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A trickling filter is a type of wastewater treatment system. It consists of a fixed bed of rocks, lava, coke, gravel, slag, polyurethane foam, sphagnum peat moss, ceramic, or plastic media over which sewage or other wastewater flows downward and causes a layer of microbial slime (biofilm) to grow, covering the bed of media. Aerobic conditions are maintained by splashing, diffusion, and either by forced air flowing through the bed or natural convection of air if the filter medium is porous.
The terms trickle filter, trickling biofilter, biofilter, biological filter and biological trickling filter are often used to refer to a trickling filter. These systems have also been described as roughing filters, intermittent filters, packed media bed filters, alternative septic systems, percolating filters, attached growth processes, and fixed film processes.
The removal of pollutants from the wastewater stream involves both absorption and adsorption of organic compounds by the layer of microbial biofilm. The filter media is typically chosen to provide a very high surface area to volume. Typical materials are often porous and have considerable internal surface area in addition to the external surface of the medium. Passage of the wastewater over the media furnishes dissolved air, the oxygen which the slime layer requires for the biochemical oxidation of the organic compounds and releases carbon dioxide gas, water and other oxidized end products. As the biofilm layer thickens, it eventually sloughs off into the treated effluent and subsequently forms part of the secondary sludge. Typically, a trickling filter is followed by a clarifier or sedimentation tank for the separation and removal of the sloughing. Other filters utilizing higher-density media such as sand, foam and peat moss do not produce a sludge that must be removed, but require forced air blowers and backwashing or an enclosed anaerobic environment.
The treatment of sewage or other wastewater with trickling filters is among the oldest and most well characterized treatment technologies.
The three basic types of trickle filters are used for:
- the treatment of small individual residential or rural sewage
- large centralized systems for treatment of municipal sewage
- systems applied to the treatment of industrial wastewater.
Septic system leach field
This is the simplest form of waste liquid disposal system, typically using pipes buried in loose sand or gravel to dissipate the liquid outflow from a septic tank. Liquid purification is performed by a biofilm which naturally forms as a coating on the sand and gravel in the absorption field and feeds on the dissolved nutrients in the waste stream.
Due to the system being completely buried and generally isolated from the surface environment, the process of waste breakdown is slow and requires a relatively large surface area to absorb and process liquid wastes. If too much liquid wastes enter the field too quickly, the wastes may pass out of the biofilm before waste consumption can occur, leading to pollution of groundwater.
In order to prolong the life of a leaching field, one method of construction is to build two fields of piping side-by-side, and use a rotating flow valve to direct waste into one field at a time, switching between fields every year or two. This allows a period of rest to let the microorganisms have time to break down the wastes built up in the gravel bed.
In areas where the ground is insufficiently absorptive (fails the percolation test) a homeowner may be required to construct a mound system which is a special engineered waste disposal bed of sand and gravel mounded on the surface of the ground with poor liquids absorption.
Leach field dosing
Generally it is better if the biofilm is permitted a period of time to rest between liquid influxes and for the liquids to be evenly distributed through the leaching bed to promote biofilm growth throughout the pipe network. Typically flows from septic systems are either small surges (handwashing) or very large surges (clothes washer emptying), resulting in highly erratic liquid outflow into the field and uneven biofilm growth concentrating primarily around the field inlet and dropping off in the outer reaches of the piping system.
For this reason it is common for engineered mound systems to include an electrically powered dosing system which consists of a large capacity underground storage tank and lift pump after the septic tank. When the tank fills to a predetermined level, it is emptied into the leaching field.
The storage tank collects small outflows such as from handwashing and saves them for dosing when the tank fills from other sources. During this fill period the field is able to rest continuously. When full, the discharge dose fills out the entire field completely to the same degree of flow, every time, promoting an even biofilm growth throughout the system.
Dosing systems have maintenance requirements over traditional non-powered surge systems. The pump and float system can break down and require replacement, and the dosing system also needs electricity. However, the system can be designed so that in the event of power failure the storage tank overflows to the field operating in the traditional surge-flow manner until power is restored or repairs can be done.
Soil compaction issues
The biofilm is most productive if the absorption field is loosely packed, to permit easy air infiltration down into the biofilm bed. Consequently the land over the leaching field is often a restricted area where large vehicles cannot be allowed to drive, because the heavy weight will compact the bed, and potentially cause system failure due to hindering of biofilm growth.
One method to help prevent compaction of the field is to place a U-shaped cover over gravel trenches in the bed, with a dosing pipe suspended above the bed by the cover. Any weight from above is passed to the sides of the trench keeping the bed directly under the cover free from compaction.
Sewage treatment trickle filters
Onsite sewage facilities (OSSF) are recognized as viable, low-cost, long-term, decentralized approaches to sewage treatment if they are planned, designed, installed, operated and maintained properly (USEPA, 1997).[full citation needed]
Sewage trickling filters are used in areas not serviced by municipal wastewater treatment plants (WWTP). They are typically installed in areas where the traditional septic tank system are failing, cannot be installed due to site limitations, or where improved levels of treatment are required for environmental benefits such as preventing contamination of ground water or surface water.
All varieties of sewage trickling filters have a low and sometimes intermittent power consumption. They can be somewhat more expensive than traditional septic tank-leach field systems, however their use allows for better treatment, a reduction in size of disposal area, less excavation, and higher density land development.
Configurations and components
All sewage trickling filter systems share the same fundamental components:
- a septic tank for fermentation and primary settling of solids
- a filter medium upon which beneficial microbes (biomass, biofilm) are promoted and developed
- a container which houses the filter medium
- a distribution system for applying wastewater to be treated to the filter medium
- a distribution system for disposal of the treated effluent or percolation ponds.
By treating septic tank effluent before it is distributed into the ground, higher treatment levels are obtained and smaller disposal means such as leach field, shallow pressure trench or area beds are required.
Systems can be configured for single-pass use where the treated water is applied to the trickling filter once before being disposed of, or for multi-pass use where a portion of the treated water is cycled back to the septic tank and re-treated via a closed loop. Multi-pass systems result in higher treatment quality and assist in removing Total Nitrogen (TN) levels by promoting nitrification in the aerobic media bed and denitrification in the anaerobic septic tank.
Trickling filters differ primarily in the type of filter media used to house the microbial colonies. Types of media most commonly used include plastic matrix material, open-cell polyurethane foam, sphagnum peat moss, recycled tires, clinker, gravel, sand and geotextiles. Ideal filter medium optimizes surface area for microbial attachment, wastewater retention time, allows air flow, resists plugging and does not degrade. Some residential systems require forced aeration units which will increase maintenance and operational costs.
Third-party verification of trickling filters has proven them to be a reliable alternative to septic systems with increased levels of treatment performance and nitrogen removal. Typical effluent quality parameters are Biochemical Oxygen Demand (BOD), Total suspended solids (TSS), Total Kjeldahl Nitrogen (TKN), and fecal coliforms.
The leading testing facility in the United States is the Massachusetts Alternative Septic System Test Center, a program of the Buzzards Bay National Estuary Program. Testing conducted here includes the stringent Environmental Technology Initiative (ETI) where systems are tested in triplicate over two years, and the Environmental Technology Verification (ETV) program which is funded by the U.S. Environmental Protection Agency (EPA) and includes stress testing as well as evaluation of nitrogen removal over 14 months. Systems are approved for installation by local, state and federal regulations and controls.
Industrial wastewater treatment trickle filters
Wastewaters from a variety of industrial processes have been treated in trickling filters. Such industrial wastewater trickling filters consist of two types:
- Large tanks or concrete enclosures filled with plastic packing or other media.
The availability of inexpensive plastic tower packings has led to their use as trickling filter beds in tall towers, some as high as 20 meters. As early as the 1960s, such towers were in use at: the Great Northern Oil's Pine Bend Refinery in Minnesota; the Cities Service Oil Company Trafalgar Refinery in Oakville, Ontario and at a kraft paper mill.
The treated water effluent from industrial wastewater trickling filters is very often subsequently processed in a clarifier-settler to remove the sludge that sloughs off the microbial slime layer attached to the trickling filter media (see Image 1 above).
Currently, some of the latest trickle filter technology involves aerated biofilters which are essentially trickle filters consisting of plastic media in vessels using blowers to inject air at the bottom of the vessels, with either downflow or upflow of the wastewater.
- Activated sludge
- Aerated lagoon
- Industrial wastewater treatment
- Rotating biological contactors
- Sewage treatment
- List of waste-water treatment technologies
- Trickle-bed reactor
- King Fahd University of Petroleum and Minerals, Course ChE 101.11 Saudi Aramco Engineering Development Program, pages 62-65 including Figure 11
- Biological filter and process U.S. patent 4,351,729, September 28, 1982, Assigned to Celanese Corporation
- Lecture by Dr. Allen Davis, Auburn University, page 6 of 8 pdf pages including schematic of packed tower trickling filter)[full citation needed]
- Beychok, Milton R. (1967). Aqueous Wastes from Petroleum and Petrochemical Plants (1st ed.). John Wiley & Sons Ltd. LCCN 67019834.
- E.H. Bryan and D.H. Moeller, Aerobic Biological Oxidation Using Dowpac, Paper 42, Conference on Biological Waste Treatment, Manhattan College, April 20, 1960. 
- Marcus Van Sperling (2007). Activated Sludge and Aerobic Biofilm Reactors. IWA Publications. ISBN 1-84339-165-1.