The presence of dissolved oxygen (DO) in biological denitrification reactors determines inhibition effects on the denitrification rate. The article shows the results of an experimental study to control the DO concentration in the pre-denitrification stage by a post-anoxic reactor. The results demonstrate that the post-anoxic reactor is very effective in improving the nitrogen removal efficiency because it causes a considerable reduction of the DO content in the mixed liquor recycle sent to the pre-denitrification reactor. This reduction is influenced by both the retention time and the F
ratio (referred to the denitrification and the oxidation-nitrification volume). In fact, a retention time and a F
ratio equal to 1.5 h and 0.130 kgBOD5
, respectively, allow to limit DO in the post-anoxic reactor at 0.31 mgO2
. Such concentration determines a DO concentration of 0.11 mgO2
in the pre-denitrification reactor and, consequently, a denitrification efficiency of 91%. Moreover, the contribution of the endogenous denitrification to the whole denitrification efficiency was found negligible. The paper contributes to the progress in nitrogen removal from sewage, a fundamental issue for a sustainable management of water resources.
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