The wastewater treatment plant Seine Aval at Achères is the largest plant in France, with an average flowrate of 25 m3/s. For the moment, this plant consists of an activated sludge process for organic carbon removal. However, due to the recent European water regulations, it will soon be necessary to remove nitrogen pollution. For this reason, tertiary nitrification prototype reactors were installed and tested, in order to examine a future upgrading of the plant. This article presents the results obtained, during the last two years, with an upflow BAF reactor utilizing a granular media with a density > 1. The objective will be to examine a key parameter of the process: the water velocity (part of the applied loading rate. The removal rate depends directly and linearly on the applied loading rate, as long as the nitrification conditions (aeration, temperature, etc …) are not limiting. The highest loading rates for nitrification, as well as for BOD5 removal and SS retention, were obtained at high water velocity (10 m3/m2.h). These results demonstrate that the hydraulic retention time, even when extremely short (10 minutes), does not influence the reactor performance. The high water velocities in this kind of filter positively influence and improve the mass transfer between the liquid environment and the bacteria.

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