Up to date, sludge filterability has been characterized by the Ruth's classical filtration theory and quantified by the well-known parameter specific cake resistance (SCR). However, the complexity of the actual phenomenon is clearly underestimated by the classical filtration theory and SCR is often not satisfactory in describing filterability. Although many scientific studies were conducted for a better analysis and understanding of the filtration theory, still a practically applicable solution to replace the classical theory for a better description of filterability has not been proposed yet. In the present study, filtration laws proposed by Hermans and Bredee, dating back to 1936, which have been extensively used in the membrane literature for the analysis of fouling phenomenon has been adopted for the analysis of filterability of sludge systems. The results clearly show that this approach is superior to the classical approach in terms of characterizing the filterability of sludge systems and provides a slurry specific characterization parameter to replace the commonly used SCR. (c) 2007 Elsevier B.V. All rights reserved.