Anaerobic Filter

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.

With this technology, suspended solids and BOD removal can be as high as 90%, but is typically between 50% and 80%. Nitrogen removal is limited and normally does not exceed 15% in terms of total nitrogen (TN).

Design Considerations

Pre- and primary treatment is essential to remove solids and garbage that may clog the filter. The majority of settleable solids are removed in a sedimentation chamber in front of the anaerobic filter. Small-scale stand-alone units typically have an integrated settling compartment, but primary sedimentation can also take place in a separate Settler or another preceding technology (e.g., existing Septic Tanks). Designs without a settling compartment are of particular interest for (Semi-) Centralized Treatment plants that combine the anaerobic filter with other technologies, such as the Anaerobic Baffled Reactor (ABR).

Anaerobic filters are usually operated in upflow mode because there is less risk that the fixed biomass will be washed out. 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. An HRT of 12 to 36 hours is recommended.

The ideal filter should have a large surface area for bacteria to grow, with pores large enough to prevent clogging. The surface area ensures increased contact between the organic matter and the attached biomass that effectively degrades it. Ideally, the material should provide between 90 to 300 m2 of surface area per m3 of occupied reactor volume. Typical filter material sizes range from 12 to 55 mm in diameter. Materials commonly used include gravel, crushed rocks or bricks, cinder, pumice, or specially formed plastic pieces, depending on local availability.

The connection between the 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.

Appropriateness

This technology is easily adaptable and can be applied at the household level, in small neighbourhoods or even in bigger catchment areas. It is most appropriate where a relatively constant amount of blackwater is generated. The anaerobic filter can be used for secondary treatment, to reduce the organic loading rate for a subsequent aerobic treatment step, or for polishing.
This technology is suitable for areas where land may be limited since the tank is most commonly installed underground and requires a small area. Accessibility by vacuum truck is important for desludging.
Anaerobic filters can be installed in every type of climate, although the efficiency is lower in colder climates. They are not efficient at removing nutrients and pathogens. Depending on the filter material, however, complete removal of worm eggs may be achieved. The effluent usually requires further treatment.

Health Aspects/Acceptance 

Under normal operating conditions, users do not come in contact with the influent or effluent. Effluent, scum and sludge must be handled with care as they contain high levels of pathogenic organisms. The effluent contains odorous compounds that may have to be removed in a further polishing step. Care should be taken to design and locate the facility such that odours do not bother community members.

Operation & Maintenance 

An anaerobic filter 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. Because of the delicate ecology, care should be taken not to discharge harsh chemicals into the anaerobic filter.

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. As well, the growing bacterial mass will 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.

Anaerobic filter tanks should be checked from time to time to ensure that they are watertight.

References

Further Readings

  • Cover image of a reference book or miscellany.

    U.S.EPA (Editor) (1980): Onsite Wastewater Treatment Systems Manual. United States Environmental Protection Agency, Office of Water Office of Research and Development. PDF

    Rather old design manual for onsite wastewater treatment options. However, valuable information on established systems such as septic tanks, sand filters, aerobic treatment units (suspended growth and fixed film), disinfection, nutrient removal as well as wastewater segregation and recycling are given. Additional information is given on disposal methods and appurtenances.

  • Cover image of a reference book or miscellany.

    MOREL, A.; DIENER, S. (2006): Greywater Management in Low and Middle-Income Countries, Review of Different Treatment Systems for Households or Neighbourhoods. Duebendorf: Swiss Federal Institute of Aquatic Science (EAWAG), Department of Water and Sanitation in Developing Countries (SANDEC). URL [Accessed: 19.05.2010]. PDF

    This report compiles international experience in greywater management on household and neighbourhood level in low and middle-income countries. The documented systems, which vary significantly in terms of complexity, performance and costs, range from simple systems for single-house applications (e.g. local infiltration or garden irrigation) to rather complex treatment trains for neighbourhoods (e.g. series of vertical and horizontal-flow planted soil filters).

  • Cover image of a reference book or miscellany.

    GUTTERER, B.; SASSE, L.; PANZERBIETER, T.; RECKERZÜGEL, T.; ULRICH, A. (Editor); REUTER, S. (Editor); GUTTERER, B. (Editor) (2009): Decentralised Wastewater Treatment Systems (DEWATS) and Sanitation in Developing Countries. Loughborough University (UK): Water Engineering and Deveopment Centre (WEDC). URL [Accessed: 20.03.2014]. PDF

    This document speaks about waste water and sanitation strategies in the developing countries. It also advocates the use of DEWATS as sustainable treatment of waste water at a local level backing it up with case studies from different countries. It describes various options available for sanitation and waste water treatment. It gives an idea of planning and executing CBS programs.

  • Cover image of a reference book or miscellany.

    ROSE, D.G. (1999): Community-Based Technologies for Domestic Wastewater Treatment and Reuse- options for urban agriculture. Ottawa: International Development Research Center Canada (IDRC). PDF

    The report suggests that emerging trends in low-cost, decentralised naturally-based infrastructure and urban wastewater management which promote the recovery and reuse of wastewater resources are increasingly relevant. Technologies for these sanitation options are presented. The concept of managing urban wastewater flows at a decentralised or "intermediate" level, based on micro watersheds, is explored. Effluent treatment standards that are currently accepted in order to protect public health and safety are reviewed.

  • Cover image of a reference book or miscellany.

    LEMOS CHERNICHARO, C.A. de (2007): Anaerobic Reactors. London: International Water Association (IWA) Publishing. URL [Accessed: 01.11.2013]. PDF

    Anaerobic Reactors is the forth volume in the series Biological Wastewater Treatment. The fundamentals of anaerobic treatment are presented in detail, including its applicability, microbiology, biochemistry and main reactor configurations. Two reactor types are analysed in more detail, namely anaerobic filters and especially UASB (upflow anaerobic sludge blanket) reactors. Particular attention is also devoted to the post-treatment of the effluents from the anaerobic reactors. The book presents in a clear and informative way the main concepts, working principles, expected removal efficiencies, design criteria, design examples, construction aspects and operational guidelines for anaerobic reactors.

Case Studies

  • Cover image of a reference book or miscellany.

    MOREL, A.; DIENER, S. (2006): Greywater Management in Low and Middle-Income Countries, Review of Different Treatment Systems for Households or Neighbourhoods. Duebendorf: Swiss Federal Institute of Aquatic Science (EAWAG), Department of Water and Sanitation in Developing Countries (SANDEC). URL [Accessed: 19.05.2010]. PDF

    This report compiles international experience in greywater management on household and neighbourhood level in low and middle-income countries. The documented systems, which vary significantly in terms of complexity, performance and costs, range from simple systems for single-house applications (e.g. local infiltration or garden irrigation) to rather complex treatment trains for neighbourhoods (e.g. series of vertical and horizontal-flow planted soil filters).

  • Cover image of a reference chapter of a book/miscellany.

    MOREL, A.; DIENER, S. (2006): Decentralised greywater reuse for irrigation in peri-urban areas. Case study from Ein Al Beida, Jordan. In: MOREL, A.; DIENER, S. (2006): Greywater Management in Low and Middle-Income Countries, Review of Different Treatment Systems for Households or Neighbourhoods. Duebendorf, 69. PDF

    The International Development Research Centre (IDRC) provided financial assistance to an applied research project on greywater treatment and reuse for home garden irrigation in 25 low-income households in Ein Al Beida village, southern Jordan. The main objective was to help the peri-urban poor in Jordan preserve precious freshwater, achieve food security and generate income, while helping to protect the environment. An anaerobic vertical flow filter (AVF) and an anaerobic up-flow filter (AUF) were tested in parallel.

  • Cover image of a reference book or miscellany.

    VIET ANH, N.; DUC HA, T.; HIEU NHUE, T.; HEINSS, U.; MOREL, A.; MOURA, M.; SCHERTENLEIB, R. (2003): Decentralised wastewater treatment – New Concepts and Technologies for Vietnamese Conditions . Luebeck: IWA and GTZ. PDF

    The authors describe results from experiments on treatment of domestic wastewater by baffled septic tanks with an anaerobic filter that could be a feasible option for on-site wastewater treatment in residential areas of Vietnam.

  • Cover image of a reference book or miscellany.

    BORDA (Editor) (2008): Decentralized Wastewater Treatment System - DEWATS. Manjuyod Public Market. Bremen: Bremen Overseas Research and Development Association (BORDA) . URL [Accessed: 26.03.2010]. PDF

    The wastewater from Manjuyod’s public market is treated in a decentralized system (DEWATS) composed of four different components: a settling tank; a anaerobic baffled reactor which reduces the BOD/COD content from 20% to 85%; a planted gravel filter; and finally a polishing pond.

Training Material

  • Cover image of a reference book or miscellany.

    NATURGERECHTE TECHNOLOGIEN, BAU- UND WIRTSCHAFTSBERATUNG (TBW) GmbH (Editor) (2001): Decentralised Wastewater Treatment Methods for Developing Countries. GTZ and GATE. PDF

    Different operation and maintenance options are presented with respect to sustainable plant operation, the use of local resources, knowledge, and manpower.

  • Cover image of a reference book or miscellany.

    EAWAG/SANDEC (Editor) (2008): Sanitation Systems and Technologies. Lecture Notes . Duebendorf: Swiss Federal Institute of Aquatic Science (EAWAG), Department of Water and Sanitation in Developing Countries (SANDEC). PDF

    Lecture notes on technical and non-technical aspects of sanitation systems in developing countries.

  • Cover image of a reference book or miscellany.

    EAWAG/SANDEC (Editor) (2008): Sanitation Systems and Technologies. Presentation. Duebendorf: Swiss Federal Institute of Aquatic Science (Eawag), Department of Water and Sanitation in Developing Countries (Sandec). PDF

    PDF presentation on the technical and non-technical aspects of sanitation systems in developing countries.