Filters and Drains

Underdrain:  Lowers groundwater by transporting it through a perforated pipe. A filter surrounding the pipe prevents small particles from entering the pipe through the perforations (which would settle in the pipe and restrict pipe flow). Toe Drain:  Underdrain that catches water seeping through and under an earth dam. The filter inhibits piping by keeping the soil in place while allowing water to pass.
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.
Water Filter:  A filter that holds particles collected as water seeps downward through the filter on its way to the collecting trough.