The objective of this investigation was to show that the low nitrate bend in the redox potential curve is in fact the result of nitrite inhibition of oxygen uptake. The oxygen sensor used indicated traces of oxygen at the time of the observed low nitrate bend which supports the hypothesis. However, an oxygen sensor with a lower measuring range is needed in order to show conclusively that this hypothesis is true. At higher oxygen concentrations it was observed that nitrite inhibition of oxygen uptake had a marked effect on the appearance of both the oxygen concentration curve and the redox potential in activated sludge. Therefore a complete model of the redox potential in activated sludge must include nitrite inhibition of oxygen uptake. As part of the investigation, the response time of the redox electrode was found to be 5 times longer than the response time of the oxygen sensor. This indicates that the diffusion barrier between the bulk liquid in the sludge and the redox electrode surface is greater than the diffusion barrier of the oxygen sensor's membrane.
Keywords: activated sludge, nitrite inhibition, nitrogen removal, ORP, redox potential