A systematic study was undertaken of the granular composition and hydraulic properties of municipal solid waste (MSW) produced by mechanical–biological pretreatment (MBP–MSW) from three different treatment plants with the aim of evaluating the potential application of MBP–MSW as an alternative barrier material for landfill final cover systems. Despite its coarse granular composition, MBP–MSW has low hydraulic conductivity. Long-term permeability tests show that the hydraulic conductivity decreases with time. The most likely explanation for the long-term changes in permeability is the swelling of organic material contained within the compost. In the case of saturated flow, the virtually impermeable plastic fragments embedded in the material impede fluid flow. In the unsaturated case, such fragments slow down the drying process by disrupting fluid flow and allowing pooling of water above horizontally oriented fragments. The larger the number and size of the plastic fragments, the greater the influence on hydraulic conductivity and shrinkage. These processes can be better understood with the newly developed conceptual model, the thin-sheet model. Based on this conceptual model, laboratory tests were undertaken to compare natural soil material with mixtures of soil material and plastic fragments. Corresponding numerical simulations of some experiments verified the influence of plastic fragments on the hydraulic properties of MBP–MSW.Key words: mechanical–biological pretreatment, municipal solid waste (MSW), thin-sheet model, plastic fragment, hydraulic conductivity, drying test.