The performance of a particulate-biofilm, expanded-bed process for nitrification of activated sludge final effluent (ASFE) is reported for a plant receiving mixed industrial and domestic wastewater. The support material for the particulate-biofilms was glassy coke, to which the nitrifying bacteria attached and formed a highly active biofilm. An average nitrification rate of 1.7+/-0.6 kg m(expanded bed)(-3)d(-1) was recorded during operation of the bioreactor, which had a hydraulic residence time of 15 min. On average, the ASFE contained 12.6+/-3.7 g m(-3) NH3-N, which was reduced to 2.6+/-3.3 g m(-3) NH3-N. Furthermore, transfer of 10-12% of the oxygen in air was achieved using counter-current aeration. This investigation has demonstrated that a high rate of nitrification can be achieved with a particulate-biofilm, expanded-bed process. It has also demonstrated that the process can operate without backwashing and still remove particulate material from the ASFE feed.