Effect of oxygen concentration on nitrification and denitrification in single activated sludge flocs.

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 microM, anoxic zones were detected within flocs with a larger diameter (approximately 3000 microm). 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 microM, and remained constant thereafter. SND was observed in the aerated activated sludge when O(2) concentration was in the range of 10 to 35 microM.