The use of a Biological Fluidized Bed (BFB) was evaluated in a large demonstration plant for nitrification of a secondary effluent at Horley STW (South of London). The BFB was operated over a range of flows from 250→600 m3/d using effluents containing 25mg NH3-N/ℓ, 18mg BOD/ℓ to define the optimum operating conditions. It was possible to reduce from 27 down to 3mg NH3-N/ℓ in a contact time of only 38 mins (a volumetric loading of 1 kg NH3-N/m3.d). The unit was then operated under diurnal flow. A 20m deep below-ground downflow-bubble-contact oxygenator was used which was capable of dissolving 60mg 02/ℓ. Following the technical evaluation, an economic comparison was made between using the add-on nitrifying BFB against the option of extending an existing activated sludge plant. The BFB had a lower capital cost but higher operating cost. The BFB was shown to be cost competitive.