Lanzetta, Anna (2024) Modeling and experimental investigations in a single-stage biofilm reactor for the simultaneous nitrogen and carbon removal from urban wastewater. [Tesi di dottorato]

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Abstract

Industrial, economic, urban, and social growth in recent decades has inevitably led to an increase in the production of nutrient-rich wastewater, particularly nitrogen (N) and phosphorus (P), which are responsible for environmental pollution phenomena such as eutrophication. This leads to serious cases of deterioration in the quality of rivers, lakes, and seas. The removal of N and P in wastewater treatment plants is, therefore, a key issue on which research efforts have been based in recent years to identify new technologies that can improve purification processes while optimizing energy efficiency. In this context, simultaneous nitrification and denitrification (SND), which involves the simultaneous implementation of aerobic nitrification and anoxic denitrification processes within a single treatment unit operating under identical conditions, offers several advantages including a reduced plant footprint, reduced capital and operating costs, minimized carbon and sludge generation, and reduced energy requirements for aeration. From this perspective, this doctoral thesis aims to demonstrates the importance of mathematical modeling as a tool to improve the comprehension of treatment processes. Specifically, using the BioWin software it was possible to predict the experimental results collected during two different experimental campaigns aimed at investigating SND and shortcut SND in moving bed biofilm reactors (MBBRs). The calibrated and validated models satisfactorily reproduced the experimental data for all experimental campaigns and within specific acceptance criteria, resulting in a suitable tool for predicting the process efficiency. Moreover, calibrated and validated data were used to test different dissolved oxygen (DO) ranges (0.6–0.8 mg O2·L−1), pH (6.5–9.0), and hydraulic retention times (HRTs) (0.5–1.0 d) to improve shortcut SND. Based on the different simulated scenarios, the intermittent DO conditions can induce and maintain the inhibition of the nitrite-oxidizing bacteria with an average N-NO3− concentration of 0.05 mg N·L−1, while an HRT of 0.9 d resulted in average effluent N-NH4+, N-NO3− and N-NO2− concentrations of 4.0, 0.02 and 0.07 mg·L−1, respectively, indicating an efficient shortcut SND process. Subsequently, (micro)aerobic carbon (C) and N removal from synthetic urban wastewater was experimentally investigated in a continuous double-column upflow aerobic granular sludge blanket (UAGSB) system under different DO ranges (0.01–6.00 mg∙L−1), feed C/N ratios (4.7–13.6), and HRTs (6–24 h). At a DO range of 0.01–0.30 mg∙L−1, feed C/N ratio of 13.6, and HRT of 24 h, the UAGSB achieved the highest chemical oxygen demand (COD), N-NH4+, and total inorganic nitrogen (TIN) removal efficiencies of 86, 99, and 84 %, respectively. A preliminary assessment of the energy and economic savings associated with the biological process was also carried out. The impact of capital and operating costs mainly related to the energy consumption of the aeration was taken into account. The assessment reveals that the capital and energy expenses of the UAGSB reactor would result in cost savings of around 14 and 7 %, respectively, compared with a modified-Ludzack–Ettinger (MLE) system. On the other hand, increased levels of N-NO3- and N-NH4+ were detected in the effluent when the C/N ratio was reduced to 4.7-8.0 and the HRT was reduced to 6 hours. This observation implies the need for post-treatment measures to reduce the residual concentrations under specific influent conditions. For this purpose, a duplicated two-chamber flow-through bioelectrochemical system (BES) was implemented for the removal of residual N. The results indicated that a complete removal of N-NO3- and N-NO2- was achieved by the BES at an HRT of 2 h after 6 days of biomass acclimatization. Furthermore, the study showed that, although heterotrophic denitrification was predominant, the use of electron donors deriving from the electrodes kept the concentrations of N-NO3- and N-NO2- consistently below the Italian standard (D. Lgs. 152/2006, Annex V, Part III) for the discharge of industrial effluents into the sewer system. Therefore, integrating autotrophic and heterotrophic denitrification within the same system could ensure consistently high removal efficiencies, considering the significant variability in feed organic concentrations. Furthermore, in terms of energy consumption, a specific energy consumption rate of 2.3·10-2 and 9.6·10-5 kWh·g NOxremoved-1 underpins the effectiveness of the BESs. This study demonstrates how the integration of biofilm reactors with mathematical modeling can have a significant impact on scientific investigations into the removal of C, N, and P from wastewater promoting the advancement of sustainable treatment technologies.

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