Filter beds are identified as “multilayer,” “in-depth,” and “mixed media” and are graded by size and density. In the filter beds, the coarse, less dense particles are at the top, while the fine, more dense particles are found at the bottom. Deep, uniform penetration by particulate matter occurs due to the downflow filtration which also permits high filtration rates and long service runs. The small particles remain at the bottom because they are more dense, meaning that there is less space between particles. Even with the occurrences of high-rate backwashing, layers remain in their proper location in the mixed media filter bed.

The shown table lists four media that are used in multilayer filtration. There are several types of mixed media combinations that have been proven to be effective. Using too many different media methods will result in backwashing complications. For instance, if the four materials listed in the table were used in the same filter, a wash rate high enough to expand the magnetite layer might wash the anthracite from the filter. This would result in a high wash water requirement.

Anthracite/sand filter beds produce all of the benefits of single-media filtration while using less backwash water than sand or anthracite alone. This can be said for anthracite/sand/garnet mixed units, as well. The greatest benefits of dual-media filtration are higher rates and longer runs. Anthracite/sand/garnet beds have operated at normal rates of approximately 5 gpm/ft² and peak rates as high as 8 gpm/ft² without losing effluent quality.