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The biofilm net filtration rate quickly stratification: first, a layer will cover an aerobic anaerobic layer,

on the other hand, the surface layers are composed of young cells in contrast to the deep layers. The bacterial autolysis depth cause the detachment of biofilm: what will be the production of excess sludge.
Interest: compared to free bacterial cultures (activated sludge) are expected fixed cultures, concentrations and specific activity higher reactor volumes therefore less important.
* Trickle

beds. It is the raw water stream, prior decanted, on a mass of porous

material or cavernosum. We can distinguish trickling filled with traditional pozzolan, slag or cinder ... (ERU rather see STEP-Redon die obsolete), and filling plastic materials

in bulk or ordered (rather net filtration rate ERI (IAA) - pre STEP DERVAL die modern).
These beds are followed by a clarifier. Recirculation of the effluent is still practiced to allow, first self-cleansing of the trickling filter, then the dilution of U.S. high BOD5, seeding raw water decanted, and avoid drying of materials in case of low output.
This technique requires a lot of engineering (large

investment), but allows a good reduction of BOD5 (90%). It is suitable for highly loaded wastewater BOD5 and of great interest to the food industry, for example, treatment of pre-admission to the public network.
* Biofilters or granular beds: one seeking specific surfaces developed well above the previous method by balls of special materials (Biolite or polystyrene). Forced ventilation is provided. Same time, ensures the retention of solids and sludge produced, avoiding the secondary clarifier. The material is periodically regenerated by washing (air-water). The wash water is returned to the primary clarifier.
Sand filters and reed bed filters for small communities are a variant of this extensive process. They are not washed, but put to rest for total consumption of organic matter. Plants for the sole purpose of maintaining the massive

hydraulically opened.
* Biological drives large diameter (2-3 m), plastic lighter, rotate around a vertical axis and partially immersed in the treated water. Requires all the coverage and can not agree stations and small (<5000 pe).
Conclusion: This traditional technique, dethroned by the activated sludge process, which would have smaller investment costs and better performance, is experiencing a resurgence of interest with improved processes (granular beds artificial ventilation).
The advantages net filtration rate are reliability, ease of operation, low power consumption (reduced traditional benefits with improved processes) and especially the compact structures allowing their coverage, which greatly limits the odors and noise, and finally the flexibility with respect to load variations.
The disadvantages are high capital costs and new processes, maintenance net filtration rate costs higher. One can also note a sensitivity to low temperatures, and the risk of clogging in certain net filtration rate processes, low yields for N and P in the channels rustic.
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treatment plant of Saint-Thibault-des-vines net filtration rate (Unit 3 in particular, but not only) and answer (make copies net filtration rate of) the following questions on the T3 (biofilters)
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