A trickling filter consists of a bed of highly permeable media on whose surface a mixed population of microorganisms is developed as a slime layer.  The word "filter" in this case is not correctly used for there is no straining or filtering action involved.  Passage of wastewater through the filter causes the development of a gelatinous coating of bacteria, protozoa and other organisms on the media.  With time, the thickness of the slime layer increases preventing oxygen from penetrating the full depth of the slime layer.  In the absence of oxygen, anaerobic decomposition becomes active near the surface of the media.  The continual increase in the thickness of the slime layer, the production of anaerobic end products next to the media surface, and the maintenance of a hydraulic load to the filter, eventually causes sloughing of the slime layer to start to form.  This cycle is continuously repeated throughout the operation of a trickling filter.  For economy and to prevent clogging of the distribution nozzles, trickling filters should be preceded by primary sedimentation tanks equipped with scum collecting devices. 

Primary treatment ahead of trickling filters makes available the full capacity of the trickling filter for use in the conversion of non-settleable, colloidal and dissolved solids to living microscopic organisms and stable organic matter temporarily attached to the filter medium and to inorganic matter temporarily attached to the filter medium and to inorganic matter carried off with the effluent.  The attached material intermittently sloughs off and is carried away in the filter effluent.  For this reason, trickling filters should be followed by secondary sedimentation tanks to remove these sloughed solids and to produce a relatively clear effluent.  

Construction and Design

The primary factors that must be considered in the design of trickling filters include:

1. the type of filter media to be used,
2. the type and dosing characteristics of the distribution system, and
3. the configuration of the underdrain system. 

Filter Media:  The ideal filter medium is a material that has a high surface area per unit volume, is low in cost, has a high durability, and does not readily clog.  The choice of filter media is more often governed by the material locally available which may include field stone, gravel, broken stone, blast furnace slag and antracite stones.  Stones less than one inch in diameter do not provide sufficient pore space and may therefore result in plugging of the media and ponding.  The tendency is to use larger sizes with 2 1/2 inches in diameter now considered the minimum size.  Large diameter stones tend to avoid ponding situations but also limit the surface area per unit volume available for the slime layer to grow.  An upper size limit of about 4 inches is therefore recommended.

Distribution System:  The rotary distributor has become standard for the trickling filter process because of its reliability and ease of maintenance.  The rotary distributor consists of a hollow vertical center column carrying two or more radial pipes or arms, each of which contains a number of nozzles or orifices for discharging the wastewater onto the bed.  All of these nozzles point in the same direction at right angles to the arms and the reaction of the discharge through them causes the arms to revolve.  The necessary reaction is furnished by a head of 18" to 24".  The speed of revolution will vary with the flow rate, but it should be in the range of one revolution in 10 minutes or less for a two-arm distributor.  A dosing tanks and siphon should be provided for standard rate trickling filters to shut off the flow when the head falls below that necessary to revolve the arms at the required speed.  In some cases positive drive mechanisms are being used.

A clearance of 6 to 9 inches should be allowed between the bottom of the distributor arm and top of the bed.  This will permit the waste streams from the nozzles to spread out and cover the bed uniformly, and it will also prevent ice accumulation from interfering with the distributor motion during freezing weather.

Fixed spray nozzles were used when trickling filters were first developed.  The nozzles were attached to pipes laid in the filter medium and were fed intermittently from a siphon controlled dosing tank.  By this method, wastewater is applied to the filter for short periods of time.  Between applications the filter has rest periods while the dosing tank is filling.  Many types and shapes of nozzles were developed and the siphon dosing tank was designed to attain the best possible even distribution of wastewater over the entire surface of the filter.  At best, the distribution was not even and there were areas of the filter on which very little wastewater was sprayed. 

In addition, due to the greater number of nozzles used for the distribution of the wastes, clogging and increased operational and maintenance problems were encountered.  

Underdrain System:  The underdrain system in trickling filters serves two purposes:  (a)  to carry the wastewater passing through the filter and the sloughed solids from the filter to the final clarification process, and (b)  to provide for ventilation of the filter to maintain aerobic conditions.  The underdrains are specially designed vitrified clay blocks with slotted tops that admit the wastewater and yet support the media.  The blocks are laid directly on the filter floor, which is sloped toward the collection channel at a 1 to 2 percent gradient.  Since the underdrains also provide ventilation for the filter it is desirable that the ventilation openings total at least 20% of the total floor area.  Normal ventilation occurs through convection currents caused by a temperature differential between the wastewater and the ambient air temperature.  In deep filters or heavily loaded filters, there may be some advantage in force ventilation.

http://water.me.vccs.edu/courses/ENV149/trickling.htm