The wastewater was treated at 25 degrees C with active archaea-enriched sludges at const. O concn. (1.3-3.2%). The highest total N removal efficiency was achieved for the treatment carried out without archaea. The addn. of archaea substrate to the wastewater treatment plant reactor (ap-prox. 4 and 8 kg during first 320 days and 8 g during next 280 days) resulted in a decrease in COD and total N concn. after increasing the archaea dosis.