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.