An anaerobic filter is a fixed-bed biological reactor with one or more filtration chambers in series. As wastewater flows through the filter, particles are trapped and organic matter is degraded by the active biomass that is attached to the surface of the filter material. Anaerobic filters are similar to septic tank, with the difference, that wastewater, once entered into the tank must flow downwards (anaerobic downflow filter) or upwards through a filter media (anaerobic upflow filter). Anaerobic filters are used as secondary treatment in household-level or decentralised wastewater treatment systems. As for any anaerobic digestion process, biogas can be recovered and transformed to energy or light.