Several different types of media have been used in media filters. Media types can be generally categorized as solid granular or absorbent. In the solid granular media category, there are three materials that have been used—sand or gravel, bottom ash, and crushed glass.
Media less than 2 millimeters in diameter is sand and anything larger is referred to as gravel. Sand and gravel have been used in media filters for decades. Sand, through which wastewater flows relatively slow, is typically used for single-pass filters. Gravel is typically used for recirculating filters which can accept larger amounts of wastewater than single-pass sand filters. Nationwide, more sand filters have been used to treat water and wastewater than probably any other advanced pretreatment technology.
Regionally, other solid granular media such as crushed glass and bottom ash have been used. Also called “slag,” bottom ash is a byproduct of coal-fired power plants. Bottom ash is still used in some Appalachian Mountain states where coal-fired power plants are common. The use of glass media has been isolated to the northwestern United States and western Canada and is used on a limited basis today. Other types of media material, such as tire chips, have been experimented with on a limited basis.
Sand media filters are one of the oldest onsite wastewater treatment technologies known. If properly designed, constructed, operated and maintained, a sand filter produces a very high quality effluent. Sand filters are beds of sand or gravel, drained from underneath so that primary treated effluent can be treated, collected, and distributed to the soil absorption field.