A practical problem with using redox potential to control nitrogen removal in activated sludge is detecting the bend in the redox curve that occurs when the nitrate concentration is reduced to low levels. In this paper, a method for detecting the bend is proposed. The method is based on fitting a third-order polynomial to the whole redox curve during a denitrification period. The bend is then the point of inflection on this curve. This is the potential when the second derivative of the polynomial is zero. The practical application of the method is demonstrated in full scale. The method successfully detected the bend in about 90% of aeration denitrification cycles. However, this method gave no significant improvement in nitrogen removal and less than 4% reduction in electricity consumption when compared with control based on a fixed redox set point. Therefore, it is recommended that a fixed set point be used, as this is a much simpler and more robust method.

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