WATER-FLOW IN FILTER-PAPER AND CAPILLARY SUCTION TIME

Flow of water in Whatman #17 chromatography paper and the interpretati on of the capillary suction time method for determining filterability of suspensions is examined. Filter paper water flow is described by we ll-established theory of water flow in porous media. This theory is ba sed on material balance for the water and Darcy's law. Assuming that t he matric potential or suction (arising from the interaction of water with the solid surfaces and their geometry) and the hydraulic conducti vity of both the filter paper and the suspension are well-defined func tions of the water content, one-dimensional capillary suction time is analysed from considerations of material balance and continuity of wat er potential on the paper/ suspension interface. Experiments agree wit h theory and illustrate that flow in the paper is unsaturated, with th e advance of a wetting front in paper a function of the interaction of the sorptivity-matric potential functions of both the paper and the s uspension. Extension of the approach to radial flow in the filter pape r is too complicated to permit simple interpretation of wetting front advance. The capillary suction time method must remain a useful empiri cal tool for practicing engineers. Its weaknesses should be recognised , however, and constant pressure filtration tests, for example, which provide both sorptivity-, and equilibrium water content- pressure rela tions simply and accurately must ultimately be preferable. (C) 1998 El sevier Science Ltd. All rights reserved.