Media filters as discussed in this text are fixed film treatment processes designed to follow primary treatment in a septic tank and to provide more highly treated effluent. The effluent produced by these filters can be disinfected and discharged under a National Pollutant Discharge Elimination System (NPDES) permit, dispersed in the subsurface soil, or reused for irrigation or nonpotable household purposes. Media filters are one of the oldest types of treatment mechanisms for both water treatment and wastewater treatment. During their early use in England, sand filters were applied to treat water prior to use as drinking water. Sand filters continue to be used in large and small applications for water and wastewater treatment where they are used as pressure filters. In these cases, the media is completely saturated and the water is ponded on top of the filters to provide adequate head to push either the treated drinking water or pretreated wastewater through the crust, or schmutzdecke (dirty floor), of material that accumulates on the filter surface. These types of filters are known as pressure filters and, although used for filtering secondary quality treated wastewater such as settled aerobic treatment plant effluent, they should not be confused with the filters used for treating septic tank effluent.