The given name, “Biological Filter”, does not portray the mechanism of the process. In fact, the denomination is incorrectly employed because no sieving or filtration operation is carried out. Briefly, the contact of the influent sewer with the biological mass contained in the Biological Filter carries out a chemical oxidation. The mechanism of the process is characterized by feeding and continuous percolation of the sewage through the support medium, which promotes the growth and adhesion of biological mass on its surface.
Under favorable conditions the biological mass added to the support medium retains the organic matter contained in the sewage through the adsorption. The synthesis of new cells promotes the increase of the biomass damaging the passage of oxygen to the innermost layers, near the surface of the support medium, where the oxidation process is carried out anaerobically. The byproducts of the oxidation of the compounds by the anaerobic and aerobic bacteria are: CO2, HNO3 and H2SO4.
Finally, the accumulated gases produced in the anaerobic layer cause detachment of the aggregated biological mass to the support medium, facilitating its drag through the sewage stream. This material constitutes the sludge, removed by sedimentation in the secondary decanters.