Anoxic N2 fluxes from freshwater sediments in the absence of oxidized nitrogen compounds
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The denitrification potential, a key parameter in nutrient removal activated sludge systems, is mathematically described in terms of mass balance expressions for different carbon sources, namely, easily biodegradable substrate, slowly biodegradable substrate and biomass. Mass balance was derived both for single-anoxic (pre-denitrification) and dual anoxic (Bardenpho) systems. Correction factors for anoxic growth were experimentally determined using respirometry for domestic sewage and meat processing wastewater. The denitrification potential expressions were evaluated for different process configurations such as pre-denitrification, Bardenpho process and University of Cape Town (UCT) process.
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Abstract Dissolved oxygen (DO) is entrained by mixed liquid recirculation (MLR) into the anoxic basin in the pre‐denitrification biological nitrogen removal (PDBNR) process and should be removed to enhance denitrification. A laboratory‐scale PDBNR reactor with an anoxic basin divided into two isometric components was built to investigate the effect of a hydrocyclone on DO removal from the MLR and the subsequent denitrification. The denitrification rate of the mixture of influent and hydrocyclone‐treated nitrified wastewater increased with increasing DO and was higher than that of the wastewater in a conventional anoxic component. More total nitrogen was removed in the anoxic component due to the application of the hydrocyclone in the MLR of a laboratory‐scale reactor and a full‐scale anoxic/aerobic reactor.
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The denitrification kinetics at 12, 20 and 30°C in nitrification denitrification biological excess phosphorus removal (NDBEPR) systems were delineated in batch tests on sludge harvested from laboratory scale M/UCT systems. In some investigations, it was found that the P release and uptake were confined exclusively (>95%) to the anaerobic and aerobic reactors respectively and the observed P removal conformed to the BEPR model of Wentzel et al. In these investigations, due to an absence of anoxic P uptake (substantiated by PHB measurements), it could be inferred that the phosphate accumulating organisms (PAOs) did not significantly contribute to the denitrification. The ordinary heterotrophic organism (OHO) and PAO groups were separated with the aid of the BEPR model of Wentzel et al. Ascribing the denitrification to the OHO group performing this process, the specific rates of denitrification associated with the utilization of slowly biodegradable COD (SBCOD) in the primary (K′2) and secondary (K′3) anoxic reactors were calculated and compared with the rates in ND systems (K2 and K3). In other investigations it was found that P release and uptake were not confined exclusively to the anaerobic and aerobic reactors respectively and the observed P removal was only about 60% of that expected from the BEPR model of Wentzel et al. In these investigations significant P uptake under anoxic conditions was observed so the PAOs may have been involved with the denitrification. However, the denitrification rates were calculated as before by attributing it exclusively to the OHOs. Widely varying K'2 rates were observed at 20°C, ranging from 0.071 to 0.335 mgNO3-N/(mgAHVSS.d). The variation in K' rate is mainly due to widely varying OHO active fraction estimates for NDBEPR systems.
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Two packed bed column (PB) systems, namely anoxic-anoxic and anoxic-oxic were investigated for treatment of wastewater containing high concentrations of nitrogen (N) and COD. The anoxic-anoxic PB removal rates 6.70 kg N/m3 x d and 26.02 kg COD/m3 x d, respectively. The responding removal rates of the anoxic-oxic PB system were 7.41 kg N/m3 and 28.00 kg COD/m3 x d, respectively. The N and COD removal efficiencies of anoxic anoxic PB system were in the range of 99.2-100% and 97.2-98.8%, respectively. The corresponding removal efficiencies of anoxic-oxic PB system were 97.5-100% and 98.6-99.4%, respectively. These findings showed that a PB system consisting of anoxic-oxic columns in series has a high capacity to remove nitrogenous and carbonaceous compounds even though the influent to the anoxic stage was oxygenated. Better system stability in terms of denitrification was, however, obtained with the anoxic-anoxic system.
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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.
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Using an A2/O process with three dissolved oxygen (DO) levels[3.0-3.5 mg·L-1(Ⅰ stage), 2.0-2.5 mg·L-1(Ⅱ stage), 1.5-2.0 mg·L-1(Ⅲ stage)], the sludge and denitrification characteristics of its aerobic unit and sedimentation unit were investigated and compared with that of an anoxic-aerobic (A/O) process with a DO content of 1.5-2.0 mg·L-1. The results showed that denitrification in the sedimentation unit was accomplished with both internal and external carbon sources, but sludge's denitrification was more efficient with the use of external carbon sources. Nitrate reductase activity and denitrification activity in the sludge in sedimentation unit were highest when DO content was 1.5-2.0 mg·L-1 under aerobic conditions, and the denitrification efficiency of the A2/O process was greatest under anoxic conditions. The residual PHB in the aerobic A/O process was higher than that in the A2/O process with experimental sludge loading. The denitrification activity of the sludge in the A/O process was higher, and the nitrate reductase activity was 1.08 times higher than that in the A/O process. After returnning of the sludge, denitrification in the anoxic A/O process was poor, although the removal of nitrate nitrogen was sufficient. In comparison, denitrification in the anoxic unit of the A2/O process was better. Denitrification of the sludge in the sedimentation unit was directly related to denitrification in the anoxic unit. Therefore, to ensure that denitrification in sedimentation unit does not seriously affect the separation of sludge and water, appropriate control of the aerobic operation and the maintenance of denitrification in the sedimentation unit will contribute more to the denitrification efficiency of the system rather than simply controlling the level of anoxia.
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