Effect of process parameters and operational mode on nitrous oxide emissions from a nitritation reactor treating reject wastewater

Reduction of greenhouse gas emissions from closed activated sludge- to aerobic granular sludge-based biosystems via gas circulation

Greenhouse gas (GHG) emissions from biological treatment units are challenging wastewater treatment plants (WWTPs) due to their wide applications and global warming. This study aimed to reduce GHG emissions (especially N2O) using a gas circulation strategy in a closed sequencing-batch reactor when the biological unit varies from activated sludge (AS) to aerobic granular sludge (AGS). Results show that gas circulation lowers pH to 6.3 ± 0.2, facilitating regular granules but elevating total N2O production. From AS to AGS, N2O emission factor increased (0.07-0.86 %) due to decreasing ammonia-oxidizing rates while the emissions of CO2 (0.3 ± 0.1 kg-CO2/kg-chemical oxygen demand) and CH4 remained in the closed biosystem. The gas circulation decreased N2O emission factor by 63 ± 15 % after granulation higher than 44 ± 34 % before granulation, which is implemented by heterotrophic denitrification. This study provides a feasible strategy to enhance heterotrophic N2O elimination in the biological WWTPs.

Analyzing greenhouse gas emissions and influencing factors of 247 actual wastewater treatment plants in China using emission factor and operational data integrated methods (ODIM)

In response to China's policies on pollution control and carbon emission (CE) reductions, more stringent regulations have been implemented to evaluate CE in wastewater treatment facilities. In this study, we have analyzed CE from China's wastewater treatment plants (WWTPs) and influencing factor. Emission factor (EF) and operational data integrated methods (ODIM) were utilized to measure emissions, using data collected from 247 WWTPs over a 1-year period across seven regions in China. The average CE intensity was 0.45 kgCO2-eq/m3, affected by region, season, influent water quality, treatment processes, effluent discharge standards, and facilities. The scale effect was obvious only in the range of 2 × 105 m3/day. Underground WWTPs exhibited significantly higher CE compared to aboveground WWTPs. In summary, the assessment of CE in 247 actual WWTPs not only identifies emission reduction potential but also provides a scientific basis for formulating targeted emission reduction measures.

Impact of aerobic granular sludge sizes and dissolved oxygen concentration on greenhouse gas N2O emission

Aerobic granular sludge (AGS) at wastewater treatment plants (WWTPs) are known to produce nitrous oxide (N2O), a greenhouse gas which has a ∼300 times higher global warming potential than carbon dioxide. In this research, we studied N2O emissions from different sizes of AGS developed at a dissolved oxygen (DO) level of 2 mgO2/L while exposing them to disturbances at various DO concentrations ranging from 1 to 4 mgO2/L. Five different AGS size classes were studied: 212-600 µm, 600-1000 µm, 1000-1400 µm, 1400-2000 µm, and > 2000 µm. Metagenomic data showed N2O reductase genes (nosZ) were more abundant in the smaller AGS sizes which aligned with the observation of higher N2O reduction rates in small AGS under anaerobic conditions. However, when oxygen was present, the activity measurements of N2O emission showed an opposite trend compared to metagenomic data, smaller AGS (212 to 1000 µm) emitted significantly higher N2O (p < 0.05) than larger AGS (1000 µm to >2000 µm) at DO of 2, 3, and 4 mgO2/L. The N2O emission rate showed positive correlation with both oxygen levels and nitrification rate. This pattern indicates a connection between N2O emission and nitrification. In addition, the data suggested the penetration of oxygen into the anoxic zone of granules might have hindered nitrous oxide reduction, resulting in incomplete denitrification stopping at N2O and consequently contributing to an increase in N2O emissions. This work sets the stage to better understand the impacts of AGS size on N2O emissions in WWTPs under different disturbance of DO conditions, and thus ensure that wastewater treatment will comply with possible future regulations demanding lowering greenhouse gas emissions in an effort to combat climate change.

Direct and indirect monitoring methods for nitrous oxide emissions in full-scale wastewater treatment plants: A critical review

Mitigation of nitrous oxide (N2O) emissions in full-scale wastewater treatment plant (WWTP) has become an irreversible trend to adapt the climate change. Monitoring of N2O emissions plays a fundamental role in understanding and mitigating N2O emissions. This paper provides a comprehensive review of direct and indirect N2O monitoring methods. The techniques, strengths, limitations, and applicable scenarios of various methods are discussed. We conclude that the floating chamber technique is suitable for capturing and interpreting the spatiotemporal variability of real-time N2O emissions, due to its long-term in-situ monitoring capability and high data acquisition frequency. The monitoring duration, location, and frequency should be emphasized to guarantee the accuracy and comparability of acquired data. Calculation by default emission factors (EFs) is efficient when there is a need for ambiguous historical N2O emission accounts of national-scale or regional-scale WWTPs. Using process-specific EFs is beneficial in promoting mitigation pathways that are primarily focused on low-emission process upgrades. Machine learning models exhibit exemplary performance in the prediction of N2O emissions. Integrating mechanistic models with machine learning models can improve their explanatory power and sharpen their predictive precision. The implementation of the synergy of nutrient removal and N2O mitigation strategies necessitates the calibration and validation of multi-path mechanistic models, supported by long-term continuous direct monitoring campaigns.

Nitrous oxide emissions from two full-scale membrane-aerated biofilm reactors

The upcoming change of legislation in some European countries where wastewater treatment facilities will start to be taxed based on direct greenhouse gas (GHG) emissions will force water utilities to take a closer look at nitrous oxide (N2O) production. In this study, we report for the first time N2O emissions from two full-scale size membrane aerated biofilm reactors (MABR) (R1, R2) from two different manufacturers treating municipal wastewater. N2O was monitored continuously for 12 months in both the MABR exhaust gas and liquid phase. Multivariate analysis was used to assess process performance. Results show that emission factors (EFN2O) for both R1 and R2 (0.88 ± 1.28 and 0.82 ± 0.86 %) were very similar to each other and below the standard value from the Intergovernmental Panel on Climate Change (IPCC) 2019 (1.6 %). More specifically, N2O was predominantly emitted in the MABR exhaust gas (NTRexh) and was strongly correlated to the ammonia/um load (NHx,load). Nevertheless, the implemented Oxidation Reduction Potential (ORP) control strategy increased the bulk contribution (NTRbulk), impacting the overall EFN2O. A thorough analysis of dynamic data reveals that the changes in the external aeration (EA)/loading rate patterns suggested by ORP control substantially impacted N2O mass transfer and biological production processes. It also suggests that NTRexh is mainly caused by ammonia-oxidizing organisms (AOO) activity, while ordinary heterotrophic organisms (OHO) are responsible for NTRbulk. Different methods for calculating EFN2O were compared, and results showed EFN2O would range from 0.6 to 5.5 depending on the assumptions made. Based on existing literature, a strong correlation between EFN2O and nitrogen loading rate (R2 = 0.73) was found for different technologies. Overall, an average EFN2O of 0.86 % N2O-N per N load was found with a nitrogen loading rate >200 g N m-3 d-1, which supports the hypothesis that MABR technology can achieve intensified biological nutrient removal without increasing N2O emissions.

The mitigation effect of free ammonia and free nitrous acid on nitrous oxide production from the full-nitrification and partial-nitritation systems

The potentials of using endogenous free ammonia (FA) and free nitrous acid (FNA) as nitrous oxide (N₂O) mitigators were investigated in treatment of both mainstream and sidestream wastewater. Although the N₂O emission factor of a sidestream partial-nitritation (PN) reactor (averaged 1.70 % ± 0.39 %, n = 30) was about 2.4 times higher than a mainstream full-nitrification (FN) reactor (averaged 0.72 % ± 0.24 %, n = 30) (P < 0.01), one-hour exposure of PN sludge to 1.5 mg HNO₂-N/L FNA could virtually abolish N₂O emission. As for FN sludge, both 45 mg NH₃-N/L FA and 0.015 mg HNO₂-N/L FNA successfully mitigated N₂O production at varying dissolved oxygen (DO) levels (50 % vs 61 %), while 1.5 mg HNO₂-N/L FNA not only reduced more N₂O (92 %) but also altered the N₂O dependency on DO. Both FNA and FA sludge treatment were effective N₂O mitigation strategies with FNA toward the end of carbon neutrality and FA being more economically appealing (2 % cost saving).

A method to estimate the direct nitrous oxide emissions of municipal wastewater treatment plants based on the degree of nitrogen removal

The greenhouse gas nitrous oxide (N₂O) is produced in activated sludge tanks as a byproduct of nitrification and heterotrophic denitrification. Insufficient knowledge on how microbiological N₂O generation and degradation pathways impact N₂O emissions in activated sludge tanks still hampers the development of effective mitigation strategies. Our research contributes to overcome this gap by quantifying N₂O emissions through extensive measurement campaigns at ten full-scale wastewater treatment plants and correlating them to relevant operating parameters by multivariate regression analysis. Measurements revealed that N₂O production depends mainly on the activity of nitrifying bacteria and is triggered by high ammonium concentrations. In contrast, well-performing heterotrophic denitrification plays a key role as a sink of N₂O in activated sludge tanks. Following these patterns, low loaded plants achieving high nitrogen removal (83-92%) exhibited the lowest N₂O emission intensity (0.0012 ± 0.001 kg N₂O-N emitted per kg TKN in the influent wastewater). The regression analysis corroborated these results by revealing a negative linear correlation between the N₂O emission factor and the total nitrogen removal degree of the plants. The regression model represents a novel estimation method that links N₂O emissions with plant performance and provides a significant improvement over approaches applying fixed N₂O emission factors.

Mitigating nitrous oxide emissions at a full-scale wastewater treatment plant

Mitigation of nitrous oxide (N₂O) emissions is of primary importance to meet the targets of reducing carbon footprints of wastewater treatment plants (WWTPs). Despite of a large amount of N₂O mitigation studies conducted in laboratories, full-scale implementation of N₂O mitigation is scarce, mainly due to uncertainties of mitigation effectiveness, validation of N₂O mathematical model, risks to nutrient removal performance and additional costs. This study aims to address the uncertainties by investigating the quantification, development and implementation of N₂O mitigation strategies at a full-scale sequencing batch reactor (SBR). To achieve this, N₂O emission dynamics, nutrient removal performance and operation of the SBR were monitored to quantify N₂O emissions, and identify the N₂O generation mechanisms. N₂O mitigation strategies centered on reducing dissolved oxygen (DO) levels were consequently proposed and evaluated using a multi-pathway N₂O production mathematical model before implementation. The implemented mitigation strategy resulted in a 35% reduction in N₂O emissions (from the emission factor of 0.89 ± 0.05 to 0.58 ± 0.06%), which was equivalent to annual reduction of 2.35 tonne of N₂O from the studied WWTP. This could be mainly attributed to reductions in N₂O generated via the NH₂OH oxidation pathway due to the lowering of DO level. As the first reported mitigation strategy permanently implemented at a full scale WWTP, it showcased that the mitigation of N₂O emissions at full-scale is feasible and that widely accepted N₂O mitigation strategies developed in laboratory studies are also likely effective in full-scale plants. Furthermore, the close agreement between the validated and predicted N₂O emission factors (0.58% vs 0.55%, respectively), showed that the N₂O mathematical model is a useful tool to evaluate N₂O mitigation strategies at full-scale. Importantly this work demonstrated that N₂O mitigation does not necessarily require additional operational cost to meet reduction targets. In contrast, the N₂O mitigation applied here reduced energy requirements for aeration by 20%. Equally important, long-term monitoring identified that N₂O mitigation did not affect the nutrient removal performance of the plant. Finally, with the knowledge acquired in this study, a standard approach for mitigating N₂O emissions from full-scale treatment plants was proposed.

Recent advances in nitrous oxide production and mitigation in wastewater treatment

Nitrous oxide (N₂O) emitted from wastewater treatment plants has caused widespread concern. Over the past decade, people have made tremendous efforts to discover the microorganisms responsible for N₂O production, elucidate metabolic pathways, establish production models and formulate mitigation strategies. The ultimate goal of all these efforts is to shed new light on how N₂O is produced and how to reduce it, and one of the best ways is to find key opportunities by integrating the information obtained. This review article critically evaluates the knowledge gained in the field within a decade, especially in N₂O production microbiology, biochemistry, models and mitigation strategies, with a focus on denitrification. Previous research has greatly deepened the understanding of the N₂O generation mechanism, but further efforts are still needed due to the lack of standardized methodology for establishing N₂O mitigation strategies in full-scale systems. One of the challenges seems to be to convert the denitrification process from a net N₂O source into an effective sink, which is recommended as a key opportunity to reduce N₂O production in this review.

Minimizing greenhouse gas emission from wastewater treatment process by integrating activated sludge and microalgae processes

A novel integrated microalgae and activated sludge (MA/AS) system was constructed to minimize greenhouse gas emission from traditional wastewater treatment plants. Its removal properties for aqueous pollutants were assessed as well. The ratio of microalgae-to-activated sludge volatile suspended solids of 1.3 and an incident light intensity of 12 W/m² provided the best performance: COD, NH₄⁺, and total phosphorus (TP) removals were up to 100%, 99.6% and 100%, respectively. Even without illumination, COD, NH₄⁺, and TP removal efficiencies were as high as 95.1%, 96.5% and 100%, respectively. In both cases, nutrient uptake by MA was proved to play an important role in nutrients removal. And no CH₄ or N₂O emissions were detected during the whole experimental period of the MA/AS system (mass ratio of 1.3:1). Only negligible CO₂ was detected up to 45 μmol with illumination and 130 μmol without illumination in the headspace of the serum bottles, which merely accounted for 2.0% and 5.8% of the initial total carbon equivalent (glucose serving as organic carbon source). Since photosynthesis by microalgae could provide oxygen to heterotrophs or nitrifying bacteria, extra energy demand (mainly from aeration units) could be greatly cut down, which would heavily reduce the total energy demands and further indirect CO₂ emission from wastewater treatment plants. Our integrated system is demonstrated to be a sustainable approach for contaminants removal from aqueous phase, restraining greenhouse gas emission and saving energy consumption contemporaneously.

A knowledge discovery framework to predict the N2O emissions in the wastewater sector

Data Analytics is being deployed to predict the dissolved nitrous oxide (N₂O) concentration in a full-scale sidestream sequence batch reactor (SBR) treating the anaerobic supernatant. On average, the N₂O emissions are equal to 7.6% of the NH₄-N load and can contribute up to 97% to the operational carbon footprint of the studied nitritation-denitritation and via-nitrite enhanced biological phosphorus removal process (SCENA). The analysis showed that average aerobic dissolved N₂O concentration could significantly vary under similar influent loads, dissolved oxygen (DO), pH and removal efficiencies. A combination of density-based clustering, support vector machine (SVM), and support vector regression (SVR) models were deployed to estimate the dissolved N₂O concentration and behaviour in the different phases of the SBR system. The results of the study reveal that the aerobic dissolved N₂O concentration is correlated with the drop of average aerobic conductivity rate (spearman correlation coefficient equal to 0.7), the DO (spearman correlation coefficient equal to -0.7) and the changes of conductivity between sequential cycles. Additionally, operational conditions resulting in low aerobic N₂O accumulation (<0.6 mg/L) were identified; step-feeding, control of initial NH₄⁺ concentrations and aeration duration can mitigate the N₂O peaks observed in the system. The N₂O emissions during aeration shows correlation with the stripping of accumulated N₂O from the previous anoxic cycle. The analysis shows that N₂O is always consumed after the depletion of NO₂^- during denitritation (after the "nitrite knee"). Based on these findings SVM classifiers were constructed to predict whether dissolved N₂O will be consumed during the anoxic and anaerobic phases and SVR models were trained to predict the N₂O concentration at the end of the anaerobic phase and the average dissolved N₂O concentration during aeration. The proposed approach accurately predicts the N₂O emissions as a latent parameter from other low-cost sensors that are traditionally deployed in biological batch processes.

Impact of coexistence of sludge flocs on nitrous oxide production in a granule-based nitrification system: A model-based evaluation

A common operational status of granule-based reactor is the inevitable coexistence of sludge flocs. Such hybrid system could have a profound impact on nitrous oxide (N₂O) production in nitrifying process. In this work, a mathematical model is employed to evaluate the key role of the coexistence of sludge flocs on N₂O production in a granule-based nitrifying system for the first time, by considering both nitrifier denitrification and hydroxylamine oxidation pathways. The modelling results show that the N₂O production gradually decreases with the increase of the percentage of sludge flocs in the total biomass (10-60%). More N₂O is tended to be generated in sludge flocs which has lower N₂O production capacity compared to granular biomass, thus lowering the total N₂O production. The relative contributions of two N₂O production pathways are only affected by bulk dissolved oxygen (DO) for the sludge flocs in the hybrid system, whereas those are affected by both bulk DO and the fractions of sludge flocs for the granular biomass. The results reveal a substantial effect of the coexistence of sludge flocs on N₂O production in granule-based nitrifying process, which should not be ignored in future design and operation.

Mixture of different Pseudomonas aeruginosa SD-1 strains in the efficient bioaugmentation for synthetic livestock wastewater treatment

Strains selection for inoculation is the key to the successful construction of a bioaugmentation system, a promising strategy for specific pollutant removal. Pseudomonas aeruginosa SD-1 wild-type (WT) strain exhibited high capacity for biofilm formation but low efficiency for nitrate (NO₃^-) removal. Meanwhile, quorum sensing deficient strain ΔlasR showed excellent efficiency for NO₃^- removal but poor capability for colonization in activated sludge. The opposite effect of biofilm formation and NO₃^- removal exist in WT or ΔlasR, which limits the construction of bioaugmentation system of strain SD-1 and its application. To solve this issue, a mixture of WT and ΔlasR (v/v = 1:1) was used to construct a bioaugmentation system. Compared with the inoculation of WT or ΔlasR alone, the mixed inoculation not only was beneficial for activated sludge development but also for pollutant removal. The indicators for activated sludge including the abundance of P. aeruginosa, the sludge volume index and the average particle size in mixed inoculated reactors were close to those of reactors with single and repeated inoculation of WT. The effluent of chemical oxygen demand (COD) and NO₃^--N were stable at 3.9-22.6 mg L^-1 and 0-5.53 mg L^-1 after d 3, respectively. This study presents a detailed case on the ecological tradeoff of colonization and pollutant removal of inoculated strains during bioaugmentation. The results provide information on the appropriate conditions for application of P. aeruginosa SD-1 for livestock wastewater treatment and further enrich our ecological understanding of bioaugmentation.

Two-stage granular sludge partial nitritation/anammox process for the treatment of digestate from the anaerobic digestion of the organic fraction of municipal solid waste

The increasing amount of source separated organic fraction of municipal solid wastes (OFMSW) treated by anaerobic digestion for energy recovery requires the implementation of cost-efficient processes for the treatment of the produced digestate, especially in terms of nitrogen removal. The autotrophic nitrogen removal process, based on the coupling of two biological processes, partial nitritation (PN) and anammox (A), appears as a suitable solution due to important savings in operational costs compared to conventional treatment processes. However, its application could be hampered by the high salinity and inhibitory potential of this kind of digestate. In this contribution, two lab-scale granular sludge reactors performing the PN and anammox processes, respectively, were used to treat (opportunely diluted) real OFMSW digestate originating from full-scale biogas plants with the aim of assessing their treatment feasibility in a two-stage PN/A configuration. The PN process was implemented in an air-lift granular sludge reactor and was able to treat a nitrogen loading rate of about 1 g N L^-1 d^-1 at 30 ± 0.5 °C; moreover, its effluent was suitable for the subsequent anammox treatment, with an appropriate effluent NO₂^-/NH₄⁺ ratio and marginal inhibiting effects. In the anammox granular sludge reactor, the anammox activity was affected by high salinity levels, nonetheless a stable reactor performance at a nitrogen removing rate of 0.83 ± 0.20 and 0.31 ± 0.04 g N L^-1 d^-1 at 35 ± 0.5 °C, were achieved when treating 50% and 30% diluted real wastewaters at a conductivity in the reactor of 9.1 and 11.2 mS cm^-1, respectively.

Reducing Cost and Environmental Impact of Wastewater Treatment with Denitrifying Methanotrophs, Anammox, and Mainstream Anaerobic Treatment

In water resource recovery facilities, sidestream biological nitrogen removal via anaerobic ammonium oxidation (anammox) is more energy and cost efficient than conventional nitrification-denitrification. However, under mainstream conditions, nitrite oxidizing bacteria (NOB) out-select anammox bacteria for nitrite produced by ammonium oxidizing bacteria (AOB). Therefore, nitrite production is the bottleneck in mainstream anammox nitrogen removal. Nitrate-dependent denitrifying anaerobic methane oxidizing archaea (n-damo) oxidize methane and reduce nitrate to nitrite. The nitrite supply challenge in mainstream anammox implementation could be solved with a microbial community of AOB, NOB, n-damo, and anammox with methane from anaerobic sludge digestion or a mainstream anaerobic membrane bioreactor (AnMBR). The cost and environmental impact of traditional nitrification/dentrification relative to AOB/anammox and AOB/anammox/n-damo systems, with and without an AnMBR, were compared with a stoichiometric model. AnMBR implementation reduced costs and emission rates at moderate to high nutrient loading by lowering aeration and sludge handling demands while increasing methane available for cogeneration. AnMBR/AOB/anammox systems reduced cost and GHG emission by up to $0.303/d/m³ and 1.72 kg equiv. CO₂/d/m³, respectively, while AnMBR/AOB/anammox/n-damo systems saw a similar reduction of at least $0.300/d/m³ and 1.65 kg equiv. CO₂/d/m³ in addition to alleviating the necessity to stop nitrification at nitrate, allowing easier aeration control.

A decade of nitrous oxide (N2O) monitoring in full-scale wastewater treatment processes: A critical review

Direct nitrous oxide (N₂O) emissions during the biological nitrogen removal (BNR) processes can significantly increase the carbon footprint of wastewater treatment plant (WWTP) operations. Recent onsite measurement of N₂O emissions at WWTPs have been used as an alternative to the controversial theoretical methods for the N₂O calculation. The full-scale N₂O monitoring campaigns help to expand our knowledge on the N₂O production pathways and the triggering operational conditions of processes. The accurate N₂O monitoring could help to find better process control solutions to mitigate N₂O emissions of wastewater treatment systems. However, quantifying the emissions and understanding the long-term behaviour of N₂O fluxes in WWTPs remains challenging and costly. A review of the recent full-scale N₂O monitoring campaigns is conducted. The analysis covers the quantification and mitigation of emissions for different process groups, focusing on techniques that have been applied for the identification of dominant N₂O pathways and triggering operational conditions, techniques using operational data and N₂O data to identify mitigation measures and mechanistic modelling. The analysis of various studies showed that there are still difficulties in the comparison of N₂O emissions and the development of emission factor (EF) databases; the N₂O fluxes reported in literature vary significantly even among groups of similar processes. The results indicated that the duration of the monitoring campaigns can impact the EF range. Most N₂O monitoring campaigns lasting less than one month, have reported N₂O EFs less than 0.3% of the N-load, whereas studies lasting over a year have a median EF equal to 1.7% of the N-load. The findings of the current study indicate that complex feature extraction and multivariate data mining methods can efficiently convert wastewater operational and N₂O data into information, determine complex relationships within the available datasets and boost the long-term understanding of the N₂O fluxes behaviour. The acquisition of reliable full-scale N₂O monitoring data is significant for the calibration and validation of the mechanistic models -describing the N₂O emission generation in WWTPs. They can be combined with the multivariate tools to further enhance the interpretation of the complicated full-scale N₂O emission patterns. Finally, a gap between the identification of effective N₂O mitigation strategies and their actual implementation within the monitoring and control of WWTPs has been identified. This study concludes that there is a further need for i) long-term N₂O monitoring studies, ii) development of data-driven methodological approaches for the analysis of WWTP operational and N₂O data, and iii) better understanding of the trade-offs among N₂O emissions, energy consumption and system performance to support the optimization of the WWTPs operation.

Nitrous oxide emission mitigation during low-carbon source wastewater treatment: effect of external carbon source supply strategy

Nitrous oxide (N₂O) generated during biological nitrogen removal in wastewater treatment processes has contributed an important proportion to the global warming effect. To evaluate the possibility of N₂O emission mitigating by changing carbon source supply strategies, nitrogen transformation characteristics and N₂O emissions with methanol one-time dosing and step dosing were investigated. Two sets of laboratory-scale sequencing batch biofilm reactors (SBBRs) were conducted to treat real domestic wastewater with low carbon source. The results revealed that reactors with methanol step dosing showed a lower N₂O emission of 0.0402 ± 0.0016 mg/(L·h), together with a higher total nitrogen and ammonia nitrogen removal efficiencies of 83.30% ± 1.21 and 93.45% ± 1.20, respectively. While N₂O emission from conventional one-time dosing reactors was 0.0741 ± 0.0025 mg/(L·h), total nitrogen and ammonia nitrogen removal efficiencies were 75.71% ± 0.54 and 88.45% ± 0.59, respectively. The N₂O emission factor of SBBR was reduced from 6.26% ± 0.21 to 3.40% ± 0.14 with methanol step dosing. Moreover, nitrification rates in aerobic phases were reduced, while denitrification rates in anoxic phases were elevated. Hence, carbon source step dosing enhanced nitrogen removal and reduced N₂O emission compared with one-time dosing, which is a simply achievable strategy for N₂O emission reduction in highly automated systems like wastewater treatment plants.

Nitrification and denitrification processes for mitigation of nitrous oxide from waste water treatment plants for biovalorization: Challenges and opportunities

Nitrous oxide (N₂O) is a potent greenhouse gas. Even though its emissions is much lesser than CO₂ but its global warming potential (GWP) is 298 times more than CO₂. N₂O emissions from wastewater treatment plants was caused due to incomplete nitrification or incomplete denitrification catalyzed by ammonia-oxidizing bacteria and heterotrophic denitrifiers. Low dissolved oxygen, high nitrite accumulation, change in optimal pH or temperature, fluctuation in C/N ratio, short solid retention time and non-availability of Cu ions were responsible for higher N₂O leakage. Regulation of enzyme metabolic pathways involved in N₂O production and reduction has also been reviewed. Sequential bioreactors, bioscrubbers, membrane biofilters usage have helped microbial nitrification-denitrification processes in succumbing N₂O production in wastewater treatment plants. Reduction of N₂O negativity has been studied through its valorization for the formation of value added products such as biopolymers has led to biorefinery approaches as an upcoming mitigation strategy.

The effect of temperature shifts on N2O and NO emissions from a partial nitritation reactor treating reject wastewater

Temperature has a known effect on ammonia oxidizing bacteria (AOB) activities, reducing its ammonia oxidizing rate (AOR) when temperature is lowered. However, little is known concerning its effect on N₂O and NO emissions which are produced during ammonia oxidation having a greenhouse effect. To study this, an AOB enriched partial nitrification sequencing batch reactor (PN-SBR) was operated within a two step-wise feed under 5 different temperatures (30-25-20-15-10 °C). A decrease on the specific AOR (sAOR) was detected when decreasing the temperature. N₂O emissions were also affected by the temperature but only the ones produced during the first aeration of the cycle, when AOBs shifted from a period of low activity to a period of high activity. N₂O emission factors (%) detected during the second aerobic phase were similar among all temperatures tested and lower than the emissions detected during the first aerated phase. The average N₂O emission factor was in the range of 0.15-0.70% N₂O-N/NH₄⁺-N oxidized in the first aeration phase and 0.14-0.15% N₂O-N/NH₄⁺-N-oxidized in the second aeration phase at 10 to 30 °C, respectively. On the other hand, NO emissions were very similar under all temperatures resulting in 0.03-0.06% of NH₄⁺-N oxidized.

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.

Approaches and processes for ammonia removal from side-streams of municipal effluent treatment plants

The main objective of this review article is to provide a comprehensive view on various conventional and emerging side-stream ammonia removal treatment options for municipal wastewater treatment plants (WWTPs). Optimization of wastewater treatment facilities from an energy and emissions stand-point necessitates consideration of the impact of the various internal side-streams. Side-streams from anaerobic sludge digesters in particular have the potential to be a significant ammonium load to the mainstream treatment process. However, the literature suggests that managing side-streams through their treatment in the mainstream process is not the most energy efficient approach, nor does it allow for practical recovery of nutrients. Furthermore, as effluent criteria become more stringent in some jurisdictions and sludge hydrolysis pre-treatment for digesters more common, an understanding of treatment options for ammonia in digester supernatant becomes more important. Given these considerations, a variety of side-stream treatment processes described in the literature are reviewed.

Environmental assessment of alternative treatment schemes for energy and nutrient recovery from livestock manure

The application of livestock manure on agricultural land is being restricted due to its significant content of phosphorus (P) and nitrogen (N), leading to eutrophication. At the same time, the growing demand for N and P mineral fertilizers is increasing their production costs and causing the depletion of natural phosphate rock deposits. In the present work, seven technologically feasible treatment schemes for energy (biogas) and nutrient recovery (e.g., struvite precipitation) and/or removal (e.g., partial nitritation/anammox) were evaluated from an environmental perspective. In general, while approaches based solely on energy recovery and use of digestate as fertilizer are commonly limited by community regulations, strategies pursuing the generation of high-quality struvite are not environmentally sound alternatives. In contrast, schemes that include further solid/liquid separation of the digestate improved the environmental profile, and their combination with an additional N-removal stage would lead to the most environmental-friendly framework. However, the preferred scenario was identified to be highly dependent on the particular conditions of each site, integrating environmental, social and economic criteria.

A review on nitrous oxide (N2O) emissions during biological nutrient removal from municipal wastewater and sludge reject water

Nitrous oxide (N₂O) is an important pollutant which is emitted during the biological nutrient removal (BNR) processes of wastewater treatment. Since it has a greenhouse effect which is 265 times higher than carbon dioxide, even relatively small amounts can result in a significant carbon footprint. Biological nitrogen (N) removal conventionally occurs with nitrification/denitrification, yet also through advanced processes such as nitritation/denitritation and completely autotrophic N-removal. The microbial pathways leading to the N₂O emission include hydroxylamine oxidation and nitrifier denitrification, both activated by ammonia oxidizing bacteria, and heterotrophic denitrification. In this work, a critical review of the existing literature on N₂O emissions during BNR is presented focusing on the most contributing parameters. Various factors increasing the N₂O emissions either per se or combined are identified: low dissolved oxygen, high nitrite accumulation, low chemical oxygen demand to nitrogen ratio, slow growth of denitrifying bacteria, uncontrolled pH and temperature. However, there is no common pattern in reporting the N₂O generation amongst the cited studies, a fact that complicates its evaluation. When simulating N₂O emissions, all microbial pathways along with the potential contribution of abiotic N₂O production during wastewater treatment at different dissolved oxygen/nitrite levels should be considered. The undeniable validation of the robustness of such models calls for reliable quantification techniques which simultaneously describe dissolved and gaseous N₂O dynamics. Thus, the choice of the N-removal process, the optimal selection of operational parameters and the establishment of validated dynamic models combining multiple N₂O pathways are essential for studying the emissions mitigation.

Effect of temperature on N2O emissions from a highly enriched nitrifying granular sludge performing partial nitritation of a low-strength wastewater

In the race to achieve a sustainable urban wastewater treatment plant, not only the energy requirements have to be considered but also the environmental impact of the facility. Thus, nitrous oxide (N₂O) emissions are a key-factor to pay attention to, since they can dominate the total greenhouse gases emissions from biological wastewater treatment. In this study, N₂O production factors were calculated during the operation of a granular sludge airlift reactor performing partial nitritation treating a low-strength synthetic influent, and furthermore, the effect of temperature on N₂O production was assessed. Average gas emission relative to conversion of ammonium was 1.5 ± 0.3% and 3.7 ± 0.5% while the effluent contained 0.5 ± 0.1% and 0.7 ± 0.1% (% N-oxidized) at 10 and 20 °C, respectively. Hence, temperature increase resulted in higher N₂O production. The reasons why high temperature favoured N₂O production remained unclear, but different theoretical hypotheses were suggested.

Pathways and Controls of N2O Production in Nitritation-Anammox Biomass

Nitrous oxide (N₂O) is an unwanted byproduct during biological nitrogen removal processes in wastewater. To establish strategies for N₂O mitigation, a better understanding of production mechanisms and their controls is required. A novel stable isotope labeling approach using ¹⁵N and ¹⁸O was applied to investigate pathways and controls of N₂O production by biomass taken from a full-scale nitritation-anammox reactor. The experiments showed that heterotrophic denitrification was a negligible source of N₂O under oxic conditions (≥0.2 mg O₂ L^-1). Both hydroxylamine oxidation and nitrifier denitrification contributed substantially to N₂O accumulation across a wide range of conditions with varying concentrations of O₂, NH₄⁺, and NO₂^-. The O₂ concentration exerted the strongest control on net N₂O production with both production pathways stimulated by low O₂, independent of NO₂^- concentrations. The stimulation of N₂O production from hydroxylamine oxidation at low O₂ was unexpected and suggests that more than one enzymatic pathway may be involved in this process. N₂O production by hydroxylamine oxidation was further stimulated by NH₄⁺, whereas nitrifier denitrification at low O₂ levels was stimulated by NO₂^- at levels as low as 0.2 mM. Our study shows that ¹⁵N and ¹⁸O isotope labeling is a useful approach for direct quantification of N₂O production pathways applicable to diverse environments.

Strategies for enhanced deammonification performance and reduced nitrous oxide emissions

Deammonification's performance and associated nitrous oxide emissions (N₂O) depend on operational conditions. While studies have investigated factors for high performances and low emissions separately, this study investigated optimizing deammonification performance while simultaneously reducing N₂O emissions. Using a design of experiment (DoE) method, two models were developed for the prediction of the nitrogen removal rate and N₂O emissions during single-stage deammonification considering three operational factors (i.e., pH value, feeding and aeration strategy). The emission factor varied between 0.7±0.5% and 4.1±1.2% at different DoE-conditions. The nitrogen removal rate was predicted to be maximized at settings of pH 7.46, intermittent feeding and aeration. Conversely, emissions were predicted to be minimized at the design edges at pH 7.80, single feeding, and continuous aeration. Results suggested a weak positive correlation between the nitrogen removal rate and N₂O emissions, thus, a single optimizing operational set-point for maximized performance and minimized emissions did not exist.

Nitrous Oxide Production in a Granule-based Partial Nitritation Reactor: A Model-based Evaluation

Sustainable wastewater treatment has been attracting increasing attentions over the past decades. However, the production of nitrous oxide (N₂O), a potent GHG, from the energy-efficient granule-based autotrophic nitrogen removal is largely unknown. This study applied a previously established N₂O model, which incorporated two N₂O production pathways by ammonia-oxidizing bacteria (AOB) (AOB denitrification and the hydroxylamine (NH₂OH) oxidation). The two-pathway model was used to describe N₂O production from a granule-based partial nitritation (PN) reactor and provide insights into the N₂O distribution inside granules. The model was evaluated by comparing simulation results with N₂O monitoring profiles as well as isotopic measurement data from the PN reactor. The model demonstrated its good predictive ability against N₂O dynamics and provided useful information about the shift of N₂O production pathways inside granules for the first time. The simulation results indicated that the increase of oxygen concentration and granule size would significantly enhance N₂O production. The results further revealed a linear relationship between N₂O production and ammonia oxidation rate (AOR) (R² = 0.99) under the conditions of varying oxygen levels and granule diameters, suggesting that bulk oxygen and granule size may exert an indirect effect on N₂O production by causing a change in AOR.

Effects of soluble microbial products (SMP) on the performance of an anammox attached film expanded bed (AAFEB) reactor: Synergistic interaction and toxic shock

The accumulation of soluble microbial production (SMP) in an anammox attached film expanded bed (AAFEB) and its effect on the reactor performance were investigated in this study. During the long-term experiment, an extended HRT resulted in the accumulation of SMP and the change of treatment performance. When the SMP increased from 10.5±1.5mgL^-1 to 31.7±6.4mgL^-1 with the increase of influent TN concentration from 313mgL^-1 to 2500mgL^-1, the TN removal efficiency was stable. However, when the influent TN concentration was 3500mgL^-1, the SMP concentration increased higher than 100mgL^-1, the reactor soon became inhibited. Bath tests indicated that both the specific anammox activity (SAA) and the substrate tolerance ability decreased during the stable operation phases, whereas the specific denitrification activity (SDA) was significantly enhanced. In addition, N₂O emissions in the anammox-denitrifier symbiotic system were greater than in the conventional nitrogen removal process.

Kinetic and microbiological characterization of aerobic granules performing partial nitritation of a low-strength wastewater at 10 °C

A granular airlift reactor enriched in ammonia oxidizing bacteria (AOB) was operated at 10 °C performing stable partial nitritation in the long-term. The reactor treated a synthetic low-strength influent during 250 days with an average nitrogen loading rate of 0.63 ± 0.06 g N L(-1) d(-1). Nitrate production was barely detected, being the average concentration in the effluent of 0.6 ± 0.3 mg N-NO3 L(-1). Furthermore, a suitable effluent for a subsequent reactor performing the anammox process was achieved. A maximum specific growth rate as high as 0.63 ± 0.05 d(-1) was determined by performing kinetic experiments with the granular sludge in a chemostat and fitting the results to the Monod model. Pyrosequencing analysis showed a high enrichment in AOB (41 and 65% of the population were identified as Nitrosomonas genus on day 98 and 233, respectively) and an effective repression of nitrite oxidizing bacteria in the long-term. Pyrosequencing analysis also identified the coexistence of nitrifying bacteria and heterotrophic psychrotolerant microorganisms in the granular sludge. Some psychrotolerant microorganisms are producers of cryoprotective extracellular polymeric substances that could explain the better survival of the whole consortia at cold temperatures.

Modeling of Nitrous Oxide Production from Nitritation Reactors Treating Real Anaerobic Digestion Liquor

In this work, a mathematical model including both ammonium oxidizing bacteria (AOB) and heterotrophic bacteria (HB) is constructed to predict N2O production from the nitritation systems receiving the real anaerobic digestion liquor. This is for the first time that N2O production from such systems was modeled considering both AOB and HB. The model was calibrated and validated using experimental data from both lab- and pilot-scale nitritation reactors. The model predictions matched the dynamic N2O, ammonium, nitrite and chemical oxygen demand data well, supporting the capability of the model. Modeling results indicated that HB are the dominant contributor to N2O production in the above systems with the dissolved oxygen (DO) concentration of 0.5-1.0 mg O2/L, accounting for approximately 75% of N2O production. The modeling results also suggested that the contribution of HB to N2O production decreased with the increasing DO concentrations, from 75% at DO = 0.5 mg O2/L to 25% at DO = 7.0 mg O2/L, with a corresponding increase of the AOB contribution (from 25% to 75%). Similar to HB, the total N2O production rate also decreased dramatically from 0.65 to 0.25 mg N/L/h when DO concentration increased from 0.5 to 7.0 mg O2/L.

Unravelling the spatial variation of nitrous oxide emissions from a step-feed plug-flow full scale wastewater treatment plant

Plug-flow activated sludge reactors (ASR) that are step-feed with wastewater are widely adopted in wastewater treatment plants (WWTPs) due to their ability to maximise the use of the organic carbon in wastewater for denitrification. Nitrous oxide (N2O) emissions are expected to vary along these reactors due to pronounced spatial variations in both biomass and substrate concentrations. However, to date, no detailed studies have characterised the impact of the step-feed configuration on emission variability. Here we report on the results from a comprehensive online N2O monitoring campaign, which used multiple gas collection hoods to simultaneously measure emission along the length of a full-scale, step-fed, plug-flow ASR in Australia. The measured N2O fluxes exhibited strong spatial-temporal variation along the reactor path. The step-feed configuration had a substantial influence on the N2O emissions, where the N2O emission factors in sections following the first and second step feed were 0.68% ± 0.09% and 3.5% ± 0.49% of the nitrogen load applied to each section. The relatively high biomass-specific nitrogen loading rate in the second section of the reactor was most likely cause of the high emissions from this section.

Identifying N2O formation and emissions from a full-scale partial nitritation reactor

In this study, N2O formation and emissions from a full-scale partial nitritation (SHARON) reactor were identified through a three-weeks monitoring campaign during which the off-gas was analysed for N2O, O2, CO2 and NO. The overall N2O emission was 3.7% of the incoming ammonium load. By fitting the N2O emission to a theoretical gas stripping profile, the N2O emissions could be assigned to aerobically formed N2O and N2O formed under anoxic conditions. This was further substantiated by liquid N2O measurements. Under standard operation, 70% of the N2O emission was attributed to anoxic N2O formation. Dedicated experiments revealed that low dissolved oxygen concentrations (<1.0 gO2·m(-3)) and longer anoxic periods resulted in an increased N2O emission. Minimising or avoiding anoxic conditions has the highest effect in lowering the N2O emissions. As an additional result, the use of the off-gas N2O concentration measurements to monitor the gas-liquid mass transfer rate coefficient (kLa) during dynamic reactor operation was demonstrated.

Effects of dissolved oxygen and pH on nitrous oxide production rates in autotrophic partial nitrification granules

The effects of dissolved oxygen (DO) and pH on nitrous oxide (N2O) production rates and pathways in autotrophic partial nitrification (PN) granules were investigated at the granular level. N2O was primarily produced by betaproteobacterial ammonia-oxidizing bacteria, mainly Nitrosomonas europaea, in the oxic surface layer (<200μm) of the autotrophic PN granules. N2O production increased with increasing bulk DO concentration owing to activation of the ammonia (i.e., hydroxylamine) oxidation in this layer. The highest N2O emissions were observed at pH 7.5, although the ammonia oxidation rate was unchanged between pH 6.5 and 8.5. Overall, the results of this study suggest that in situ analyses of PN granules are essential to gaining insight into N2O emission mechanisms in a granule.

Minimization of nitrous oxide emission from CASS process treating low carbon source domestic wastewater: Effect of feeding strategy and aeration rate

Nitrous oxide (N2O) emission during wastewater treatment can be mitigated by improving operational conditions, e.g., organic carbon supply and dissolved oxygen. To evaluate the control parameters for N2O emission in the low carbon source domestic wastewater treatment process, N2O emissions from Cyclic Activated Sludge System (CASS) under different feeding strategies and aeration rates were investigated. Results showed that continuous feeding enhanced nitrogen removal and reduced N2O emission compared to batch feeding, while a higher aeration rate led to less N2O emission. N2O was mainly produced during non-aeration phases in batch feeding CASS and the amount of N2O generated from denitrification decreased under continuous feeding, indicating that carbon source in the continuous influent relieved the electron competition between denitrification reductases during non-aeration phase. Moreover, taxonomic analysis based on high-throughput 16S rRNA gene sequencing revealed higher abundance of denitrifying bacteria, especially N2O-reducing bacteria in continuous feeding CASS.

Nitrogen removal performance and microbial distribution in pilot- and full-scale integrated fixed-biofilm activated sludge reactors based on nitritation-anammox process

Nitritation-anammox process was successfully established in pilot- and full-scale integrated fixed-film activated sludge (IFAS) reactors. An average nitrogen removal efficiency of 80% was achieved under ammonium loading rate of 0.7-1.3kgN/(m(3)d) in the pilot-scale reactor (12m(3)). Moreover, molecular analysis showed that ammonium oxidizing bacteria (AOB) were more abundant in the activated sludge while anammox bacteria were primarily located in the biofilm. The segregation of AOB and anammox bacteria enhanced the nitrogen removal rate and operational stability. Furthermore, a full-scale IFAS reactor of 500m(3) was set-up to treat sludge dewatering liquors. An average nitrogen removal efficiency of 85% and a nitrogen removal rate of 0.48kgN/(m(3)d) were achieved after inoculation. It was noted that high influent suspended solids would seriously affect the performance of the IFAS system. Therefore, a pre-treatment was proposed to reduce suspended solid in the full-scale application.

Minimizing N2O emissions and carbon footprint on a full-scale activated sludge sequencing batch reactor

A continuous, on-line quantification of the nitrous oxide (N2O) emissions from a full-scale sequencing batch reactor (SBR) placed in a municipal wastewater treatment plant (WWTP) was performed in this study. In general, N2O emissions from the biological wastewater treatment system were 97.1 ± 6.9 g N2O-N/Kg [Formula: see text] consumed or 6.8% of the influent [Formula: see text] load. In the WWTP of this study, N2O emissions accounted for over 60% of the total carbon footprint of the facility, on average. Different cycle configurations were implemented in the SBR aiming at reaching acceptable effluent values. Each cycle configuration consisted of sequences of aerated and non-aerated phases of different time length being controlled by the ammonium set-point fixed. Cycles with long aerated phases showed the largest N2O emissions, with the consequent increase in carbon footprint. Cycle configurations with intermittent aeration (aerated phases up to 20-30 min followed by short anoxic phases) were proven to effectively reduce N2O emissions, without compromising nitrification performance or increasing electricity consumption. This is the first study in which a successful operational strategy for N2O mitigation is identified at full-scale.

Hydroxylamine diffusion can enhance N₂O emissions in nitrifying biofilms: a modeling study

Wastewater treatment plants can be significant sources of nitrous oxide (N2O), a potent greenhouse gas. However, little is known about N2O emissions from biofilm processes. We adapted an existing suspended-growth mathematical model to explore N2O emissions from nitrifying biofilms. The model included N2O formation by ammonia-oxidizing bacteria (AOB) via the hydroxylamine and the nitrifier denitrification pathways. Our model suggested that N2O emissions from nitrifying biofilms could be significantly greater than from suspended growth systems under similar conditions. The main cause was the formation and diffusion of hydroxylamine, an AOB nitrification intermediate, from the aerobic to the anoxic regions of the biofilm. In the anoxic regions, hydroxylamine oxidation by AOB provided reducing equivalents used solely for nitrite reduction to N2O, since there was no competition with oxygen. For a continuous system, very high and very low dissolved oxygen (DO) concentrations resulted in lower emissions, while intermediate values led to higher emissions. Higher bulk ammonia concentrations and greater biofilm thicknesses increased emissions. The model effectively predicted N2O emissions from an actual pilot-scale granular sludge reactor for sidestream nitritation, but significantly underestimated the emissions when the NH2OH diffusion coefficient was assumed to be minimal. This numerical study suggests an unexpected and important role of hydroxylamine in N2O emission in biofilms.

Would a two-stage N-removal be a suitable technology to implement at full scale the use of anammox for sewage treatment?

Sewage treatment with anammox could be implemented through a two-step reactor system, where the first reactor would be devoted to partial nitritation. A process design was sketched including control loops. The control strategy regulates the flow-rate of the rich ammonium sidestream produced after dewatering the digested sludge, to keep the ammonium concentration at a set point in the partial nitritation reactor by DOsing the SIde Stream (DOSIS). A second control loop manages the ammonium concentration set point based on the measurement of the total nitrogen in the partial nitritation reactor. A mathematical model was developed to assess the amount of sidestream required. Even in the case of a strong diurnal variability, simulations show how the control strategy is correctly performing, demonstrating the potential of the proposed technology.

Heterotrophic denitrification plays an important role in N₂O production from nitritation reactors treating anaerobic sludge digestion liquor

Nitrous oxide (N2O) emissions from nitritation reactors receiving real anaerobic sludge digestion liquor have been reported to be substantially higher than those from reactors receiving synthetic digestion liquor. This study aims to identify the causes for the difference, and to develop strategies to reduce N2O emissions from reactors treating real digestion liquor. Two sequencing batch reactors (SBRs) performing nitritation, fed with real (SBR-R) and synthetic (SBR-S) digestion liquors, respectively, were employed. The N2O emission factors for SBR-R and SBR-S were determined to be 3.12% and 0.80% of the NH4(+)-N oxidized, respectively. Heterotrophic denitrification supported by the organic carbon present in the real digestion liquor was found to be the key contributor to the higher N2O emission from SBR-R. Heterotrophic nitrite reduction likely stopped at N2O (rather than N2), with a hypothesised cause being free nitrous acid inhibition. This implies that all nitrite reduced by heterotrophic bacteria was converted to and emitted as N2O. Increasing dissolved oxygen (DO) concentration from 0.5 to 1.0 mg/L, or above, decreased aerobic N2O production from 2.0% to 0.5% in SBR-R, whereas aerobic N2O production in SBR-S remained almost unchanged (at approximately 0.5%). We hypothesised that DO at 1 mg/L or above suppressed heterotrophic nitrite reduction thus reduced aerobic heterotrophic N2O production. We recommend that DO in a nitritation system receiving anaerobic sludge digestion liquor should be maintained at approximately 1 mg/L to minimise N2O emission.

Influence of operating conditions on nitrous oxide formation during nitritation and nitrification

Nitrous oxide (N2O), a strong greenhouse gas, can be produced by ammonium-oxidizing bacteria (AOB) as a by-product of ammonium oxidation and can potentially be formed in all types of nitrification processes. However, partial nitritation has been reported to cause significantly higher N2O emissions than complete nitrification. In the study presented here, the mechanisms and factors that drive N2O formation by AOB were investigated with respect to different operational strategies to achieve nitrite accumulation base on combined evaluation of oxygen uptake rate (OUR) and N2O formation rate. On the one hand, N2O formation during partial nitritation and nitrification in a continuously stirred tank reactor (CSTR) with continuous aerobic conditions was observed. On the other hand, the effect of intermittent aeration on N2O formation during nitrification was investigated. The presence of nitrite, the extend of sludge-specific ammonium loading, low oxygen concentration, and transition from aerobic to anoxic conditions significantly increased N2O formation in this reactor independently from each other, indicating that different formation pathways, supposedly via nitrite or hydroxylamine, were active.

Evaluation of process conditions triggering emissions of green-house gases from a biological wastewater treatment system

In this study, methane (CH4) and nitrous oxide (N2O) emission dynamics of a plug-flow bioreactor located in a municipal full-scale wastewater treatment plant were monitored during a period of 10 weeks. In general, CH4 and N2O gas emissions from the bioreactor accounted for 0.016% of the influent chemical oxygen demand (COD) and 0.116% of the influent total Kjeldahl nitrogen (TKN) respectively. In order to identify the emission patterns in the different zones, the bioreactor was divided in six different sampling sites and the gas collection hood was placed for a period of 2-3 days in each of these sites. This sampling strategy also allowed the identification of different process perturbations leading to CH4 or N2O peak emissions. CH4 emissions mainly occurred in the first aerated site, and were mostly related with the influent and reject wastewater flows entering the bioreactor. On the other hand, N2O emissions were given along all the aerated parts of the bioreactor and were strongly dependant on the occurrence of process disturbances such as periods of no aeration or nitrification instability. Dissolved CH4 and N2O concentrations were monitored in the bioreactor and in other parts of the plant, as a contribution for the better understanding of the transport of these greenhouse gases across the different stages of the treatment system.