In sewage treatment systems, an anaerobic filter (AF) is a form of anaerobic digester. The digestion tank contains a filter medium where anaerobic microbial populations — organisms that live in the absence of oxygen — can establish themselves. Such filters are commonly employed in the treatment of waste water. These reactors are gaining in popularity versus more established aerobic waste-water treatment systems because they produce a less solid residue than do other types of filter.
^AF Reactor Paper Francisco, O., Garrido, J. M., Arrojo, B. & Mendez, R. (2003) Anaerobic filter reactor performance for the treatment of complex diary waste water at industrial scale, Water Research 37, 4099-4108[dead link]