The ability of supernatant obtained after sludge disintegration treatment to bind Cd or Pb as well as the composition of the supernatant were studied. Different processes of sludge reduction such as sonication, heating at 95 degrees C or 120 degrees C at lab-scale, and predation by fungi in situ were investigated. Organic and to a lesser extent mineral sludge supernatant composition showed significant differences after the disintegration processes and to a lesser extent after predation by fungi. The use of size exclusion chromatography confirmed a significant difference in molecular size distribution in the supernatant of treated and untreated sludge. The changes in supernatant composition after treatment led to a significant increase in its ability to bind Cd and Pb (by increasing the number of fixation sites). Pb displayed a higher affinity for sludge supernatant than Cd. (C) 2011 Elsevier B.V. All rights reserved.