Abstract
A fine (10 nun) plastic medium particle used to fix the nitrifying bacteria was implemented as a submerged biological filter in a tertiary nitrification pilot plant. A specific nitrification rate of 0.8 kg NH4+‐N.m3.d‐1 at 20°C was steadily obtained. Suspended Solids filtration and Chemical Oxygen Demand elimination were studied. The hydraulic behavior of the medium seems to be the hindering factor in the optimal plant operation, mainly because of the poor hydraulic replacement of substrates at the biofilm surface when the medium is not in motion.