Trickling Filter in Wastewater Treatment

Trickling filters are strange-looking wastewater treatment systems that can deliver good reduction rates with less maintenance and on-site operation What is a Trickling Filter in Wastewater Treatment?

Trickling filters are used to remove organic matter from wastewater an aerobic treatment system that uses microbes attached to a filter medium rock, gravel, or plastic that removes organic matter from wastewater sprayed on it, followed by a recirculation tank It can handle peak shock loads to loads.

A Trickling filter uses physical forms of filtration, adsorption, and assimilation for the removal of contaminants from wastewater. Wastewater should flow in a thin film over the media to allow time for treatment.

Trickling Filter in Wastewater Treatment


A trickling filter uses filtration, adsorption, and assimilation for removal of contaminants from wastewater. Wastewater should flow in a thin film over the media to allow time for treatment.

The media serves as a substrate where a biological film grows and is fed by the nutrients contained in the wastewater.

Trickling FilterTrickling filters are considered On-Site Sewage facilities (OSSF) to provide aerobic treatment of wastewater. Wastewater is generally pumped from a recirculation tank or compartment, dispersed or sprayed over a media bed, and allowed to drain back into the recirculation tank.

The wastewater is aerated as it flows through the media bed, which may consist of a variety of media such as tire chips, stone, and rigid plastics configured into a number of shapes (i.e., honeycomb blocks, rings, or cylinders).

A trickling filter uses filtration, adsorption, and assimilation for removal of contaminants from wastewater. Wastewater should flow in a thin film over the media to allow time for treatment.

Two of the most commonly used systems for biological waste treatment are the activated sludge system and the Biological Film System.
Trickling filters became the dominant secondary treatment process in the U.S. by the 1950s. Trickling filters fell out of favor in the 1970s when the U.S. Environmental Protection Agency issued its definition of secondary treatment standards — activated sludge was better able to meet the new standards.
Improvements in secondary clarifier design and the introduction of coupled processes — such as trickling filter/solids contact — and nitrifying trickling filters have led to a resurgence of the technology.
Distribution system: provides for even distribution of wastewater over the media.
Containment structure: provides structural support to contain the media within the bioreactor.
Filter media: Rock media commonly are found in older trickling filters.
Almost all new installations use plastic media. Rock media are more likely to plug due to smaller voids, which also restrict airflow. Plastic media, available as cross-flow or vertical, provide for better oxygen transfer, and are lighter than rock, so media can be deeper.
Vertical-flow media are used in roughing and industrial applications. Underdrain: collects treated wastewater and creates a plenum that facilitates oxygen transfer throughout the trickling filter. Ventilation: Trickling filters for biochemical oxygen demand (BOD) removal and nitrification are aerobic processes and require air exchange to sustain aeration.
In the Activated Sludge Sys­tem, the wastewater is brought into contact with a diverse group of micro-or­ganisms in the form of a flocculent suspension in a mixed liquid aerated tank.
In the biological film system, also known as trickling filters, the wastewater is brought into contact with a mixed microbial population in the form of a film of slime attached to the surface of a solid support system. In both cases, the or­ganic matter is broken down and metabolized to more stable inorganic forms.

The media serves as a substrate where a biological film grows and is fed by the nutrients contained in the wastewater. As the biological film establishes and continues to grow, it will eventually exceed its own ability to cling to the media surface.


Trickling filter systems usually follow primary treatment. They generally include a secondary settling tank or clarifier. Widely used for the treatment of domestic and industrial wastes, the trickling filter biological treatment process is a fixed film method designed to remove BOD and suspended solids. Wastewater containing organic contaminants comes in contact with a population of microorganisms attached, or fixed, to the surface of filter media.


Fist-sized stone or redwood, synthetic materials such as plastic, or any other substance capable and hold up in bad weather conditions for many years are typically used as filter media. The primary effluent is disbursed over the top of the filter media by a fixed distributor system or a rotating distribution arm.

The wastewater forms a thin layer as it flows down through the filter media and across the microorganism layer on the media surfaces.

As the distributor’s arm rotates, a flow of wastewater and air alternate contact with the microorganism layer (the zoogleal slime). The spaces between the media allow air to circulate easily, maintaining aerobic conditions favorable to the microbes.

The biological slime absorbs and consumes the wastes trickling through the media bed, while the organisms aerobically decompose the solids, producing more organisms and stable wastes. These wastes either become part of the slime or return to the wastewater flowing over the media.

Zoogleal slime comprises mostly bacteria, but may also contain algae, protozoa, worms, snails, fungi, and insect larvae. As the slime accumulates, it builds up and occasionally sloughs off the media materials.

The sloughings are collected at the bottom of the trickling filter with the treated wastewater and pass on to the secondary settling or sedimentation tank for removal. In a fixed distributor system, the wastewater flow cycles on and off at a specific dosing rate, ensuring that the microorganisms receive an important mixture of oxygen supply. The trickling filter rate of removal and overall performance depends on hydraulic and organic loading, temperatures, and recirculation.

As the layer thickens with microbial growth, oxygen cannot penetrate the media face, and anaerobic organisms develop.  As the biological film continues to grow, the microorganisms next to the surface lose their ability to cling to the media, and a portion of the slime layer falls off the media filter. This is known as sloughing and is the main source of solids picked up by the underdrain system.


What is an Oxidation Pond in Wastewater Treatment?

Oxidation Ponds are large basins where wastewater is treated by natural processes using bacteria & algae at a slow rate.

Aerobic Pond-bacteria use oxygen
Anaerobic Ponds-bacteria use no oxygen
Facultative Ponds-use both bacteria
Maturation or Tertiary Ponds- aerobic bacteria used for polishing effluents ………………………………………………………………………………………………… Read more



What are the 3 Stages of Wastewater Treatment?

  1. Primary Stage-separates settable organic solids & inorganic material that won’t degrade
  2. Secondary Stage-removes suspended & soluble solids converting them to settable solids using Biological Oxidation
  3. Tertiary stage uses chemical & physical treatment to create H2O closer to potable quality .……………………………………………………………………………. Read more



Filters Trickling Waste Water Media


Trickling filters are aerobic meaning the bacteria that work on the media use oxygen to perform. The filter develops good adaptability to handle peak shock loads and the ability to function satisfactorily after a short period of time. 

Milk processing, paper mill, and pharmaceutical wastes are among those treated by tricking filters. Conventional trickling fil­ters normally consist of a rock bed gravel or plastic media, 1 to 3 meters in depth, with enough open­ings between rocks to allow oxygen to circulate through the media easily.

The biological film material that breaks off and washes down through the media bed, will either return to the recirculation tank or settle out in a clarifier tank. The clarifier typically contains a solids return pump dedicated to moving accumulated biological materials up to the head works of the septic tank for decomposition.

Excessive biological growth or sloughing will Reduce organic loading; increase hydraulic loading to increase sloughing; use a high-pressure stream of water to flush the filter surface and rinse the solids through the filter. Rotary distributors are very reliable and easy to maintain.

A clearance of 15 to 23 cm (6 to 9 inches) is needed between the bottom of the distributor’s arm and the top of the media bed to allow the wastewater from the nozzles to spread out correctly.

Activate sludge contains less than 1 percent solids, while the sludge from trickling filters is a little better at about 2 percent solids. The common unit operations of sludge treatment and disposal involve concentration or thickening, digestion, conditioning, dewatering, oxidation, and finally safe disposal.

There are many ways to treat sewage and plant designs vary greatly from one system to the next.  But no matter how a plant is designed, the results are the same.  Some type of mechanical or biological treatment of wastewater occurs. The supernate is drawn off, treated for contamination of bacteria, and aerated before being released.  Solid waste or bio-solids is produced from sludge and is graded for land application or dumping.  In nearly all types of wastewater treatment, organic waste is consumed by microbial action.  This microbial action can be divided into two categories: free-swimming or attached.

Packaged plants and oxidation ditches are an example of the first type of microbial action.  The microorganisms are free-swimming in the water, so they must be cycled through the system.  After being used to break down B.O.D., they are removed from the wastewater in a clarifier and returned as mixed liquor to the aeration chamber or oxidation ditch.  

In contrast, fixed media filters like the Trickling Filter use microorganisms attached to a medium (rocks, plastic, metal, etc.)  The microorganisms stay in place and do not need to be cycled through the system.  Instead, wastewater is circulated past and to the fixed microorganisms.  

A fixed media filter mimics the treatment method used in a healthy stream in which microorganisms produce a slick coating on rocks and pebbles.  This coating of microorganisms is able to trap and consume B.O.D. and ammonia in the water.

Rotary distributors are very reliable and easy to maintain. A clearance of 15.2-22.9 centimeters (6-9 inches) is needed between the bottom of the distributor’s arm and the top of the medium bed to allow the wastewater from the nozzles to spread out and cover the bed uniformly. This also helps prevent ice from accumulating during freezing weather.

Care should be taken to prevent any leaks. Follow the manufacturer’s operation and maintenance (O&M) instructions on pumps, bearings, and motors. All equipment must be tested and calibrated as recommended by the equipment manufacturer.

A routine O&M schedule should be developed and followed for any (TF)  system. It is critical that a Trickling filter system be pilot-tested prior to installation to ensure that it will meet effluent discharge permit requirements for that particular site.

Trickling filters are very efficient at removing B.O.D. and ammonia from wastewater, and they use a minimal amount of power.  The cost to remove B.O.D. is only a few dollars per ton.

Although many trickling filters have hydraulically driven distributors, some distributors are equipped with mechanical or other speed controls. As a result, distributors previously operated at 30 seconds to 2 minutes per revolution. Speed-control distributors now often allow normal operational speeds ranging from 4 to 10 min. per revolution.

More importantly, many speed-control distributors allow specifying a regular (daily or weekly) “flushing” speed rate that may slow the distributor to more than 60 minutes per revolution.

The combination of slow distributor speed and pumped flow results in dosing rates sometimes referred to as “Spulkraft” that are higher than previously possible. Suggested dosing rates — which vary with media type, depth, and loading — for regular operation range from 25 to 200 mm (1 to 8 in.) per pass; suggested flushing rates range from 100 to 600 mm (4 to 24 in.) per pass

The maintenance of good growth of organisms living on the filter media is crucial to a successful operation. The term “filter” is rather misleading because it indicates that solids are separated from the liquid by a straining action.  Passage of wastewater through the filter causes the creation of a gelatinous coating of bacteria, protozoa, and other organisms living on the media.

This growth of organisms absorbs and uses much of the suspended, colloidal, and dissolved organic matter from the wastewater as it passes over the growth in a rather thin film. Part of the material is used as food for the production of new cells, while another portion is oxidized to carbon dioxide and water. The partially decomposed organic matter together with excess and dead film is continuously or periodically washed (sloughed) off and passes from the filter with the effluent.


Slow Rate Trickling Filters


Low-rate filters are commonly used for loadings of less than 40 kilograms five-day biochemical oxygen demand (BOD5 )/100 meters cubed per day (25 lb BOD5 /1000cu ft/day). These systems have fewer problems than other filters with regard to filter flies, odors, and medium plugging because of the lower loading rate. Low-rate filters with a rock medium range in depth from 0.9 to 2.4 meters (3- 8 ft.). Most low-rate filters are circular with rotary distributors, but some filters currently in use are rectangular.

The sloughed solids from a low-rate filter are generally well-digested and as a result, these filters yield fewer solids than higher-rate filters. Secondary quality effluent is readily achievable if the low-rate trickling filter design incorporates filter media with flocculation capabilities or good secondary clarification.

Intermediate rate filters can be loaded up to 64 kg BOD5 /100 m3 -d (40 lb BOD5 /1000cu ft/day). In order to ensure good distribution and thorough blending of the filter and secondary effluent, the system should recirculate the trickling filter effluent. The biological solids that slough from an intermediate trickling filter are not as well digested as those using a low-rate filter. 


Medium-High Rate Trickling Filters


High-rate filters High-rate filters are generally loaded at the maximum organic loading capabilities of the filter and receive total BOD5 loading ranging from 64 to 160 kg BOD5 /100 m3 -d (40 to 100 lb. BOD5 /1000cu ft/day). Achieving a secondary quality effluent is less likely for a high-rate filter without a second-stage process. As a result, high-rate filters are often used with combined processes.

Roughing filters are designed to allow a higher amount of soluble BOD to bleed through the trickling filter. Filters of this type generally have a design load ranging from 160-480 kg BOD5 /100 m 3 -d (100 to 300 lb. BOD5 /1000cu ft/day). 

Recent efforts have been made to combine fixed film reactors with suspended growth processes to efficiently remove organic materials from wastewater. For example, the combination of a trickling filter with an activated sludge process has allowed for the elimination of shock loads to the more sensitive activated sludge while providing a highly polished effluent that could not be done by a trickling filter alone.

 Although the Trickling filter process is typically reliable, there is still a chance for mechanical problems. Some of the common problems are attributed to increased growth of biofilm, improper design, changing wastewater characteristics, or equipment failure.

Excessive biological growth can Reduce organic loading; and increase hydraulic loading to increase sloughing;

  • use a high-pressure stream of water to flush the filter surface (recycled plant water);
  • maintain 1 to 2 mg/L residual chlorine on the filter for several hours;
  • flood the filter for 24 hours;
  • shut down the filter to dry out media;
  • replace media if necessary
  • remove debris



Ponding on Filter Media with excessive biological growth;

  • Reduce organic loading; increase hydraulic loading to increase sloughing
  • Use a high-pressure stream of water to flush the filter surface (recycled water)
  • Maintain 1 to 2 mg/L residual chlorine on the filter for several hours
  • flood filter for 24 hours; shut down filter to dry out media
  • Replace media if necessary
  • Remove debris



What is the Difference Between BOD and COD?

The main difference between BOD and COD is that BOD is the amount of oxygen that is consumed by bacteria while decomposing organic matter under aerobic conditions whereas COD is the amount of oxygen required for the chemical oxidation of total organic & inorganic matter in H2O.…………………………………………………………………………………………. Read more


JimGalloway Author/Editor 




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