S.15 Anaerobic Filter

An Anaerobic Filter (AF) can efficiently treat many different types of wastewater. An AF 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 biofilm that is attached to the surface of the filter material.

This technology is widely used as a secondary treatment for black or greywater and improves the solid removal compared to Septic Tanks S.13 or Anaerobic Baffled Reactors S.14 . The treatment process is anaerobic making use of biological treatment mechanisms. Suspended solids and biochemical oxygen demand (BOD) removal can be up to 90 %, but is typically between 50 % and 80 %. Nitrogen removal is limited and normally does not exceed 15 % in terms of total nitrogen.

Design Considerations

Pre-Treatment (PRE) is essential to remove solids and solid waste that may clog the filter. The majority of settleable solids are removed in a sedimentation chamber in front of the AF. Small-scale, standalone units typically have an integrated settling compartment, but primary sedimentation can also take place in a separate Settler T.1 or another preceding technology, e.g. Septic Tank S.13 . AFs are usually operated in upflow mode because there is less risk that the fixed biomass will be washed out and treatment efficiency reduced. The water level should cover the filter media by at least 0.3 m to guarantee an even-flow regime. The hydraulic retention time (HRT) is the most important design parameter influencing filter performance and a HRT of 12–36 hours is recommended. The ideal filter should have a large surface area for bacteria to grow, with large pore volume to prevent clogging. The surface area ensures increased contact between organic matter and attached biomass that effectively degrades it. Ideally, the material should provide between 90 to 300 m2 of surface area/m3 of occupied reactor volume. The connection between chambers can be designed either with vertical pipes or baffles. Accessibility to all chambers (through access ports) is necessary for maintenance. The tank should be vented to allow for controlled release of odorous and potentially harmful gases. Where kitchen wastewater is connected to the system, a grease trap must be incorporated into the design before the Settler.


An AF can be made of concrete, sand, gravel, cement, steel, as well as fibreglass, PVC or plastic and can be prefabricated. Typical filter material should ideally range from 12 to 55 mm in diameter, decreasing in diameter from bottom to top. Filter materials commonly used include gravel, crushed rocks or bricks, cinder, pumice, shredded glass or specially-formed plastic pieces (even crushed PVC plastic bottles can be used).


AFs are not suitable for the acute response stage of an emergency because the biological environment within the AF takes time to establish. AFs are more suited for stabilisation and recovery periods, and are longterm solutions. The neighbourhood scale is most suitable, but AFs can also be implemented at the household level,in larger catchment areas or in public buildings (e.g. schools). Even though AFs are watertight, it is not recommended to construct them in areas with high groundwater tables or where there is frequent flooding. However, prefabricated modules can be placed above ground. AFs can be installed in every type of climate, although efficiency will be lower in colder climates. Pathogen and nutrient reduction is low in AFs; if high effluent standards are to be achieved, an additional treatment technology should be added, e.g. the Anaerobic Baffled Reactor S.14 , Waste Stabilisation Ponds T.5 or Constructed Wetlands T.6 .

Operation and Maintenance

An AF requires a start-up period of 6 to 9 months to reach full treatment capacity since the slow-growing anaerobic biomass first needs to be established on the filter media. To reduce start-up time, the filter can be inoculated with anaerobic bacteria, e.g. by spraying Septic Tank sludge onto the filter material. The flow should be gradually increased over time. Scum and sludge levels need to be monitored to ensure that the tank is functioning well. Over time, solids will clog the pores of the filter. Also the growing bacterial mass can become too thick, break off and eventually clog pores. When the efficiency decreases, the filter must be cleaned. This is done by running the system in reverse mode (backwashing) or by removing and cleaning the filter material. AF tanks should be checked from time to time to ensure that they are watertight.

Health and Safety

Effluent, scum and sludge must be handled with care as the effluent still contains pathogens and should be treated further if reused in agriculture, directly used for fertilisation and irrigation or discharged properly. Full personal protective equipment must be worn during the desludging and cleaning of the AF.


The capital cost of an AF is medium and the operational costs are low. The costs of the AF depend on the conveyance technology and treatment used, and also on local availability and thus costs of construction materials (sand, gravel, cement, steel), or cost of the prefabricated modules, and labor costs. The main operation and maintenance costs are related to the removal of primary sludge and cost of electricity if pumps are required for discharge (in the absence of the gravity flow option).

Social Considerations

Usually, AF treatment systems are a well-accepted technology. Because of the delicate ecology in the system, awareness raising on eliminating the use of harsh chemicals for the users is necessary.


  • Low O & M requirements and costs
  • Robust and stable treatment performance (Resistant to organic and hydraulic shock loadings)
  • No electrical energy is required
  • High reduction of BOD and solids


  • Limited reduction of pathogens and nutrients
  • Requires expert design and construction
  • Removing and cleaning the clogged filter media is cumbersome
  • Long start-up time

Still have questions?

You could not find the information you were looking for? Please contact our helpdesk team of experts for direct and individual support.