Trickling Filters

1. Description: Another type of reactor has been developed which uses a kind of a permanent biological growth support. These reactors are called assisted biological growth reactors. Trickling filters belong to this type of assisted growth reactors.

A trickling filter is a filling covered with biological silt through which sewage flow seep. Usually sewage is distributed in the form of a uniform spray over the filler bed by means of a rotary distributor of flow. Residual water migrates downward through the filling media and the effluent is collected at the bottom. The layer of slime that forms along the filling has a total thickness between 0.1 and 2.0 mm and is made up of an aerobic sublayer and another anaerobic one.

A percolating filter consists of a tank containing a bed of a thick material, composed in the majority of cases of synthetic materials or stones of various shapes and a high area/volume ratio, on which is applied sewage water by way of fixed or mobile distributing arms. Around this bedding a bacterial population that breaks down the waste water is lain which percolates down to the bottom of the tank. After some time, the bacterial layer becomes very thick and is hydraulically detached from the bed of stones to then be moved to a secondary clarifier where the separation from the sludge is made.

The biological filtration process can be defined as a system of beds, composed in the majority of cases of synthetic materials or stones of various forms and of a high area/volume ratio on which sewage is continuously or intermittently applied by fixed or mobile dispersement arms. A product of the application of the sewage to the filter material, the micro-organisms formed as a bio-film attached to this material can come into contact with organic loads to begin the purification process. On aerobic bed, conditions are maintained by means of the air flow through the bed, which can be done by natural means, induced by the existing temperature gradients between the temperature of the air in the bed and the environmental temperature and by forced aeration, using equipment similar to exhaust fans. Having access to sewage, rich in organic matter can absorb the necessary oxygen for cell synthesis (bacterial growth), the bio-film of aerobic microorganisms begins the unfolding of organic matter obtaining in the same way as other biological processes for wastewater treatment the removal of organic matter by its conversion to cell mass, CO₂ and H₂O, which translates into a sewage cleaning that make up the new effluent that, depending on the case, may require subsequent treatments if technical specifications so require. The product of bacterial growth in the filter medium, will reach a limit in which bacteria will receive neither oxygen nor the necessary nutrients for survival so it will die off and end up detaching the bio-film of the medium. This fact requires a process of sedimentation that will take care of the detached material.

2. Ranges of design: In principle this technology does not have an upper limit for its use, as there are proofs of trickling filters dealing with wastewater for populations of more than 100,000 inhabitants. If a lower limit could be used, given a certain complexity of the technology, which could be about 500 inhabitants, the following types could be offered:

     a. Low load filters.

     b. High load filters.

     c. Conventional or low rate trickling filters.

     d. Medium rate trickling filters.

     e. High rate filters.

     f. Super high rate filters.

3. Important factors to be considered in the design:

     a. Geometry.

     b. Means of support.

     c. Filter depth.

     d. Configuration.

     e. Recirculation.

     f. Ventilation.

     g. Distribution of flow.

4. Benefits:

     a. Equipment with excellent chemical resistance.

     b. Low operation and maintenance cost.

     c. Neutralization of pollutants prior to their release into the atmosphere.