Simultaneous nitrification and denitrification (SND) was investigated in the single aeration tank of a municipal wastewater treatment plant. Microelectrode measurements and batch experiments were performed to test for the presence of SND. Microelectrodes recorded the presence of O-2 concentration gradients in individual activated sludge flocs. When the O-2 concentration in the bulk liquid was <45 μM, anoxic zones were detected within flocs with a larger diameter (approximately 3000 pm). The O-2 penetration depth in the floc was found to be dependent on the O-2 concentration in the bulk liquid. Nitrification was restricted to the oxic zones, whereas denitrification occurred mainly in the anoxic zones. The nitrification rate of the activated sludge increased with increasing O-2 concentration in the bulk liquid, up to 40 μM, and remained constant thereafter. SND was observed in the aerated activated sludge when O-2 concentration was in the range of 10 to 35 μM. (C) 2003 Wiley Periodicals, Inc.