A simple yet relatively complete model and numerical solution method for describing the effects of osmotic pressure on the compressibility of materials bearing surface charges, and its dependence on bulk solution properties is presented. The basis of this thermodynamic model is Donnan equilibrium and surface charge regulation. The model is capable of explaining effects that were observed in [1] with regard to the effect of salt addition on wastewater treatment sludge compressibility. The simulation results suggest that the filtrate conductivity changes with the level of compression, which was confirmed experimentally. Furthermore, the simulation results suggest a dependence of compressibility on the dewatering method, i.e. on the fate of the expelled water. In case the solution that is expelled from the compressed gel stays in contact with the gel (e.g. centrifugation), the compressibility properties can be different from the properties in case the expelled water is completely removed (e.g. filtration). (C) 2010 Elsevier B.V. All rights reserved.