Tower filters are typically 20 to 30 feet high and have a smaller diameter than the filters with rock media. Most systems use a rotating distributor to spray effluent onto the surface layer of the media, but some use a fixed distributor. Rotating distributors spread the effluent more evenly than fixed distributors, but they require more energy to operate. The distribution of the effluent can either be intermittent or continuous depending on the system. When continuous, a portion of the wastewater is recirculated back to the distribution system.
Sewage first goes to a settling tank where much of the solid matter settles out of the wastewater.
The wastewater is then pumped to the distributor, which sprays it onto the surface of the media. The media act as a substrate to which microorganisms attach themselves. Empty space between the media allows for the presence of air, creating an aerobic environment for the microorganisms. Plastic media have significantly more empty space, thus allowing for a greater oxygen transfer. As the wastewater passes over the media, these microbes feed upon organic material.