Developed for tertiary nitrification, this biofilter also removed carbonaceous BOD (cBOD) and (SS). Because the biofilter is expanded, it cannot clog, and therefore does not require backflushing; yet, it removed a significant proportion of the influent SS. This unanticipated capability was due to the activities of heterotrophic bacteria, protozoa, and metazoa (nematode and oligochaete worms). The expanded bed is an intensified process, which is based on natural immobilization of microbes to small support particles. Using glassy coke as the support material, an attached layer of microbes develops, forming particulate biofilms having a superficial surface area of 1800 m(2) m(-3). Autotrophic nitritifiers (Nitrosomonas spp.) were detected in the biofilm using rRNA-based molecular methods and were likely responsible, at least in part, for reducing the ammonia concentration by up to 99% (to 0.1 mg L-1), while the other organisms reduced cBOD and SS by up to 56% and 62 respectively. Furthermore, the influent concentrations of Escherichia coli, coliform and heterotrophic bacteria were reduced by over 80 It thereby provides a single process solution for combined tertiary nitrification and solids removal. Operating the process to consistently achieve <0.5 mg NH3N L-1 and at the same time removing a significant fraction of cBOD and SS, it can replace processes such as SAFs or NTFs followed by a sandfilter.