Filters for suspended particle removal can be made of graded sand, granular
synthetic material, screens of various materials, and fabrics. The most
widely used are rapid-sand filters in tanks. In these units, gravity holds
the material in place and the flow is downwards. The filter is periodically
cleaned by a reversal of flow and the discharge of backflushed water into
a drain. Cartridge filters made of fabric, paper, or plastic material are
also common and are often much smaller and cheaper and are disposable.
Filters are available in several ratings depending on the size of particles
to be removed. Activated carbon filters, described earlier, will also remove
turbidity, but would not be recommended for that purpose only.