A three-stage biofiltration process: Performances of a pilot plant

A three-stage biological aerated filter (BAF) pilot plant has been run, fed with a mechanically pre-treated municipal wastewater. The pilot plant cons isted 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 nitrifi cation and a biological anoxic filter (BaF DN) for post-denitrification wit h methanol as an external organic carbon source. The main objective of the experiments was to study the process limits of this pilot plant under overl oading conditions, and to evaluate the time necessary for recovery of norma l operation after a period of strong overloading. The pilot plant was first operated at a nominal wastewater flow rate of 12 m(3)/h (according to prev ious conventional design rules for first generation of up-flow BAF), which was progressively increased up to 25 m(3)/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 r ate of 25 m(3)/h, was 2 times higher than the nominal applied load at a was tewater flow rate of 12 m(3)/h. In the second experimental period, short-te rm strong overloadings were applied to the pilot plant operating in steady state conditions at a nominal flow rate, and the limiting process parameter s were detected. A lower short-term overloading (32 m(3)/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/m(3).d (BAF C), 1.6 kg NH4-N/m(3).d (BAF N) and 3.2 kg NO3-N/m(3).d (B aF DN). These loadings represent respectively 1.33 times the maximum applie d load found in the first set of experiments and 2.66 times the nominal loa d at a wastewater flow rate of 12 m(3)/h. A higher short-term overloading ( 40 m(3)/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/m(3).d for the BAF C) was partially removed in the BAF N, decreasing thus its nitrificati on capacity. The recovery of normal operation after the period of overloadi ng was almost immediate. This study showed that it is very important to mai ntain the specific populations in separate reactors, in order to reduce the competition between different bacterial species and to enable the microorg anisms to grow under optimal conditions. The tested treatment configuration is particularly suited for treating peak flow with high removal rates. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.