Data from a range of filtration experiments on dilute suspensions are used to determine the parameters that describe the physics of suspension flow during compaction. The range of solids volume fractions used is 0.00001 < phi < 0.1; zeta-potentials vary between O and 50 mV. Relevant physical data are extracted from an analysis of the initial stages of experiments at various phi and zeta. Theoretical considerations on suspension flow are presented to argue that the physical character of the flow at relatively dense, strongly interacting conditions is significantly different from that of dilute systems. The latter are dominated by fluctuations in the particle velocity near the septum to give gas-type diffusive behavior while in the former the particles are more or less localized. This observation has implications for the diffusion coefficient which is predicted to be quadratic in the filtration pressure for very dilute suspensions and which is roughly independent of pressure for mixtures containing strongly interacting particles. Experiments are described and analyzed, and these reinforce the main theoretical insights.