GREENN2 · Greener biological nitrogen removal: minimization of N2O emissions and optimization of the integration issues of the nitritation / Anammox process for main stream wastewater treatment

Greener biological nitrogen removal: minimization of N2O emissions and optimization of the integration issues of the nitritation / Anammox process for main stream wastewater treatment

PROJECT CONTEXT AND OBJECTIVES The main objective of the GreenN2 research project is to achieve a well performing and fully integrated nitritation-Anammox system (PN/AMX) for mainstream wastewater treatment at minimal N2O emission rates (see Fig. 1). The autotrophic nitrogen removal in unit 3 (Fig. 1) is composed of two different processes: nitritation and Anammox; both are combined in a single oxygen-limited reactor. The biomass is retained in fast-settling, compact granules of a coculture of nitrite oxidizing bacteria (AOB) and Anammox, in a special type of biofilm named granular sludge. The following partial objectives can be distinguished: (i) To study how the microbial community interactions and operating conditions contribute to N2O emissions in single-stage nitritation-Anammox for main stream wastewater treatment. (ii) To define the best strategy for the integration of the nitritation-Anammox process for mainstream wastewater treatment, determining the potential domain of operating conditions to minimize N2O emissions, whereas an efficient nitrogen removal is maintained. (iii) To demonstrate the improvements in terms of nitrogen removal and reduction of N2O emissions with experimental tests. MAIN RESULTS AND CONCLUSIONS Microbial community interactions in PN-AMX granular sludge Competition for oxygen between ammonia-oxidizing bacteria (AOB) and NOB was found to be essential for NOB repression even when the suppression of nitrite oxidation is assisted by nitrite reduction by anammox (AMX). The operational window for efficient NOB repression was sketched with the aid of the model (see Figure 2). At low temperatures, efficient NOB repression can be obtained if bulk ammonium concentrations are above certain level (denoted as (SNH4+)min). Operating the reactor at a bulk ammonium concentration higher than (SNH4+)min increases the nitritation rate, which leads to less oxygen available for nitrite oxidation, triggering NOB repression. If the bulk DO concentration is not low enough, AMX activity is hampering NOB repression, because it decreases the nitritation rate. NOB repression is AMX assisted due to a decrease in the nitratation rate, but still an adequate excess of ammonium is required. In the whole range of DO tested, NOB are effectively outcompeted by AOB, and never directly outcompeted by AMX through nitrite unavailability. A similar modelling study was used to assess what could be the advantages of using a hybrid system for PN/AMX in which partial nitritation would be carried out mainly in suspension, while anammox would be retained in carriers, as done for reject water by some widespread technologies. In that case, the model was used to show how the anammox biofilm requires of thin layer of AOB that contributes ca. one third of the total nitritation capacity. Assessment of the two-stage approach for PN/AMX in mainstream conditions A possibility to maintain efficient NOB repression at low temperatures with granular sludge reactors is to split the nitrogen removal in two stages, one devoted to partial nitritation and one devoted to anaerobic ammonium oxidation (anammox) (Figure 3; Table 1). Modelling N2O emissions from granular sludge performing nitritation A mathematical model was used to investigate N2O emissions in biofilm reactors (and in particular aerobic granular sludge performing partial nitritation). The main findings point out to the importance of an intermediate, hydroxylamine (NH2OH), which could diffuse in the biofilm and enhance the N2O emissions in this type of systems. The theoretical findings were supported by experimental data obtained in a granular sludge reactor performing partial nitritation. The abiotic N2O emissions due to the reaction between NH2OH and nitrite during nitritation were found to be of importance during the treatment of reject water. Note: a version including tables and figures is included as attaachement

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