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In this type of treatment technology, known also as biological or trickling filters, the biomass is in the form of a thin film, which is attached to a solid surface, such as gravel, stone or plastic materials. There is a large variety of types of biofilm support media, which have large specific area in order to provide a better contact between the wastewater and the biofilm. The voids within the filter media allow the wastewater to trickle slowly downwards, and also provide passage for the air, which circulates in opposite direction to the water flow. Thus, in the classic applications, aeration is provided by natural means. In order for a biological filter to function properly, it requires a well-distributed flow, watering at regular intervals, and a relatively high hydraulic load; the last is necessary for the removal of excess sludge from the biofilm layer. For this purpose, recirculation (by a pump) of the treated effluent must be provided.
In the illustration shown on Figure 12, the biological filter unit is located after a septic tank for preliminary sedimentation of solids, in the upper part of the treatment unit. There are dosing pumps to lift the effluent from the septic tank to the top. The bottom part (conical) serves as sedimentation tank. The excess sludge may be pumped out by a submerged pump at the bottom of the tank, which also serves for recirculation of the treated effluent. The unit has a ventilation pipe to supply the necessary air for the biological process.
Figure 12 A scheme of a bio-filter (trickling filter)
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