A centrifugation method for the assessment of low pressure compressibility of particulate suspensions

This work describes an easy methodology for assessing the compressibility of particulate suspensions at relatively low pressures. The material used in this study was an anaerobically digested waste water sludge. Using a centrifuge, the equilibrium sediment bed height was recorded as a function of rotational speed during the centrifugation. This approach avoids errors due to expansion of the sediment bed after the centrifugation is stopped, a phenomenon typically seen with waste water sludges. A straightforward numerical method was used to solve a one-dimensional centrifugation model describing the equilibrium sediment bed height as a function of rotational speed. It has been shown that this approach yields realistic results. The one-dimensional model is able to describe the compressibility of the anaerobically digested sludge. Besides yielding a close fit for the measured sediment bed heights, extrapolating these results to a different initial solids volume fraction resulted in a good prediction of the bed height as well. Different constitutive equations for the relation between solids pressure and solids volume fraction were assessed. The typical power law functions [F. Tiller, W. Leu, Basic data fitting in filtration, journal of the Chinese Institute of Chemical Engineers 11 (1980) 61-70], often used in filtration modelling, were the most adequate for describing the compressional behaviour of the sludge. A functional relation based on the osmotic pressure within the solids network, suggested for describing the phenomenology of filtration dewatering [K. Keiding, M. Rasmussen, Osmotic effects in sludge dewatering, Advances in Environmental Research 7 (2003) 641-645], could not be used to adequately describe the behaviour of the sludge at the low pressures generated during centrifugation. (c) 2008 Elsevier B.V. All rights reserved.