A moving‐bed biofilm reactor has been developed, where the biofilm grows on small, free‐floating plastic elements with a large surface area and a density slightly less than 1.0 g/cm3. Nitrification of preprecipitated wastewater in a once‐through system and raw wastewater in a recycled system with predenitrification has been studied in a pilot plant with an active, specific biofilm surface area of approximately 310 m2/m3. Maximum nitrification rates under comparable operating conditions were 20% to 25% higher with preprecipitated wastewater in the once‐through system than with raw wastewater in the recycled system. A temperature coefficient ofθ= 1.09 was found. However, under oxygen‐limited conditions, the apparent temperature effect was insignificant in the temperature range from 7 to 18°C because of increased dissolved oxygen levels at low temperatures. At 10°C volumetric nitrification rates from 300 to 400 g NH4‐N/m3·d can be expected.