Nitrous oxide emissions from one-step partial nitritation/anammox processes

A comprehensive literature mining and analysis of nitrous oxide emissions from different innovative mainstream anammox-based biological nitrogen removal processes

The biological nitrogen removal (BNR) process in wastewater treatment plants generates a substantial volume of nitrous oxide (N2O), which possesses a potent greenhouse gas effect. A limited number of studies have systematically investigated the N2O emissions of anammox-based systems with different BNR processes under mainstream conditions. Based on extensive big data statistical analysis, it had been revealed that simultaneous nitritation, anammox and denitrification (SNAD), partial nitritation anammox (PNA) and partial denitrification anammox (PDA), exhibit significantly lower N2O emission factors when compared to traditional BNR processes. The median values for N2O emission factors were determined to be 1.01 %, 1.15 % and 1.43 % for SNAD, PNA and PDA, respectively. Based on nitrogen removal data and N2O emission factors, the N2O emissions from PNA, SNAD and PDA processes were calculated to be 0.016 g·d-1, 0.037 g·d-1 and 0.008 g·d-1, respectively. Furthermore, the machine learning models (SVM and ANN) exhibited excellent predictive performance for N2O emissions in the BNR processes. However, after removing environmental factors, the R2 value of the SVM model sharply decreased. The SHAP feature analysis demonstrated the significant impact of environmental factors on the accuracy of predictive performance in machine learning models. Spearman correlation analysis was employed to investigate the relationship between N2O emissions and operational factors as well as microbial communities. The results demonstrated a negative correlation between HRT, temperature and C/N with N2O emissions. Moreover, strong associations were observed between Nitrosomonas, Nitrospira, Denitratisoma, Thauera species and N2O emissions. The contribution of N2O production via AOB pathways played a key role that was quantitatively calculated to be 93 %, 80 % and 48 % in the PNA, SNAD and PDA processes, respectively. These findings highlight the potential of these innovative BNR processes in mitigating N2O emissions.

Sustainable nitrogen removal in anammox-mediated systems: Microbial metabolic pathways, operational conditions and mathematical modelling

Anammox-mediated systems have attracted considerable attention as alternative cost-effective technologies for sustainable nitrogen (N) removal from wastewater. This review comprehensively highlights the importance of understanding microbial metabolism in anammox-mediated systems under crucial operation parameters, indicating the potentially wide applications for the sustainable treatment of N-containing wastewater. The partial nitrification-anammox (PN-A), simultaneous PN-A and denitrification (SNAD) processes have demonstrated sustainable N removal from sidestream wastewater. The partial denitrification-anammox (PD-A) and denitrifying anaerobic methane oxidation-anammox (DAMO-A) processes have advanced sustainable N removal efficiency in mainstream wastewater treatment. Moreover, N2O production/emission hotspots are extensively discussed in anammox-based processes and are related to the dominant ammonia-oxidizing bacteria (AOB) and denitrifying heterotrophs. In contrast, N2O is not produced in the metabolism pathways of AnAOB and DAMO-archaea; Moreover, the actual contribution of N2O production by dissimilatory nitrate reduction to ammonium (DNRA) and DAMO-bacteria in their species remains uncertain. Thus, PD-A and DAMO-A processes would achieve reduction in greenhouse gas production, as well as energy consumption for the reliability of N removal efficiencies. In addition to reaction mechanisms, this review covers the mathematical models for simultaneous anammox, partial nitrification and/or denitrification (i.e., PN-A, PD-A, and SNAD). Promising NO3- reduction technologies by endogenous PD, sulfur-driven autotrophic denitrification, and DNRA by anammox are also discussed. In summary, this review provides a better understanding of sustainable N removal in anammox-mediated systems, thereby encouraging future investigation and exploration of the sustainable N bio-treatment from wastewater.

Nitrous Oxide Emission from Full-Scale Anammox-Driven Wastewater Treatment Systems

Wastewater treatment plants (WWTPs) are important contributors to global greenhouse gas (GHG) emissions, partly due to their huge emission of nitrous oxide (N₂O), which has a global warming potential of 298 CO₂ equivalents. Anaerobic ammonium-oxidizing (anammox) bacteria provide a shortcut in the nitrogen removal pathway by directly transforming ammonium and nitrite to nitrogen gas (N₂). Due to its energy efficiency, the anammox-driven treatment has been applied worldwide for the removal of inorganic nitrogen from ammonium-rich wastewater. Although direct evidence of the metabolic production of N₂O by anammox bacteria is lacking, the microorganisms coexisting in anammox-driven WWTPs could produce a considerable amount of N₂O and hence affect the sustainability of wastewater treatment. Thus, N₂O emission is still one of the downsides of anammox-driven wastewater treatment, and efforts are required to understand the mechanisms of N₂O emission from anammox-driven WWTPs using different nitrogen removal strategies and develop effective mitigation strategies. Here, three main N₂O production processes, namely, hydroxylamine oxidation, nitrifier denitrification, and heterotrophic denitrification, and the unique N₂O consumption process termed nosZ-dominated N₂O degradation, occurring in anammox-driven wastewater treatment systems, are summarized and discussed. The key factors influencing N₂O emission and mitigation strategies are discussed in detail, and areas in which further research is urgently required are identified.

Optimization of the Aeration Strategies in a Deammonification Sequencing Batch Reactor for Efficient Nitrogen Removal and Mitigation of N2O Production

In deammonification systems, nitrite-oxidizing bacteria (NOB) suppression and nitrous oxide (N₂O) mitigation are two important operational objectives. To carry out this multivariable analysis of response, a comprehensive model for the N cycle was developed and evaluated against experimental data from a laboratory-scale deammonification granular sludge sequencing batch reactor. Different aeration strategies were tested, and the manipulated variables comprised the dissolved oxygen (DO) set point in the aerated phase, aeration on/off frequency (F), and the ratio (R) between the non-aerated and aerated phase durations. Experimental results showed that a high ammonium utilization rate (AUR) in relation to the low nitrate production rate (NPR) (NPR/AUR = 0.07-0.08) and limited N₂O emissions (E_N2O < 2%) could be achieved at the DO set point = 0.7 mg O₂/L, R ratio = 2, and F frequency = 6-7 h^-1. Under specific operational conditions (biomass concentration, NH₄⁺-N loading rate, and temperature), simulation results confirmed the feasible aeration strategies for the trade-offs between the NOB suppression and N₂O emission. The intermittent aeration regimes led to frequent shifts in the predominating N₂O production pathways, that is, hydroxylamine (NH₂OH) oxidation (aerated phase) versus autotrophic denitrification (non-aerated phase). The inclusion of the extracellular polymeric substance mechanism in the model explained the observed activity of heterotrophs, especially Anaerolineae, and granule formation.

Mainstream Ammonium Recovery to Advance Sustainable Urban Wastewater Management

Throughout the 20th century, the prevailing approach toward nitrogen management in municipal wastewater treatment was to remove ammonium by transforming it into dinitrogen (N₂) using biological processes such as conventional activated sludge. While this has been a very successful strategy for safeguarding human health and protecting aquatic ecosystems, the conversion of ammonium into its elemental form is incompatible with the developing circular economy of the 21st century. Equally important, the activated sludge process and other emerging ammonium removal pathways have several environmental and technological limitations. Here, we assess that the theoretical energy embedded in ammonium in domestic wastewater represents roughly 38-48% of the embedded chemical energy available in the whole of the discharged bodily waste. The current routes for ammonium removal not only neglect the energy embedded in ammonium, but they can also produce N₂O, a very strong greenhouse gas, with such emissions comprising the equivalent of 14-26% of the overall carbon footprint of wastewater treatment plants. N₂O emissions often exceed the carbon emissions related to the electricity consumption for the process requirements of WWTPs. Considering these limitations, there is a need to develop alternative ammonium management approaches that center around recovery of ammonium from domestic wastewater rather than deal with its "destruction" into elemental dinitrogen. Current ammonium recovery techniques are applicable only at orders of magnitude above domestic wastewater strength, and so new techniques based on physicochemical adsorption are of particular interest. A new pathway is proposed that allows for mainstream ammonium recovery from wastewater based on physicochemical adsorption through development of polymer-based adsorbents. Provided adequate adsorbents corresponding to characteristics outlined in this paper are designed and brought to industrial production, this adsorption-based approach opens perspectives for mainstream continuous adsorption coupled with side-stream recovery of ammonium with minimal chemical requirements. This proposed pathway can bring forward an effective resource-oriented approach to upgrade the fate of ammonium in urban water management without generating hidden externalized environmental costs.

Nitrous oxide emissions of a mesh separated single stage deammonification reactor

It is widely accepted that partial nitrification by ANAMMOX has the potential to become one of the key processes in wastewater treatment. However, large greenhouse gas emissions have been panobserved in many cases. A novel mesh separated reactor, developed to allow continuous operation of deammonification at smaller scale without external biomass selection, was compared to a conventional single-chamber deammonification sequencing batch reactor (SBR), where both were equally-sized pilot-scale reactors. The mesh reactor consisted of an aerated and an anoxic zone separated by a mesh. The resulting differences in the structure of the microbial community were detected by next-generation sequencing. When both systems were operated in a sequencing batch mode, both systems had comparable nitrous oxide emission factors in the range of 4% to 5% of the influent nitrogen load. A significant decrease was observed after switching from sequencing batch mode to continuous operation.