N2O emissions from full-scale nitrifying biofilters

Synergies from off-gas analysis and mass balances for wastewater treatment - Some personal reflections on our experiences

Looking back at over a decade of research by herself and her group, the author advocates the added value of gas phase measurements and the application of mass balances, as well as the synergetic benefits obtained when combining both. The increased application of off-gas measurements for greenhouse gas emission monitoring offers a great opportunity to look at other components in the gas phase, particularly oxygen. Mass balances should not be strictly reserved for modellers but also prove useful while conducting lab experiments and studying full-scale measurement data. Combining off-gas measurements with mass balances may serve not only to quantify greenhouse gas emission factors and aeration efficiency but also to follow dynamic concentration profiles of dissolved components without dedicated sensors and/or to calculate other unmeasured variables. Mass-balance-based data reconciliation allows for obtaining reliable and accurate data, and even more when combined with off-gas analysis.

Soil N2O emissions during dry fallow periods

Shang et al. (2024) recently suggested to include nitrous oxide (N2O) emissions during the fallow period to better estimate N2O emission factors (EFs). We however highlighted several pitfalls of the proposed adjusted EFs for croplands in the specific case of dry subhumid, semiarid, and arid regions with dry fallow periods, these regions covering about 47% of the Earth's terrestrial area.

Impact of urbanism on source separation systems: A life cycle assessment

This study aims to assess the effect of different urban configuration regarding the choice of wastewater management of the district with source separation systems. Understanding this link can guide researchers, and also urban actors, in order to choose the best source separation solution to implement in a specific urban configuration. For this purpose, an integrated modelling approach was used to model the district with different types of urban planning, the water resources recovery facility (WRRF) and create a life cycle inventory to carry out a life cycle assessment (LCA). Six different urban configurations were tested with three different source separation scenarios and compared with an advanced WRRF with high level of nutrients and organic matter recovery. This study concludes that urine source separation is beneficial compared to advanced WWRF for all the urban configurations. Sewer construction was identified as the main contributor to environmental impact for the low-density configuration (pavilions), limiting the benefits of source separation in this urban settlement. Blackwater separation with a decentralised treatment is only beneficial for high densely populated area. Treatment of blackwater and greywater for reuse, has greater impact than reference scenario, in all urban configurations, due to high energy consumption for greywater treatment. Future research should therefore explore technical solutions for limiting the energy consumption.

Synchronous Achievement of Advanced Nitrogen Removal and N2O Reduction in the Anoxic Zone in the AOA Process for Low C/N Municipal Wastewater

Continuous flow processes for the in situ determination of N2O emissions during low C/N municipal wastewater treatment have rarely been reported. The anaerobic/aerobic/anoxic (AOA) process has recently shown promising potential in energy savings and advanced nitrogen removal, but it still needs to be comprehensively explored in relation to N2O emissions for its carbon reduction advantages. In this study, a novel gas-collecting continuous flow reactor was designed to comprehensively evaluate the emissions of N2O from the gas and liquid phases of the AOA process. Additionally, the measures of enhancing endogenous denitrification (ED) and self-enriching anaerobic ammonium oxidation (Anammox) were employed to optimize nitrogen removal and achieve N2O reduction in the anoxic zone. The results showed that enhanced ED coupled with Anammox led to an increase in the nitrogen removal efficiency (NRE) from 67.65 to 81.96%, an enhancement of the NO3- removal rate from 1.76 mgN/(L h) to 3.99 mgN/(L h), and the N2O emission factor in the anoxic zone decreased from 0.28 to 0.06%. Impressively, ED eliminated 91.46 ± 2.47% of the dissolved N2O from the upstream aerobic zone, and the dissolved N2O in the effluent was reduced to less than 0.01 mg/L. This study provides valuable strategies for fully evaluating N2O emissions and N2O reduction from the AOA process.

Modelling the benefits of urine source separation scenarios on wastewater treatment plants within an urban water basin

Stringent discharge regulations are encouraging researchers to create innovative and sustainable wastewater treatment solutions. Urine source separation (USS) is among the potent approaches that may reduce nutrient peak loads in the influent wastewater and improve nutrient recovery. A phenomenological model was used to simulate dynamic influent properties and predict the advantages gained from implementing USS in an urban water basin. Several scenarios were investigated assuming different levels of deployment: at the entire city, or specifically in office buildings for men's urine only, or for both men and women employees. The results confirmed that all scenarios of urine source separation offered benefits at the treatment plant in terms of reducing nitrogen influent load. The economic benefits in terms of reducing energy consumption for nitrification and decreasing methanol addition for denitrification were quantified, and results confirmed environmental advantages gained from different USS scenarios. Despite larger advantages gained from a global USS rate in an entire city, implementation of a specific USS in office buildings would remain more feasible from a logistical perspective. A significant benefit in terms of reducing greenhouse gas emissions is demonstrated and this was especially due to the high level of N₂O emissions avoided in nitrifying biological aerated filter.

Advancing greenhouse gas emission factors for municipal wastewater treatment plants in China

Estimations of greenhouse gas (GHG) emissions from municipal wastewater treatment plants (MWTPs) remain significant uncertainties in China owing to a lack of reliable emission factors (EFs). This study developed a framework to obtain multi-level (technology, province, and nation) GHG EFs of MWTPs using a database containing 3107 MWTPs in China and published site-specific monitoring data. Results show that GHG EFs of different technologies range widely from 180.0 to 615.7 g CO₂-eq/t wastewater, and significant differences are also observed among different provinces in China (190.5-600.3 g CO₂-eq/t wastewater), which are generally lower than the previous estimates. It confirms the importance of more detailed technology classification and considering the technological disparity of different provinces in refining GHG estimations of MWTPs. To test the feasibility of the developed EFs, we compared GHG emissions from MWTPs based on multi-level EFs at different spatial and temporal scales. Similar estimation results imply that selecting corresponding EF depending on the availability of activity data would simplify GHG estimations of MWTPs without sacrificing much accuracy. This study contributes a set of well-developed EFs to improve the estimates of GHG emissions from MWTPs, and also offers a method to develop GHG EFs for other sectors.

Estimation of countrywide N2O emissions from wastewater treatment in Switzerland using long-term monitoring data

Nitrous oxides (N₂O) emissions contribute to climate change and stratospheric ozone depletion. Wastewater treatment is an important, yet likely underestimated, source of N₂O emissions, as recent, long-term monitoring campaigns have demonstrated. However, the available data are insufficient to representatively estimate countrywide emission due to the brevity of most monitoring campaigns. This study showed that the emission estimates can be significantly improved using an advanced approach based on multiple continuous, long-term monitoring campaigns. In monitoring studies on 14 full-scale wastewater treatment plants (WWTPs), we found a strong variability in the yearly emission factors (EFs) (0.1 to 8% of the incoming nitrogen load) which exhibited a good correlation with effluent nitrite. But countrywide data on nitrite effluent concentrations is very limited and unavailable for emission estimation in many countries. Hence, we propose a countrywide emission factor calculated from the weighted EFs of three WWTP categories (carbon removal, EF: 0.1-8%, nitrification only: 1.8%, and full nitrogen removal: 0.9%). However, EF of carbon removal WWTPs are still highly uncertain given the expected variability in performance. The newly developed approach allows representative, country-specific estimations of the N₂O emissions from WWTP. Applied to Switzerland, the estimations result in an average EF of 0.9 to 3.6% and total emissions of 410 to 1690 tN₂O-N/year, which corresponds to 0.3-1.4% of the total greenhouse gas emissions in Switzerland. Our results demonstrate that better data availability and an improved understanding of long-term monitoring campaigns is crucial to improve current emission estimations. Finally, our results confirm several measures to mitigate N₂O emissions from wastewater treatment; year-round denitrification, limiting nitrite accumulation, and stringent control of sludge age in carbon removal plants.

Temporal triggers of N2O emissions during cyclical and seasonal variations of a full-scale sequencing batch reactor treating municipal wastewater

To investigate the major triggers of nitrous oxide (N₂O) production in a full-scale wastewater treatment plant, N₂O emissions and wastewater characteristics (ammonia, nitrite, nitrate, total nitrogen, dissolved inorganic carbon, dissolved organic carbon, pH, temperature, dissolved oxygen and specific oxygen uptake rate), the results of variations in the cycling of a sequential batch reactor (SBR, where only full nitrification was performed), were monitored seasonally for 16 months. Major triggers of N₂O production were investigated based on a seasonal measured database using a random forest (RF) model and sensitivity analysis, which was applied to identify important input variables. As the result of seasonal monitoring in the full-scale SBR, the N₂O emission factor relative to daily total nitrogen removal ranged from 0.05 to 2.68%, corresponding to a range of N₂O production rate from 0.02 to 0.70 kg-N/day. Results from the RF model and sensitivity analysis revealed that emissions during nitrification were directly or indirectly related to nitrite accumulation, temperature, ammonia loading rate and the specific oxygen uptake rate ratio between ammonia oxidizing bacteria and nitrite oxidizing bacteria (sOUR-ratio). However, changes in the microbial community did not significantly impact N₂O emissions. Based on these results, the sOUR-ratio could represent the major trigger for N₂O emission in a full-scale BNR system: a higher sOUR-ratio value with an average of 3.13 ± 0.23 was linked to a higher N₂O production rate with an average value of 1.27 ± 0.12 kg-N/day (corresponding to 3.96 ± 1.20% of N₂O emission factor relative to daily TN removal), while a lower sOUR-ratio with an average value of 2.39 ± 0.27 was correlated with a lower N₂O production average rate of 0.17 ± 0.11 kg-N/day (corresponding to 0.74 ± 0.69% of N₂O emission factor) (p-value = 0.00001, Mann-Whitney test).

Metagenomic insights into the "window" effect of static magnetic field on nitrous oxide emission from biological nitrogen removal process at low temperature

This study aimed to explore whether the "window" effect of static magnetic field (SMF) on nitrous oxide (N₂O) emission from the biological nitrogen removal process at low temperature existed and reveal its biological mechanism at the gene level. Four sequencing batch reactors (SBRs) with SMFs of 0, 10, 45, and 75 mT were operated continuously for 110 days at 10 °C and the lowest N₂O-Gas cumulative emission (5.50 mg N/day) and N₂O conversion rate (4.28 %) in 45 mT SMF-SBR verified the existence of the "window" effect. In 45 mT SMF-SBR, nearly all enzymatic activities related to N₂O reduction and corresponding functional gene abundances improved significantly. Metagenomic high-throughput sequencing analysis revealed that Alicycliphilus denitricans, Paracoccus denitrificans, Rhodopseudomonas palustris, Pseudomonas stutzeri, and Dechloromonas aromatica, as species related to N₂O reduction, could be separately enriched by applying suitable SMF intensity. Gene functions annotation based on KEGG and CAZy databases indicated that SMF not only accelerated the rate of free ammonia into ammonia-oxidizing bacteria and electrons delivered to the corresponding denitrification reductases, but also enhanced the degradation of complex organic matter into smaller molecules, and thus reducing the production of N₂O via nitrifier denitrification and incomplete denitrification pathways at 10 °C. These findings provided a guideline and presented a blueprint of ecophysiology for the future application of magnetic field to the reduction of N₂O emission in wastewater treatment plants in the cold region.

A predictive model for N2O production in anammox-granular sludge reactors: Combined effects of nitrite/ammonium ratio and organic matter concentration

Once the use of anammox reactors has been increasing on a global scale, it is important to understand the mechanisms of N₂O emissions and how to minimise the emissions by optimising the operating conditions. In this study, the influence of chemical oxygen demand (COD) (from 0 mgO₂ L^-1 to 100 mgO₂ L^-1) and nitrite/ammonium ratio from 0.79 to 2.21 (maintaining ammonium at 100 mgN L^-1 and varying nitrite from 79 mgN L^-1 to 221 mgN L^-1) in the N₂O emissions from anammox-granular sludge reactor was investigated in two steps. Step 1 consisted of batch tests, using central composite design, and Step 2, long-term operation of a 6.5 L continuous up-flow reactor. The results showed that the N₂O emissions were minimized by controlling, in the influent, the NO₂^--N/NH₄⁺-N ratio from 1.1 to 1.3 and maintaining the COD concentration below 100 mgO₂ L^-1. TN removal efficiencies were higher than 70% in all conditions tested".

Pineapple peel biochar and lateritic soil as adsorbents for recovery of ammonium nitrogen from human urine

Human urine is a rich source of nitrogen which can be captured to supplement the existing sources of nitrogen fertilizers thus contributing to enhanced crop production. However, urine is the major contributor of macronutrients in municipal wastewater flows resulting into eutrophication of the receiving water bodies. Herein, pineapple peel biochar (PPB), and lateritic soil (LS) adsorbents were prepared for the safe removal of ammonium nitrogen (NH₄⁺-N) from human urine solutions. Physicochemical properties of PPB, and LS were characterized by scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA) to investigate the relationship of their properties with NH₄⁺-N adsorption. Langmuir, Freundlich, and Dubinin-Radushkevich (D-R) isotherm models were employed to correlate the experimental equilibrium adsorption data. The effect of contact time and initial concentration of NH₄⁺-N adsorption was also evaluated. The D-R isotherm model best described the behaviour of NH₄⁺-N adsorption on both PPB and LS based on the coefficient of correlation values. This model showed that the adsorption of NH₄⁺-N on both samples was a physical process with PPB and LS having mean surface adsorption energies of 1.826 × 10^-2, and 1.622 × 10^-2 kJ/mol, respectively. The PPB exhibited a slightly higher adsorption capacity for NH₄⁺-N (13.40 mg/g) than LS (10.73 mg/g) with the difference attributed to its higher surface area and porosity. These values are good indicators for assessing the effectiveness of the materials for adsorption of NH₄⁺-N from human urine.

Evaluating the roles of coexistence of sludge flocs on nitrogen removal and nitrous oxide production in a granule-based autotrophic nitrogen removal system

Certain levels of sludge flocs would always coexist in granule-based reactors due to the biomass detachment from granules. Such inevitable coexistence could affect both total nitrogen (TN) removal and nitrous oxide (N₂O) production in autotrophic nitrogen removal systems. This work utilized a mathematical approach to systematically study the influence of the coexisting sludge flocs on TN removal and N₂O production in a granular nitritation-anaerobic ammonium oxidation (Anammox) process for the first time, based on a 2-pathway N₂O production model concept. The modelling results reveal that the highest TN removal efficiency decreases from ca. 87-88% to ca. 41-49% as the fraction of sludge flocs in the system increases from 10% to 40%, while the N₂O production rate gradually increases with such increase. Meanwhile, both bulk dissolved oxygen (DO, 0.05-0.3 mg/L) and the size of granule (200-400 μm) could also influence the TN removal efficiency and N₂O production. As the fraction of sludge flocs increases from 10% to 40%, the contribution of granular biomass to total N₂O production is reduced due to increase of N₂O-producing ammonia-oxidizing bacteria (AOB) in the sludge flocs, and the increase of granule size could intensify such decrease. In addition, the hydroxylamine oxidation pathway dominates the nitrifier denitrification pathway in both granules and sludge flocs under various testing conditions, whereas the increasing contribution of the latter would occur at a certain DO range, higher fraction of sludge flocs and smaller granule size. These results disclose an important influence of the coexisting sludge flocs on the performance of granular nitritation-Anammox systems.

Impact of gas-water ratios on N2O emissions in biological aerated filters and analysis of N2O emissions pathways

Biological aerated filter (BAF) is a widely applied biofilm process for wastewater treatment. However, characteristics of nitrous oxide (N₂O) production in BAF are rarely reported. In this study, two tandem BAFs treating domestic wastewater were built up, and different gas-water ratios were controlled to explore N₂O production pathway. Results showed that N₂O production increased with increasing gas-water ratio in both BAFs; higher gas-water ratio promoted more N₂O releasing from hydroxylamine oxidation process. To improve nitrogen removal performance and reduce N₂O emission, the optimal gas-water ratios for BAF1 and BAF2 were 5:1 and 1.5:1, respectively. Most of N₂O was produced from ammonia oxidizing bacteria (AOB) denitrification and hydroxylamine oxidation in BAF1, and heterotrophic denitrification contributed to relieve N₂O emission. In BAF2, N₂O was emitted from AOB denitrification and hydroxylamine oxidation by 87.8% and 12.2%, respectively. Heterotrophic denitrification is a N₂O sink in BAF, causing BAF1 produced less N₂O than BAF2 with the same gas-water ratio. Enhancing heterotrophic denitrification and anaerobic ammonium oxidation (Anammox) activity could reduce the release of N₂O in BAFs.

Past and Future Trajectories of Human Excreta Management Systems: Paris in the Nineteenth to Twenty-First Centuries

This chapter addresses the fate of nutrients in agro-food systems after their ingestion by humans. Depending on how human urine and faeces are managed, they can become a source of pollution to the environment, or they can be used as a resource, notably as fertilisers, thus contributing to closing the loop of nutrients. Taking the city of Paris as a case study from the nineteenth to the twenty-first century, we analyse the fate of human excreta through the evaluation of corresponding nitrogen and phosphorus mass flows. We put forward two major phases concerning the management of human excreta:

This increase in linearity came together with increased dependency of agro-systems on fossil resources. Ongoing climate change is also putting the current system under pressure since the dilution capacity of the Seine River is decreasing, while the population of Paris is increasing. We therefore analyse three scenarios of future human excreta management (incineration, end-of-pipe recycling and source separation) and show that source separation of human excreta may offer the perspective of a sustainable human excreta management system.

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 emissions from a full-scale biological aerated filter (BAF) subject to seawater infiltration

The increase of salt concentrations in influent wastewaters will be a consequence of the sea level rises in coastal areas due to climate change and the future use of seawater to flush toilets as a cost-attractive option for alternative water resources. Yet, little is known about the salinity effect on full-scale wastewater treatment plants (WWTPs) performance and on greenhouse gas (GHG) emissions, such as nitrous oxide (N₂O). This study aimed at quantifying the N₂O emissions of a full-scale biological aerated filter (BAF) and to correlate the dynamic behavior of the emissions with the process conditions and the periods of infiltration of seawater. A full-scale BAF was monitored for 3 months to assess both their gaseous and liquid N₂O fluxes. The total average daily N₂O emissions of the plant were 6.16 g N-N₂O/kg of NH₄-N removed. For the first time at full-scale, a correlation between the N₂O emissions and the wastewater influent conductivity (salinity) was found, in which the increase in seawater infiltration in the sewer at high tide augments the daily N₂O production and emission to 13.78 g N-N₂O/kg of NH₄-N removed. The proportional increase in influent conductivity and the N₂O emission factor in this WWTP suggested that periods of high conductivity could serve as an indicator of increased N₂O emissions by the plant. Furthermore, the operational conditions and the wastewater influent characteristics that influence the N₂O emissions were identified as being the dissolved oxygen (DO) dynamics due to the filter washing steps, leading to rapid transitions from oxic to sub-oxic conditions, as well as the (re-)adaptation of microbial consortia due to the dynamics of the biofilm thickness associated to the daily washing process. This study shows the impact that the washing process and seawater infiltration has on the N₂O emissions of a BAF and contributes to a better understanding of the operational conditions impacting the emissions in WWTPs.

Considering the plug-flow behavior of the gas phase in nitrifying BAF models significantly improves the prediction of N2O emissions

Nitrifying biologically active filters (BAFs) have been found to be high emitters of nitrous oxide (N₂O), a powerful greenhouse gas contributing to ozone layer depletion. While recent models have greatly improved our understanding of the triggers of N₂O emissions from suspended-growth processes, less is known about N₂O emissions from full-scale biofilm processes. Tertiary nitrifying BAFs have been modeled at some occasions but considering strong simplifications on the description of gas-liquid exchanges which are not appropriate for N₂O prediction. In this work, a tertiary nitrifying BAF model including the main N₂O biological pathways was developed and confronted to full-scale data from Seine Aval, the largest wastewater resource recovery facility in Europe. A mass balance on the gaseous compounds was included in order to correctly describe the N₂O gas-liquid partition, thus N₂O emissions. Preliminary modifications of the model structure were made to include the gas phase as a compartment of the model, which significantly affected the prediction of nitrification. In particular, considering gas hold-up influenced the prediction of the hydraulic retention time, thus nitrification performances: a 3.5% gas fraction reduced ammonium removal by 13%, as the liquid volume, small in such systems, is highly sensitive to the gas presence. Finally, the value of the volumetric oxygen transfer coefficient was adjusted to successfully predict both nitrification and N₂O emissions.

The link between nitrous oxide emissions, microbial community profile and function from three full-scale WWTPs

Few attempts have been made in previous studies to link the microbial community structure and function with nitrous oxide (N₂O) emissions at full-scale wastewater treatment plants (WWTPs). In this work, high-throughput sequencing and reverse transcriptase-qPCR (RT-qPCR) was applied to activated sludge samples from three WWTPs for two seasonal periods (winter and summer) and linked with the N₂O emissions and wastewater characteristics. The total N₂O emissions ranged from 7.2 to 937.0 g N-N₂O/day, which corresponds to an emission factor of 0.001 to 0.280% of the influent NH₄-N being emitted as N₂O. Those emissions were related to the abundance of Nitrotoga, Candidatus Microthrix and Rhodobacter genera, which were favored by higher dissolved oxygen (DO) and nitrate (NO₃^-) concentrations in the activated sludge tanks. Furthermore, a relationship between the nirK gene expression and N₂O emissions was verified. Detected N₂O emission peaks were associated with different process events, related to aeration transition periods, that occurred during the regular operation of the plants, which could be potentially associated to increased emissions of the WWTP. The design of mitigation strategies, such as optimizing the aeration regime, is therefore important to avoid process events that lead to those N₂O emissions peaks. Furthermore, this study also demonstrates the importance of assessing the gene expression of nosZ clade II, since its high abundance in WWTPs could be an important key to reduce the N₂O emissions.

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.

Impact of the treatment of NH3 emissions from pig farms on greenhouse gas emissions. Quantitative assessment from the literature data

In order to limit ammonia (NH₃) emissions from pig farms, various air cleaning solutions are widely applied. However, the literature data report that these systems (chemical scrubbers, bioscrubbers and biofilters) can be both inefficient and promote nitrous oxide (N₂O) production. As air cleaning technologies should not contribute to secondary trace gases that may have a stronger environmental impact than the raw gas compounds themselves, the objective of this study was to quantify the effect of NH₃ treatment in pig farms on greenhouse gas (GHG) emissions. GHGs (carbon dioxide, methane and nitrous oxide) emitted at the outlet of three different cleaning systems ("chemical scrubber", "bioscrubber" and "bioscrubber + denitrification step") were assessed and compared with the emissions generated by the exhaust air with "no treatment". The calculations show that the chemical scrubber has no effect whereas biological treatments can increase GHG emissions. The use of bioscrubbers alone for NH₃ removal can remain acceptable provided that less than 3% of the NH₃ entering the apparatus is converted into N₂O. In such cases, a maximum increase of 1.9% in GHG emissions could be obtained. Conversely, the addition of a denitrification step to a bioscrubber must be avoided. Increases in overall GHG emissions of up to 25.8% were calculated but more significant increases could occur. With regard to GHG emissions, it is concluded that the use of a chemical scrubber is more suitable than a bioscrubber to treat exhaust air from pig farms.

Linking nitrous oxide emissions to population dynamics of nitrifying and denitrifying prokaryotes in four full-scale wastewater treatment plants

Ammonia-oxidizing bacteria (AOB), ammonia oxidizing archaea (AOA) and N₂O-reducing denitrifiers were measured by quantitative real-time PCR (qPCR) in activated sludge samples from four full-scale wastewater treatment plants (WWTPs) in South Spain, and their abundances were linked to the generation of N₂O in the samples using multivariate analysis (Non-metric multidimensional scaling, MDS, and BIO-ENV). The average abundances of AOA remained in similar orders of magnitude in all WWTPs (10⁶ copies amoA/L activated sludge mixed liquor), while significant differences were detected for AOB (10⁵-10⁹copies amoA/L) and N₂O-reducers (10⁷-10¹⁰copies nosZ/L). Average N₂O emissions measured in activated sludge samples ranged from 0.10 ± 0.05 to 6.49 ± 8.89 mg N₂O-N/h/L activated sludge, and were strongly correlated with increased abundances of AOB and lower counts of N₂O-reducers. A significant contribution of AOA to N₂O generation was unlikely, since their abundance correlated negatively to N₂O emissions. AOB abundance was favoured by higher NO₃^- and NO₂^-concentrations in the activated sludge.

Leachate treatment in landfills is a significant N2O source

The importance of methane (CH₄) emissions from landfills has been extensively documented, while the nitrous oxide (N₂O) emissions from landfills are considered negligible. In this study, three landfills were selected to measure CH₄ and N₂O emissions using the static chamber method. Dongbu (DB) and Dongfu (DF) landfills, both located in Xiamen city, Fujian Province, were classified as sanitary. The former started to receive solid waste from Xiamen city in 2009, and the latter was closed in 2009. Nanjing (NJ) landfill, located in Nanjing county, Fujian Province, was classified as managed. Results showed that for the landfill reservoirs, CH₄ emissions were significant, while N₂O emissions occurred mainly in operating areas (on average, 16.3 and 19.0mgN₂Om^-2h^-1 for DB and NJ landfills, respectively) and made a negligible contribution to the total greenhouse gas emissions in term of CO₂ equivalent. However, significant N₂O emissions were observed in the leachate treatment systems of sanitary landfills and contributed 72.8% and 45.6% of total emissions in term of CO₂ equivalent in DB and DF landfills, respectively. The N₂O emission factor (EF) of the leachate treatment systems was in the range of 8.9-11.9% of the removed nitrogen. The total N₂O emissions from the leachate treatment systems of landfills in Xiamen city were estimated to be as high as 8.55gN₂O-Ncapita^-1yr^-1. These results indicated that N₂O emissions from leachate treatment systems of sanitary landfills were not negligible and should be included in national and/or local inventories of greenhouse gas emissions.

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.