At the high-rate anaerobic treatment of domestic sewage, both biological and physical processes play an important role. Therefore, the anaerobic biodegradability of raw, paper-filtered and membrane-filtered sewage and black water has been investigated in batch experiments. Additionally, the effect of anaerobic digestion on physical characteristics, like particle size, surface tension and zetapotential, of the present particles is studied. The biodegradability of domestic sewage and black water at 30 degreesC is almost similar (71-74%). Moreover, a high methanogenesis of the colloidal fraction in domestic sewage (86 +/- 3%) is achieved, showing that the low removal of colloidal particles in continuous high-rate anaerobic reactors is due to low physical removal rather than biodegradability. The lowest biodegradability is demonstrated for the disserved fraction (62%). The results show that after anaerobic digestion the average radius of particles with diameter <4.4 and <0.45 mum increased for domestic sewage, while it decreased for black water. Part of the surface-active components in domestic sewage is not biodegraded during anaerobic batch digestion, as indicated by the development of the surface tension. The negative seta-potential of all particles hardly changes during digestion, showing that colloidal interactions were not affected by anaerobic digestion. (C) 2001 Elsevier Science Ltd. All rights reserved.