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.