Abstract
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:M ratio (referred to the denitrification and the oxidation-nitrification volume). In fact, a retention time and a F:M ratio equal to 1.5 h and 0.130 kgBOD5 kgMLVSS−1·day−1, respectively, allow to limit DO in the post-anoxic reactor at 0.31 mgO2·L−1. Such concentration determines a DO concentration of 0.11 mgO2·L−1 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.
Highlights
Anthropic activities such as industrial plants, intensive agriculture, zootechny, uncontrolled waste disposal and domestic wastewater discharge introduce many pollutants in the environment
The comparison between the results of Pilot plant 1 and 2 highlights the effect of the post-anoxic reactor on (i) the dissolved oxygen (DO) concentrations in the pre-denitrification tank and (ii) the denitrification performance. It stands out the higher denitrification efficiency and the lower content of DO in denitrification stage (DEN) achieved with Pilot Plant 2 compared to Pilot Plant 1
Small concentrations of dissolved oxygen are always present in biological pre-denitrification reactors
Summary
Anthropic activities such as industrial plants, intensive agriculture, zootechny, uncontrolled waste disposal and domestic wastewater discharge introduce many pollutants in the environment. The activated sludge of the oxidation reactor containing NO3-N is recirculated with high flow rate in the anoxic pre-denitrification tank in order to remove nitrates from wastewater. Where rDEN 20 °C = 2.9 ÷ 3.0 gNO3-N kgMLVSS−1·h−1 is the denitrification rate at 20 °C and θ is the temperature coefficient (θ = 1.026; θ = 1.07) Such a value can be significantly reduced by the presence of DO in the denitrification reactor. Preliminary batch tests with mixed liquor samples from an oxidation-nitrification stage were carried out in order to verify the DO behavior in anoxic conditions, while a pilot-plant with two parallel lines has been used to compare the denitrification performances with and without the presence of a post-anoxic reactor
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