A trickling filter consists of a fixed bed of rocks, gravel, slag, polyurethane foam, sphagnum peat moss, or plastic media over whichsewage or other wastewater flows downward and causes a layer or film of microbial slime to grow, covering the bed of media. Aerobicconditions 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 process mechanism, or how the removal of waste from the water happens, involves both absorption andadsorption of organic compounds within the sewage or other wastewater by the layer of microbial slime. Diffusion 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 slime layer thickens, it becomes more difficult for air to penetrate the layer and an inner anaerobic layer is probably formed. This slime layer continues to build until it eventually sloughs off, breaking off longer growth into the treated effluent as a sludge that requires subsequent removal and disposal. 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 terms trickle filter, trickling bio-filter, bio-filter, 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 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.

Filter Classification:

Trickling filters are classified by hydraulic or organic loading, as high-rate or low-rate.

The organic load on a filter is the BOD content in pounds applied to the filter.  This is usually expressed as pounds of BOD per day per 1000 cubic feet of filter medium or pounds of BOD per day per acre foot.  The hydraulic load, including recirculation flow if used, is the gallons of flow per acre of filter surface per day.

Low-rate filters are relatively simple treatment units that normally produce a consistent effluent quality even with varying influent strength.  Depending upon the dosing system, wastewater is applied intermittently with rest periods which generally do not exceed five minutes at the designed rate of waste flow.  With proper loadings the low-rate trickling filter, including primary and secondary sedimentation units, should remove from 80 to 85 percent of the applied BOD.  While there is some unloading or sloughing of solids at all times, the major unloadings usually occur several times a year for comparatively short periods of time.

High-rate filters are usually characterized by higher hydraulic and organic loadings than low-rate filters.  The higher BOD loading is accomplished by applying a larger volume of waste per acre of surface area of the filter.

One method of increasing the efficiency of a trickling filter is to incorporate recirculation.  Recirculation is a process by which the filter effluent is returned to and reapplied onto the filter.  This recycling of the effluent increases the contact time of the waste with the microorganisms and also helps to “seed” the lower portion of the filter with active organisms.

When recirculation is used, the hydraulic loading per unit area of filter media is increased.  As a result, higher flow velocities will usually occur causing a more continuous and uniform sloughing of excess growths.  Recirculation also helps to minimize problems with ponding and restriction of ventilation.

Recirculation can be continuous or intermittent.  Return pumping rates can either be constant or variable.  Sometimes recycling can be practiced during periods of low flow to keep the distributors in motion, to prevent the drying of the filter growths, and to prevent freezing during colder temperatures.  Also, recirculation in proportion to flow may be utilized to reduce the organic strength of the incoming wastes, and to smooth out diurnal flow variations.

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.

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) are ideally suited for 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 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.

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 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