A geotextile support is used for wastewater treatment by aerobic, attached-growth microorganisms. The relative process which is developed in this paper, works like a submerged bed. The geotextile medium makes easier the installation and operation of treatment plants in comparison with granular biofilters. The colonization of the selected medium takes place in a pilot plant of 0.75 m3. The process does not need a secondary clarifier. The reactor works in cycles varying between 2 and 12 days interrupted by backwashing and air scour. The maximum efficiency, measured as inlet volumetric loading, is about 7 kg COD/m3.day . Under these conditions, a cycle of 3–4 days assures an effluent quality characterized as level “e” by French standards (TSS 30 mg/l, BOD5 30 mg/l, COD 90 mg/l). Two phases can be distinguished during the operation of the system : In phase A, the biomass and the TSS are held on the geotextile. Thus we avoid the use of a secondary settlement tank. The phase A stands for a working cycle. When backwashing does not take place, the operation of the biological reactor carries on in phase B where, by a saturation effect, the geotextile cannot hold any more biomass. In this way, an abrupt increase of the TSS is observed in the effluent. During phase A, the particulate loading of the treated water depends, at the same time, on biofilm detachment, which is a function of the water flowrate, and of the growth of the suspended bacteria. The efficiency of the treatment is controlled by the TSS at the outlet of the reactor.

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