A three-stage biological aerated filter (BAF) pilot plant has been run, fed with a mechanically pre-treated municipal wastewater. The pilot plant consisted of a physico-chemical lamella sedimentation unit for suspended solids removal and soluble phosphorus precipitation, followed by three stages of up-flow biofilters: a BAF C for organic carbon removal, a BAF N for nitrification and a biological anoxic filter (BaF DN) for post-denitrification with methanol as an external organic carbon source. The main objective of the experiments was to study the process limits of this pilot plant under overloading conditions, and to evaluate the time necessary for recovery of normal operation after a period of strong overloading. The pilot plant was first operated at a nominal wastewater flow rate of 12 m3/h (according to previous conventional design rules for first generation of up-flow BAF), which was progressively increased up to 25 m3/h, a maximum value still enabling us to achieve the final effluent standards. Beyond this flow rate, a high organic carbon load applied to the BAF C was only partially removed by this filter, the rest had to be eliminated in the BAF N unit. Consequently, the factor directly limiting the tested pilot plant was the COD overloading of the BAF C, which indirectly influenced the nitrification in the BAF N. The maximum applied load eliminated by the pilot plant, at a wastewater flow rate of 25 m3/h, was 2 times higher than the nominal applied load at a wastewater flow rate of 12 m3/h. In the second experimental period, short-term strong overloadings were applied to the pilot plant operating in steady state conditions at a nominal flow rate, and the limiting process parameters were detected. A lower short-term overloading (32 m3/h = 2.66 times the nominal flow rate) had no effect on the quality of the final effluent, and different filters were able to remove short-term loadings as high as 25 kg COD/m3.d (BAF C), 1.6 kg NH4-N/m3.d (BAF N) and 3.2 kg NO3-N/m3.d (BaF DN). These loadings represent respectively 1.33 times the maximum applied load found in the first set of experiments and 2.66 times the nominal load at a wastewater flow rate of 12 m3/h. A higher short-term overloading (40 m3/h = 3.33 times the nominal flow rate) led to the same consequences as in the first period: a high organic load (up to 32 kg COD/m3.d for the BAF C) was partially removed in the BAF N, decreasing thus its nitrification capacity. The recovery of normal operation after the period of overloading was almost immediate. This study showed that it is very important to maintain the specific populations in separate reactors, in order to reduce the competition between different bacterial species and to enable the microorganisms to grow under optimal conditions. The tested treatment configuration is particularly suited for treating peak flow with high removal rates.

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