Trickling Filter

A trickling filter, is a fixed-bed, biological reactor that operates under (mostly) aerobic conditions. Pre-settled wastewater is continuously ‘trickled’ or sprayed over the filter. As the water migrates through the pores of the filter, organics are [no-ecompendium]aerobically [no-ecompendium]degraded by the biofilm covering the filter material.

The trickling filter is filled with a high specific surface area material, such as rocks, gravel, shredded PVC bottles, or special pre-formed plastic filter media. A high specific surface provides a large area for biofilm formation. Organisms that grow in the thin biofilm over the surface of the media oxidize the organic load in the wastewater to carbon dioxide and water, while generating new biomass.

The incoming pre-treated wastewater is ‘trickled’ over the filter, e.g., with the use of a rotating sprinkler. In this way, the filter media goes through cycles of being dosed and exposed to air. However, oxygen is depleted within the biomass and the inner layers may be anoxic or anaerobic.

Design Considerations

The filter is usually 1 to 2.5 m deep, but filters packed with lighter plastic filling can be up to 12 m deep.

The ideal filter material is low-cost and durable, has a high surface to volume ratio, is light, and allows air to circulate. Whenever it is available, crushed rock or gravel is the cheapest option. The particles should be uniform and 95% of them should have a diameter between 7 and 10 cm. A material with a specific surface area between 45 and 60 m2/m3 for rocks and 90 to 150 m2/m3 for plastic packing is normally used. Larger pores (as in plastic packing) are less prone to clogging and provide for good air circulation. Primary treatment is also essential to prevent clogging and to ensure efficient treatment.

Adequate air flow is important to ensure sufficient treatment performance and prevent odours.

The underdrains should provide a passageway for air at the maximum filling rate. A perforated slab supports the bottom of the filter, allowing the effluent and excess sludge to be collected. The trickling filter is usually designed with a recirculation pattern for the effluent to improve wetting and flushing of the filter material.

With time, the biomass will grow thick and the attached layer will be deprived of oxygen; it will enter an endogenous state, will lose its ability to stay attached and will slough off. High-rate loading conditions will also cause sloughing. The collected effluent should be clarified in a settling tank to remove any biomass that may have dislodged from the filter. The hydraulic and nutrient loading rate (i.e., how much wastewater can be applied to the filter) is determined based on the characteristics of the wastewater, the type of filter media, the ambient temperature, and the discharge requirements.


This technology can only be used following primary clarification since high solids loading will cause the filter to clog. A low-energy (gravity) trickling system can be designed, but in general, a continuous supply of power and wastewater is required.

Compared to other technologies (e.g., Waste Stabilization Ponds), trickling filters are compact, although they are still best suited for peri-urban or large, rural settlements.

Health Aspects/Acceptance

Odour and fly problems require that the filter be built away from homes and businesses. Appropriate measures must be taken for pre- and primary treatment, effluent discharge and solids treatment, all of which can still pose health risks.

Operation and Maintenance

A skilled operator is required to monitor the filter and repair the pump in case of problems. The sludge that accumulates on the filter must be periodically washed away to prevent clogging and keep the biofilm thin and aerobic. High hydraulic loading rates (flushing doses) can be used to flush the filter. Optimum dosing rates and flushing frequency should be determined from the field operation.

The packing must be kept moist. This may be problematic at night when the water flow is reduced or when there are power failures.

Snails grazing on the biofilm and filter flies are well known problems associated with trickling filters and must be handled by backwashing and periodic flooding.


Further Readings

  • 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.

    U.S.EPA (Editor) (2000): Trickling Filters Nitrification. United States Environment Protection Agency. URL [Accessed: 17.05.2012]. PDF

    Short factsheet on nitrification in trickling filters (form the United States Environment Protection Agency (EPA).

  • 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.

    TCHOBANOGLOUS, G.; BURTON, F. L.; STENSEL, H. D.; METCALF & EDDY Inc. (Editor) (2003): Wastewater Engineering, Treatment and Reuse. New York: McGraw-Hill Companies, Inc.. PDF

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

    UNEP (Editor) (2004): Chapter 4. Wastewater Technologies. In: UNEP (Editor) (2004): A Directory of Environmentally Sound Technologies for the Integrated Management of Solid, Liquid and Hazardous Waste for SIDS in the Caribbean Region. Nairobi, 63. PDF

    Comprehensive overview (in form of factsheets) on the different components of wastewater treatment systems (collection, transfer, onsite treatment, centralised and decentralised treatment, reuse, sludge management and disposal) adapted to the Caribbean Region. Industrial wastewater treatment is also discussed.

  • Cover image of a reference book or miscellany.

    UNEP (Editor); MURDOCH UNIVERSITY (Editor) (2004): Environmentally sound technologies in wastewater treatment for the implementation of the UNEP/GPA "Guidelines on Municipal Wastewater Management". The Hague: United Nations Environment Programme Global Programme of Action (UNEP/GPA), Coordination Office. PDF

    Technical information on environmentally sound technologies in wastewater treatment.

  • Cover image of a reference book or miscellany.

    WSP (Editor) (2008): Technology Options for Urban Sanitation in India. A Guide to Decision-Making. pdf presentation. New Delhi: Water and Sanitation Program (WSP). URL [Accessed: 26.03.2010]. PDF

    These guidance notes are designed to provide state governments and urban local bodies with additional information on available technologies on sanitation. The notes also aid in making an informed choice and explain the suitability of approaches.

Case Studies

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

    HOCHHEIMER, J.N.; WHETON, F.W. (1998): Biological Filters: Trickling and RBC Design. In: LIBEY, G.S. (Editor); TIMMONS, M.B. (Editor) (1998): Proceedings of the Second International Conference on Recirculating Aquaculture Roanoke. Virginia, 291. URL [Accessed: 18.03.2010]. PDF

    The application of biological filters (fixed film processes) for the removal of ammonia from brackish or salt wastewaters from fish farms is discussed. The biological removal of ammonia in fixed-film processes is explained and design examples for ammonia removal in either trickling filters or rotating biological contactors are discussed.

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

    MOREL A.; DIENER S. (2006): Ecosan Greywater Demonstration Project. Case study from Kuching, Malaysia. In: MOREL, A.; DIENER, S. (2006): Greywater Management in Low and Middle-Income Countries, Review of Different Treatment Systems for Households or Neighbourhoods. Duebendorf, 76.

    The city of Kuching is currently lacking a wastewater treatment plant, and the local subsurface conditions make a conventional centralised wastewater system expensive to implement. Most buildings are equipped with two separate wastewater outlets, one outlet for blackwater and one for greywater. The proposed system treats greywater from nine households, and consists of a baffled septic tank, followed by a dosing chamber from where the greywater flows into four vertical down-flow, single-pass aerobic biofilters before reaching a subsurface horizontal-flow planted filter. Finally, the treated greywater is discharged into a stormwater drain.

Training Material

  • Cover image of a reference book or miscellany.

    LESIKAR, B. (2000): Trickling Filter Design. Izmir: Dokuz Eylul University. URL [Accessed: 24.02.2010]. PDF

    Short PowerPoint Presentation on the design of trickling filters available on the Tropak Hompage hosted by the Turkish Dokuz Eylul University in Izmir.

  • Cover image of a reference book or miscellany.

    SPERLING, M. von; LEMOS CHERNICHARO, C.A. de (2005): Biological Wastewater Treatment in Warm Climate Regions Volume 1. London: International Water Association (IWA) Publishing. URL [Accessed: 01.11.2013]. PDF

    Biological Wastewater Treatment in Warm Climate Regions gives a state-of-the-art presentation of the science and technology of biological wastewater treatment, particularly domestic sewage. The book covers the main treatment processes used worldwide with wastewater treatment in warm climate regions given a particular emphasis where simple, affordable and sustainable solutions are required. The 55 chapters are divided into 7 parts over two volumes: Volume One: (1) Introduction to wastewater characteristics, treatment and disposal; (2) Basic principles of wastewater treatment; (3) Stabilisation ponds; (4) Anaerobic reactors; Volume Two (also available in the SSWM library): (5) Activated sludge; (6) Aerobic biofilm reactors; (7) Sludge treatment and disposal.