In-home water treatment, filtration devices are most commonly used to remove particles of sand or iron, manganese, or sulfur. Any bacteria from water may also be eliminated by filtration. Water flows into a substrate such as fabric or sand in mechanical filtration systems. Particles become trapped on the surface of or within the medium. The size of the pore, or space between media granules or fibers, determines what size a filter will extract particles. Based on the smallest particle they can trap, filters are scored. The opening size of the filter to be used depends on the content that the filter can eliminate. A smaller scale will meet the requirements of removal but will require more regular filter cleaning or replacement. Follow the manufacturer’s recommendations when these filters are used to pre-treat water for other water treatment systems, such as a reverse osmosis unit. The particles at or very near the filter surface are filtered by surface or film filters. They work very much like a screen; the surface is held by particles of a certain size and larger, while smaller ones pass through the holes. Depth filters provide a dense medium for filtering. Throughout the dense filter mat, particles are preserved. Depth filters have a filter media size gradation to hold the larger particles at or above the filter level, whereas smaller particles are eventually captured deeper in the filter where the filter media becomes smaller.