- mation regarding the permeability of non-compressible Peter J. Harbour (corresponding author) Nevii J. Anderson CSIRO Molecular Science, Private Bag 10, Clayton South MDC, Victoria, Austraiia 3169 E-mail: peter.harbour®csiro.au AInul A, A. Aziz David R, DIxon Peter J. Scales Anthony D, stickland Particulate Fluids Processing Centre, Department of Chemical and Biomoiecular Engineering, University of Melbourne, victoria, Australia 3010 Peter Hlllls United utilities, Lingley Mere, Lingley Green Avenue, Warrington WAS 3LP, UK Martin Tillotson Yorkshire Water, Halifax Road, Bradford BD6 21^, UK sludges but with compressible materials such as water treatment sludges the measurements are highly dependent on the applied pressure and the initial solids fraction. To fully characterise such materials requires testing at a range of pressures up to the required operating pressure cover- ing a range of initial volume fractions. The standard method for SRF determination uses a vacuum to establish the pressure gradient and the accuracy of the measure- ment is limited by difficulties in maintaining a constant pressure differential throughout the entire filtration. Neither of these techniques gives information regarding the extent of filtration or the maximum solids achievable (Novak et al. 1999). Whilst it is noted that, for determining the effects of flocculants on a specific sludge, both these methods may