|Gravel Pack: A filter that stops particles from entering a well as ground water flows through the soil, through the surrounding screen and pump, and the the water system at the surface.
||Sand Filter: Sewage (with most of the solids already settled out), flows through the upper pipes and into the filter through perforations. It then proceeds through the filter and through more perforations into the lower pipes where it is carried to a stream or lake. Particles (which are orgranic) caught in the filter are consumed by bacteria. When properly designed, it can last indefinitely without cleaning.