Trickling filter

  A trickling filter consists of a fixed bed of rocks, gravel, slag, polyurethane foam, sphagnum peat moss, or plastic media over which sewage or other wastewater flows downward and is contacted with a layer or film of microbial slime covering the bed media. Aerobic conditions are maintained either by forced air flowing through the bed or natural convection of air if the filter medium is porous. The process mechanism involves adsorption of organic compounds in the sewage or other wastewater by the layer of microbial slime, diffusion of air into the slime layer to furnish the oxygen required for the biochemical oxidation of the organic compounds to release carbon dioxide gas, water and other oxidized end products. As the slime layer thickens, it becomes more difficult for air to penetrate the layer and an inner anaerobic layer is probably formed. For some plastic-mesh material filters this slime layer will build and eventually slough off the smooth plastic walls into the treated effluent as a sludge that requires subsequent removal and disposal. Other filters utilizing higher-density media such as sand, foam and peat moss do not produce a sludge that must be removed.

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 intermittent filters, packed media bed filters, alternative septic systems, percolating filters, attached growth processes, and fixed film processes.

The treatment of sewage or other wastewater with trickling filters is among the oldest and most well characterized treatment technologies.

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Types

Two of the basic types of trickle filters are those applied to the treatment of sewage and those applied to the treatment of industrial wastewater.

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).

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.

Sites with a high water table, high bedrock, heavy clay, small land area, or which require minimal site destruction (for example, tree removal) is desired are ideally suited to trickling filters.

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.

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.

Regulatory approvals

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.

System considerations

There are many considerations to take into account when choosing a sewage trickling filter:

• System footprint – will there be sufficient space at the installation site to accommodate the treatment unit?
• Robustness – how will the system be affected by seasonal use or in the event of a power outage?
• Treatment performance – will the system’s expected level of treatment ensure the receiving environment is not affected?
• Nitrogen removal - is there a need to protect the receiving environment from high levels of Nitrogen? Is the system approved for nitrogen reduction?
• Service requirements – are there many moving parts that will require regular maintenance and replacement?
• Electricity usage – what is the cost of operating the system? How many pumps, fans, or aeration units does it require?
• Aerobic sludge production – will the treatment unit produce unwanted aerobic sludge that must be trucked off site at the owner’s expense?
• Costs – is the cost of the system justified by its effectiveness?
• Installation costs – is the system difficult to install? How much excavation is required for installation?
• Aesthetics – how will the system impact the aesthetics of the site? Are above-ground systems attractive? How many risers do below-ground systems require? Will there be an unsightly raised disposal bed?

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.^[1]

• Vertical towers filled with plastic packing or other media.^[2][3]

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.^[4] 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.^[5]

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 of 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.^[6]

Regulatory requirements

Many countries regulate the composition of treated water effluents from industrial facilities. For example, in the United States, the Clean Water Act mandates a National Pollutant Discharge Elimination System (NPDES), which regulates industrial point sources that discharge pollutants into rivers, lakes, and oceans. All U.S. industrial facilities that discharge liquid effluents must obtain effluent discharge permits under that system.

See also

• Activated sludge
• Aerated lagoon
• Rotating biological contactors
• Biofilters
• Industrial wastewater treatment
• Sewage treatment
• Biolytix

References