When belt filters or sludge beds are dimensioned, the chosen volumetric load and medium resistance must be considered carefully to ensure fast dewatering and high final dry matter content. This can be done if the settling rate, cake compressibility, and specific cake resistance are determined and the process modeled. We present a numerical model of the gravity drainage of biological sludge. As sludge forms highly compressible cakes, cake compressibility is included in the model. Model simulations indicate that cake compression greatly influences drainage; for example, drainage time decreases with squared volumetric load because specific cake resistance increases linearly with load. Drainage rate increases with medium resistance; however, at high medium resistance, cakes are more loosely packed so specific cake resistance is lower. Cakes form more rapidly and are more compressed if particles settle, so the average specific cake resistance increases and the drainage rate decreases at a high settling rate. (C) 2011 Elsevier B.V. All rights reserved.