As more particles are removed, the headloss in the filter will increase. Backwashing consists of reversing the flow through the filter at a high rate, suspending and scouring the filter media. At the end of backwashing, the media will settle back into the normal configuration. Filter media are chosen for their headloss and settling velocity characteristics. Important filter media properties include grain size distribution, particle density, shape, and porosity.
Filters often contain more than one media type.
For example, dual-media, rapid sand filters have a layer of anthracite coal overlying sand. The effectiveness of disinfection is a function of dosage, contact time, and pathogen type.