Multimedia filters consist of two or more layers of different granular materials, with different densities. Usually, anthracite coal, sand, and gravel are used. The different layers combined may provide more versatile collection than a single sand layer. Because of the differences in densities, the layers stay neatly separated, even after backwashing. Diffusion towards media granules occurs for very small particles, such as viruses.
Particles move randomly about within the fluid, due to thermal gradients.
This mechanism is only important for particles with diameters Attachment by inertia occurs when larger particles move fast enough to travel off of their streamlines and bump into media grains. Interception of particles is common for large particles. A streamline may lie very close to the media surface. If a large enough particle follows the streamline, it will hit the media grain and be captured. Hydrodynamic attachment, due to local turbulences and pressure fluctuations gets small particles caught up in eddies to spend a lot of time in the filter bed and eventually get captured.