The clarification of dilute suspensions by filtration through granular filter beds was investigated at constant flowrate. A two-dimensional transparent model filter bed was designed in which the deposition of particles could be observed and photographed in the course of filtration. Measurements of pressure drop were related quantitatively and qualitatively to these observations. A non-consolidated filter bed of glass spheres and a consolidated sintered bed were also used. Suspensions of angular polystyrene or quartz particles were used in most of the experiments, but a few were made with spherical polystyrene particles.

It was found that there was a critical velocity above which no deposition took place in the filter bed. A simplified theoretical expression was derived for this velocity by considering the forces acting on a particle resting in a channel;​ experimental results showed encouraging agreement.

Two modes of deposition were observed:​ the one caused a gradual narrowing of flowpaths;​ the other, more frequently occurring resulted in the blocking of flowpaths. Depending on flowrate and particle size, clarification runs terminated in one of two ways:​ either all the flowpaths became blocked and particles no longer penetrated into the filter bed, but instead began to form a cake on the surface;​ or else only a limited number of flowpaths became blocked so that the velocity in the remaining flowpaths exceeded the critical value and no further deposition took place i.e. the filter bed became "non-retaining". Theoretical relationships between pressure drop and filtration time in the laminar regime for the latter process were derived and experimentally verified on both the model filter bed and the granular filter bed.

Experiments were also conducted in the granular filter bed under semi-turbulent conditions. The results suggest that the degree of turbulence decreases as blocking proceeds.