Multimedia filtration works by density separation. After backwash, the lightest media settles on top and the densest on the bottom — anthracite over sand over garnet. Water passes from coarse to fine, so large floc is captured near the surface and progressively finer particles deeper down. That is depth filtration, and it is why a multimedia bed runs longer than single-media sand at the same influent quality.
The whole mechanism depends on the densities being correctly spaced relative to the particle sizes. Get that wrong and the layers intermix on the first backwash and never separate again. The bed becomes an expensive single-media filter.
The arithmetic
Anthracite has a specific gravity around 1.5, silica sand 2.65, garnet 4.0. Because settling velocity depends on both size and density, a coarser but lighter grain can settle at the same rate as a finer but denser one. When two adjacent layers have matching settling velocities, they mix.
How to avoid it
- Specify the bed, not the layers. Give the vessel diameter, design filtration rate and target effluent, and let the supplier return a layer schedule.
- Source the whole bed from one supplier so densities and gradations are matched against each other.
- Check layer separation visually after the commissioning backwash, before the vessel is closed and returned to service.