Internationally, phosphorus limitation in nitrification of municipal wastewater has been given little attention. However, pre-precipitation of low alkalinity wastewater normally results in phosphate concentrations significantly below 0.10 mg PO4-P/l. Tests have been carried out for tertiary nitrification in rotating biological contactors (RBCs), with different influent phosphorus concentrations. These tests showed that tertiary nitrification in biofilm reactors was clearly phosphate limited at influent concentrations below approximately 0.15 mg PO4-P/l. This means that with todays practice, extension of Scandinavian chemical treatment plants with downstream biofilm reactors for nitrogen removal (i.e. nitrification and post-denitrification) will be hampered by phosphorus limitation in the biological stages. Actions to increase the phosphate concentrations in these biological reactors must be taken.