Airlift biofilm reactors are used for treating domestic and industrial waste water. The biomass is attached on small, in the water phase suspended carrier particles (diameter 200-300 µm). Due to this attached growth, a high biomass concentration (30 - 40 kg/m3 ) can be maintained in the reactor which allows high volumetric loadings. At loadings of about 8 kg COD/m3.day , COD removal and nitrification do occur simultaneously. Integration of an anoxic compartment in the reactor, results in denitrification as well. Compact/smooth biofilms will develop if the specific surface load does not exceed the level of about 10 g COD/m2.day.