Higher N2O Production Research Articles

Lithology-driven soil properties control of N2O production by ammonia oxidizers in subtropical forest soils

Lithology can strongly influence soil's physical and chemical properties, significantly affecting soil nitrogen (N) transformation rates. Ammonia-oxidizing archaea (AOA), ammonia-oxidizing bacteria (AOB) and complete ammonia oxidizers (comammox Nitrospira) constitute the major producers of soil nitrous oxide (N2O), but the importance of different lithology on their relative contributions still poorly understood, especially in terms of newly discovered and widespread comammox. To address this knowledge gap, three nitrification inhibitors, acetylene (ammonia oxidizers inhibitor), 1-octyne (AOB inhibitor) and 3,4-dimethylpyrazole phosphate (DMPP) (both AOB and comammox inhibitor), were used to selectively inhibit the activity of different ammonia oxidation groups in order to discriminate their respective roles in N2O production in forest soils developed on different lithologies (limestone and clastic rock). The results showed that AOB dominated N2O production (63.5 ± 3.3 %) in limestone soils with ammonium amendment, followed by AOA (34.5 ± 7.6 %), while comammox contributed 1 % of total N2O production with N2O yield of 0.03 ± 0.04 %. Higher N2O production by AOB in limestone soils is associated with higher soil pH, exchangeable calcium/magnesium and lower short-range order iron/aluminum minerals. However, the N2O yield by comammox (0.12 ± 0.12 %) in clastic rock soils was comparable to that by AOA (0.15 ± 0.14 %) and AOB (0.20 ± 0.13 %). Based on the results of the correlation analysis, comammox in the clastic rock soils produce greater amounts of N2O is closely related to the higher soil organic carbon and available phosphorus. The present study provided evidence that the lithology-driven soil properties markedly affect the N2O production from different ammonia oxidizers.

Municipal wastewater driven partial-denitrification (PD) aggravated nitrous oxide (N2O) production

Partial Denitrification (PD) provides a new route for nitrate-rich wastewater treatment by combining with energy-efficient anammox. This process was reported to hardly produce N2O despite high nitrite accumulation using acetate as carbon source. This study, however, demonstrated significant N2O would be produced when feeding with real municipal wastewater, the NTR-N2O (NO3−-to-N2O transformation ratio) reached as high as 10%. The supply of feed with a high COD/NO3−-N ratio was unable to alleviate N2O production but was severely aggravated after nitrate was depleted. Excess carbon source resulted in accumulated nitrite further being reduced with NTR-N2O largely improved to 63.4%. Besides, it was observed that acid pH inhibited PD activity and resulted in N2O production significantly increased with the NTR-N2O of 39.9% at an initial pH of 5.5. The increase of pH to alkaline conditions cannot mitigate N2O production. A low feeding proportion of real municipal wastewater for low nitrate wastewater treatment can drastically lower N2O production. The NTR-N2O was only 0.1% at a nitrate concentration of 10 mgN/L (10%), much lower than that at 30 mgN/L with a 30% feeding proportion of municipal wastewater (9.4%). Complex organics and ammonium present in municipal wastewater were supposed to be the primary reasons for the high N2O production. Overall, this work offered a new insight into N2O production during the PD process using the complex organics from wastewater itself.

Factors Affecting Nitrous Oxide Emissions from Activated Sludge Wastewater Treatment Plants—A Review

Nitrous oxide (N2O) is a greenhouse gas contributing to ozone layer depletion and climate change. Wastewater treatment plants (WWTPs) contribute significantly to the global anthropogenic N2O emissions. The main factors affecting N2O emissions are the dissolved oxygen concentration (DO), the nitrite accumulation, the rapidly changing process conditions, the substrate composition and COD/N ratio, the pH, and the temperature. Low DO in the nitrification process results in higher N2O emissions, whereas high aeration rate in the nitration/anammox process results in higher N2O production. High DO in the denitrification inhibits the N2O reductase synthesis/activity, leading to N2O accumulation. High nitrite accumulation in both the nitrification and denitrification processes leads to high N2O emissions. Transient DO changes and rapid shifts in pH result in high N2O production. Ammonia shock loads leads to incomplete nitrification, resulting in NO2− accumulation and N2O formation. Limiting the biodegradable substrate hinders complete denitrification, leading to high N2O production. A COD/N ratio above 4 results in 20–30% of the nitrogen load being N2O emissions. Maximum N2O production at low pH (pH = 6) was observed during nitrification/denitrification and at high pH (pH = 8) during partial nitrification. High temperature enhances the denitrification kinetics but produces more Ν2O emissions.

Chemolithoautotrophic denitrification intensifies nitrogen loss in the Eastern Arabian Sea Shelf waters during sulphidic events

The Eastern Arabian Sea Shelf i.e. Western Indian Continental Shelf (WICS) – a known biogeochemical hotspot is characterized by monsoonal upwelling, seasonal O2 deficiency, extremely high N2O build-up and sulphidic events. The frequency and duration of the sulphidic events have increased over the last two decades, but their impact on the pelagic N cycling, N budget, and N2O dynamics is poorly constrained. Thus, to address these problems and assess their implications on WICS biogeochemistry, we carried out physico-chemical measurements, 15N-labeled incubations and bag incubations on five transects over the shelf during the sulphidic event (September-October) of 2011. We observed very high rates of sulphide-driven chemolithotrophic denitrification (1885–5825 nM N2 d−1) in the sulphidic, nitrate-depleted waters, and its potential occurrence in the sulphide-free, nitrate-replete waters (460–3137 nM N2 d−1), along with high transient N2O production, and comparably low rates of anammox (0–119 nM N2 d−1) and DNRA (0–45 nM N d−1). Despite the predominant cloud cover during the monsoon season, we could for the first time show the satellite image of a large colloidal sulphur (S0) plume associated with the sulphidic event off Western India providing further evidence of extensive sulphide oxidation coupled to denitrification. Sulphide-driven denitrification (mean rate = 2697 nM N2 d−1) appeared to be the dominant N loss process during the anoxic regime (September-October) replacing the chemoorganotrophic (i.e. heterotrophic) denitrification (342 nM N2 d−1) that predominates during the preceding suboxic regime (July-August). Overall, the highest sulphide-driven denitrification rate over the WICS was found to be the second highest among the anoxic coastal systems of the world. Furthermore, simultaneous consumption of NOx− and S2− at a ratio close to the theoretical value in the anaerobic incubations of chemocline waters indicated that the sulphide-driven denitrifiers were fixing carbon. The estimated dark C production (0.21 g C m−2 d−1) due to chemolithoautotrophic denitrification was 18% of the photoautotrophic production and accounted for 15% of the total column productivity. Based on our conservative estimates, the chemolithoautotrophic denitrification was responsible for the removal of 0.4 Tg of fixed N and 0.57 Tg of sulphide, and fixation of 0.1 Tg of carbon annually in the shelf waters. Thus, the sulphidic event impacted the biogeochemistry and ecology, and modulated the N loss pathways and rates over the WICS. With the expansion and intensification of OMZs induced by global climate change, and the spreading of dead zones due to increasing anthropogenic activities, chemolithoautotrophic denitrification is likely to become increasingly significant in oceanic N cycle and impact the N budgets of shallow marine systems particularly.

Evaluation of nitrogen removal and nitrous oxide turnovers in granule-based simultaneous nitrification and denitrification system

Nitrous oxide (N2O) is an inevitable intermediate generated during the nitrogen removal process of granule-based simultaneous nitrification and denitrification (SND) system. In order to alleviate N2O production while maintaining a desired total nitrogen (TN) removal level in this system, a comprehensive evaluation of the contribution pathways and process parameters affecting N2O turnovers is keenly required. Therefore, mathematical models were applied to evaluate the impact of operating conditions and unravel potential mechanisms on TN removal performance and N2O production. Simulation results show that higher N2O production (11.6 %–14.2 %) occurs at higher dissolved oxygen (DO) concentrations, lower chemical oxygen demand (COD) levels, longer hydraulic retention time (HRT) and larger granule size in the granular SND system. The relative conversion rates of nitrogenous components in different regions within the granule influence N2O turnovers, with the nitrification process occurring only in the region 200 μm inward from the granule surface and denitrification working throughout the entire granule. In the inner region of the granule (0–300 μm), the heterotrophic bacteria (HB) denitrification pathway dominates N2O production as a source of N2O. While in the outer region (300–450 μm), HB denitrification acts as a sink for N2O and regulates N2O turnovers (i.e. production and reduction of N2O) together with the hydroxylamine (NH2OH) pathway that is the main contributor of N2O production. Moreover, simultaneous adjustment of multiple operating parameters within a certain range can lower the N2O production factor (<0.5 %) while achieving the desired TN removal efficiency (>80 %), resulting in a feasible N2O mitigation strategy.

Higher N2O production in sequencing batch reactors compared to continuous stirred tank reactors: effect of feast-famine cycles

Higher N2O production in sequencing batch reactors compared to continuous stirred tank reactors: effect of feast-famine cycles

Rapid Permafrost Thaw Removes Nitrogen Limitation and Rises the Potential for N2O Emissions

Ice–rich Pleistocene permafrost deposits (Yedoma) store large amounts of nitrogen (N) and are susceptible to rapid thaw. In this study, we assess whether eroding Yedoma deposits are potential sources of N and gaseous carbon (C) losses. Therefore, we determined aerobic net ammonification and nitrification, as well as anaerobic production of nitrous oxide (N2O), carbon dioxide (CO2), and methane (CH4) in laboratory incubations. Samples were collected from non-vegetated and revegetated slump floor (SF) and thaw mound (TM) soils of a retrogressive thaw slump in the Lena River Delta of Eastern Siberia. We found high nitrate concentrations (up to 110 µg N (g DW)−1) within the growing season, a faster transformation of organic N to nitrate, and high N2O production (up to 217 ng N2O-N (g DW)−1 day−1) in revegetated thaw mounds. The slump floor was low in nitrate and did not produce N2O under anaerobic conditions, but produced the most CO2 (up to 7 µg CO2-C (g DW)−1 day−1) and CH4 (up to 65 ng CH4-C (g DW)−1 day−1). Nitrate additions showed that denitrification was substrate limited in the slump floor. Nitrate limitation was rather caused by field conditions (moisture, pH) than by microbial functional limitation since nitrification rates were positive under laboratory conditions. Our results emphasize the relevance of considering landscape processes, geomorphology, and soil origin in order to identify hotspots of high N availability, as well as C and N losses. High N availability is likely to have an impact on carbon cycling, but to what extent needs further investigation.

Sources of nitrous oxide and the fate of mineral nitrogen in subarctic permafrost peat soils

Abstract. Nitrous oxide (N2O) emissions from permafrost-affected terrestrial ecosystems have received little attention, largely because they have been thought to be negligible. Recent studies, however, have shown that there are habitats in the subarctic tundra emitting N2O at high rates, such as bare peat (BP) surfaces on permafrost peatlands. Nevertheless, the processes behind N2O production in these high-emission habitats are poorly understood. In this study, we established an in situ 15N-labeling experiment with two main objectives: (1) to partition the microbial sources of N2O emitted from BP surfaces on permafrost peatlands and (2) to study the fate of ammonium and nitrate in these soils and in adjacent vegetated peat (VP) surfaces showing low N2O emissions. Our results confirm the hypothesis that denitrification is mostly responsible for the high N2O emissions from BP. During the study period, denitrification contributed ∼ 79 % of the total N2O emissions from BP, whereas the contribution from ammonia oxidation was less (about 19 %). Both gross N mineralization and gross nitrification rates were higher in BP than in VP, with high C/N ratios and a low water content likely limiting N transformation processes and, consequently, N2O production in the latter soil type. Our results show that multiple factors contribute to high N2O production in BP surfaces on permafrost peatlands, with the most important factors being the absence of plants, an intermediate to high water content and a low C/N ratio, which all affect the mineral-N availability for soil microbes, including those producing N2O. The process understanding produced here is important for the development of process models that can be used to evaluate future permafrost–N feedbacks to the climate system.

Influences of granule properties on the performance of autotrophic nitrogen removal granular reactor: A model-based evaluation

This work studied the impacts of key granule properties on the granular reactor performing partial nitritation/anammox from the modeling perspective. The results could guide not only future reliable modeling but also practical startup/operation of the reactor. To achieve high total nitrogen (TN) removal whilst avoiding significant N2O production, inoculated granules should be big and anammox-enriched. The optimum boundary layer thickness for maximum TN removal increased with the decreasing diffusivity of soluble components in the granule structure. Even though a thick boundary layer could protect anammox bacteria from elevated dissolved oxygen (DO) (e.g., 0.5 g-O2/m3) and obtain high TN removal (>90.0%) and low N2O production (<1.8%), even complete removal of the boundary layer would fail to provide sufficient substrate for anammox and therefore couldn’t increase TN removal to 90.0% and decrease N2O production to <2.4% at insufficient DO (e.g., 0.3 g-O2/m3 in the presence of lifted influent NH4+ concentration).

Potential greenhouse gases emissions by different plant communities in maritime Antarctica

Antarctic plant communities show a close relationship with soil types across the landscape, where vegetation cover changes, biological influence, and soil characteristics can affect the dynamic of greenhouse gases emissions. Thus, the objective of this study was to evaluate greenhouse gases emissions in lab conditions of ice-free areas along a topographic gradient (from sea level up to 300 meters). We selected 11 distinct vegetation compositions areas and assessed greenhouse gases production potentials through 20 days of laboratory incubations varying temperatures at -2, 4, 6, and 22 °C. High N2O production potential was associated with the Phanerogamic Community under the strong ornithogenic influence (phosphorus, nitrogen, and organic matter contents). Seven different areas acted as N2O sink at a temperature of -2 °C, demonstrating the impact of low-temperature conditions contributing to store N in soils. Moss Carpets had the highest CH4 emissions and low CO2 production potential. Fruticose Lichens had a CH4 sink effect and the highest values of CO2. The low rate of organic matter provided the CO2 sink effect on the bare soil (up to 6 °C). There is an overall trend of increasing greenhouse gases production potential with increasing temperature along a toposequence.

Short and long term continuous hydroxylamine feeding in a granular sludge partial nitritation reactor

Hydroxylamine is a nitrogen intermediate of ammonium oxidizing bacteria (AOB) that can transiently accumulate during nitrification. The impact of hydroxylamine on aerobic ammonium oxidations is still obscure. In the present study the short and long term impact of hydroxylamine on partial nitritation granular sludge was investigated. Dissolved oxygen was the governing factor determining the hydroxylamine impact in short term studies with continuous hydroxylamine feeding. Continuous short term hydroxylamine feeding together with low dissolved oxygen resulted in higher hydroxylamine accumulation, higher N2O production and decreased or maintained ammonium consumption. Instead, high dissolved oxygen reduced hydroxylamine accumulation and N2O production and increased ammonium consumption. Long term continuous hydroxylamine feeding reduced ammonium consumption rate while the constant nitrite production rate indicated that dosed hydroxylamine was mainly transformed to nitrite. This indicates that hydroxylamine was preferred over ammonium as substrate. Nitrosomonas sp. was shown to be predominant during continuous hydroxylamine feeding while the side community shifted.

Impact of Periodic Polarization on Groundwater Denitrification in Bioelectrochemical Systems.

Nitrate contamination is a common problem in groundwater around the world. Nitrate can be cathodically reduced in bioelectrochemical systems using autotrophic denitrifiers with low energy investment and without chemical addition. Successful denitrification was demonstrated in previous studies in both microbial fuel cells and microbial electrolysis cells (MECs) with continuous current flow, whereas the impact of intermittent current supply (e.g., in a fluidized-bed system) on denitrification and particularly the electron-storing capacity of the denitrifying electroactive biofilms (EABs) on the cathodes have not been studied in depth. In this study, two continuously fed MECs were operated in parallel under continuous and periodic polarization modes over 280 days, respectively. Under continuous polarization, the maximum denitrification rate reached 233 g NO3--N/m3/d with 98% nitrate removal (0.6 mg NO3--N/L in the effluent) with negligible intermediate production, while under a 30 s open-circuit/30 s polarization mode, 86% of nitrate was removed at a maximum rate of 205 g NO3--N/m3/d (4.5 mg NO3--N/L in the effluent) with higher N2O production (6.6-9.3 mg N/L in the effluent). Conversely, periodic polarization could be an interesting approach in other bioelectrochemical processes if the generation of chemical intermediates (partially reduced or oxidized) should be favored. Similar microbial communities dominated byGallionellaceaewere found in both MECs; however, swapping the polarization modes and the electrochemical analyses suggested that the periodically polarized EABs probably developed a higher ability for electron storage and transfer, which supported the direct electron transfer pathway in discontinuous operation or fluidized biocathodes.

Restoring wetlands on intensive agricultural lands modifies nitrogen cycling microbial communities and reduces N2O production potential

The concentration of nitrous oxide (N2O), an ozone-depleting greenhouse gas, is rapidly increasing in the atmosphere. Most atmospheric N2O originates in terrestrial ecosystems, of which the majority can be attributed to microbial cycling of nitrogen in agricultural soils. Here, we demonstrate how the abundance of nitrogen cycling genes vary across intensively managed agricultural fields and adjacent restored wetlands in the Sacramento-San Joaquin Delta in California, USA. We found that the abundances of nirS and nirK genes were highest at the intensively managed organic-rich cornfield and significantly outnumber any other gene abundances, suggesting very high N2O production potential. The quantity of nitrogen transforming genes, particularly those responsible for denitrification, nitrification and DNRA, were highest in the agricultural sites, whereas nitrogen fixation and ANAMMOX was strongly associated with the wetland sites. Although the abundance of nosZ genes was also high at the agricultural sites, the ratio of nosZ genes to nir genes was significantly higher in wetland sites indicating that these sites could act as a sink of N2O. These findings suggest that wetland restoration could be a promising natural climate solution not only for carbon sequestration but also for reduced N2O emissions.

Microbial pathways account for the pH effect on soil N2O production

Clarifying the responses of different N2O production pathways to pH change is helpful to better understand the effect of pH on N2O emission from soils. A15N tracing study was carried out on subtropical forest (SF) and cropland (SC) soil in 30-day pH manipulation treatments to quantify the effect of pH on autotrophic nitrification, heterotrophic nitrification and denitrification-derived N2O. Gene abundances of autotrophic nitrification (AOA-amoA, AOB-amoA) and denitrification (nirK, nirS, nosZ) -related N2O production were determined to identify the microbial mechanism behind the pH effect on N2O production. The results showed that the total N2O production rate (N2Ot) of the SF soil was 4.56 and 4.86 μg N kg−1 day−1 at pH 3.5 and 4.5, respectively, which was significantly higher than the other treatments. Similarly, the highest N2Ot of the SC soil was also observed in pH 3.5 treatment (7.15 μg N kg−1 day−1). Both N2O production rate via denitrification (N2Od) and heterotrophic nitrification (N2Oh) markedly increased, while the N2O production rate via autotrophic nitrification (N2Oa) markedly decreased by decreasing pH in both the SF and SC soil. Thus, denitrification and heterotrophic nitrification were responsible for the high N2O production under strongly acidic conditions. The abundance of nosZ gene was significantly and negatively correlated with N2Od (P < 0.05), and the abundance of AOB-amoA was significantly and positively correlated with N2Oa (P < 0.01). The results of the structural equation modeling (SEM) analysis revealed that soil pH generated a more direct effect on the denitrifying and ammonia-oxidizing gene abundance, in turn, affected N2Od and N2Oa, respectively. In addition, soil pH exerted a significant effect on both the bacterial 16S rRNA and fungal ITS rDNA gene abundance (P < 0.05). However, the role of soil fungi and bacteria on N2Oh could not be determined in this study.

N2O reduction during denitrifying phosphorus removal with propionate as carbon source.

Denitrifying phosphorus removal was realized in sequencing batch reactors using different carbon sources (acetate, propionate, and a mixture of acetate/propionate). Nutrient removal and nitrous oxide (N2O) production were investigated, and the factors affecting N2O production were explored. Nitrogen removal was 40.6% lower when propionate was used as the carbon source instead of acetate, while phosphorus removal was not significantly different. N2O production was greatly reduced when propionate was used as the carbon source instead of acetate. The emission factor in the propionate system was only 0.43%, while those in the acetate and mixed-carbon source system were 16.3% and 1.9%, respectively. Compared to the propionate system, ordinary heterotrophic organisms (i.e., glycogen-accumulating organisms) were enriched in the acetate system, explaining the higher N2O production in the acetate system. The lower nitrite accumulation in the propionate system compared to the acetate system was the dominant factor leading to the lower N2O production.

Nitrite build-up effect on nitrous oxide emissions in a laboratory-scale anaerobic/aerobic/anoxic/aerobic sequencing batch reactor

Biological wastewater treatment processes with biological nitrogen removal are potential sources of nitrous oxide (N2O) emissions. It is important to expand knowledge on the controlling factors associated with N2O production, in order to propose emission mitigation strategies. This study therefore sought to identify the parameters that favor nitrite (NO2-) accumulation and its influence on N2O production and emission in an anaerobic/aerobic/anoxic/aerobic sequencing batch reactor with biological nitrogen removal. Even with controlled dissolved oxygen concentrations and oxidation reduction potential, the first aerobic phase promoted only partial nitrification, resulting in NO2- build-up (ranging from 29 to 57%) and consequent N2O generation. The NO2- was not fully consumed in the subsequent anoxic phase, leading to even greater N2O production through partial denitrification. A direct relationship was observed between NO2- accumulation in these phases and N2O production. In the first aerobic phase, the N2O/NO2- ratio varied between 0.5 to 8.5%, while in the anoxic one values ranged between 8.3 and 22.7%. Higher N2O production was therefore noted during the anoxic phase compared to the first aerobic phase. As a result, the highest N2O fluxes occurred in the second aerobic phase, ranging from 706 to 2416 mg N m-2 h-1, as soon as aeration was triggered. Complete nitrification and denitrification promotion in this system was proven to be the key factor to avoid NO2- build-up and, consequently, N2O emissions.

Insight into the mechanism of chemoautotrophic denitrification using pyrite (FeS2) as electron donor

In this study, denitrification was performed using pyrite as sole electron donor. The nitrate reducing rate ranged from 0.61 to 0.95 mM/d. The production of nitrous oxide (N2O) was observed, accounting for 20% of the total nitrate reduction. The isotope fractionation indicated that N2O production was mainly caused by the bacterial denitrification, instead of chemical denitrification by Fe(Ⅱ). Thiobacillus was the predominant genus, of which relative abundance decreased after the incubation with pyrite. Conversely, other genera belonging to Actinobacteria, like Rhodococcus, increased by more than 10 times. These Actinobacteria-like bacteria lack nitrous oxide reductase, which might be the reason for high N2O production. Furthermore, the predicted microbial functions analysis by PICRUSt2 showed that the genes (menC/E/G) involved in the biosynthesis of electron shuttles (menaquinone-related redox-active molecule), which were remarkably enriched during the process, suggesting that the first step of pyrite oxidation might be driven by the microbial derived electron shuttles.

Nitric oxide and nitrous oxide production in anaerobic/anoxic nitrite-denitrifying phosphorus removal process: effect of phosphorus concentration.

Nitric oxide (NO) and nitrous oxide (N2O) production in biological nutrient removal has been studied widely due to the strong negative effects on the environment. Nitrite-denitrifying phosphorus removal (N-DPR), as a significant source of NO and N2O production, has received great attention. However, the mechanism of NO and N2O production at different phosphorus concentrations is not well understood. Therefore, this study was conducted to investigate the effect of phosphorus concentration on pollutant removal, as well as NO and N2O production during the N-DPR process. The results showed that the phosphorus removal efficiency was improved with the increase of phosphorus concentration, which is caused by the enrichment of denitrifying phosphorus accumulating organisms (DPAOs) at high phosphorus concentration. High NO production was observed at phosphorus concentration of 0.5 mg L-1, which is mainly attributed to the slow recovery of reductase activity and low abundance of DPAOs. The maximal N2O accumulation of 31.45 mg L-1 was also achieved at phosphorus concentration of 0.5 mg L-1. The possible reason is that fewer poly-β-hydroxyalkanoates (PHAs) were synthesized by glycogen accumulating organisms (GAOs) at low phosphorus concentration, which could intensify the electron competition among different reductases. In addition, free nitrous acid (FNA) inhibition was another significant reason for high N2O production.

Application of acidic conditions and inert-gas sparging to achieve high-efficiency nitrous oxide recovery during nitrite denitrification

Nitrogen removal with energy recovery through denitrification dependent N2O production is garnering recent attention due to its cost advantages. The most effective current method requires alternating COD and nitrite to achieve high N2O production making it incompatible with typical wastewaters and consequently difficult to use in most settings. The work described here introduces a robust and highly efficient N2O recovery approach which has the potential to work with wastewaters containing COD and nitrite simultaneously. This method relies on low pH incubation and inert gas sparging (IGS) to shift a community of mainly N2 producing nitrite denitrifiers to a community that accumulates N2O when incubated in the absence of IGS. Before experiencing IGS, samples from activated sludge incubated at a pH of 4.5 and 6.0 only achieved a maximum N2O production efficiency (PE_N2O) of ∼26%. After IGS the PE_N2O values increased to ∼97.5% and ∼80.2% for samples from these same pH 4.5 and pH 6.0 reactors, respectively. IGS did not lead to N2O production in a pH 7.5 bioreactor. Meta-omics analysis revealed that IGS resulted in an increase in bacteria utilizing the clade I nitrous oxide reductase (nosZI) relative to bacteria utilizing the clade II nitrous oxide reductase (nosZII). This likely results from IGS flushing out N2O leaving nitrite as the principal nitrogen oxide available for respiration, favoring nosZI utilizing bacteria which are more likely to be complete denitrifiers. Metatranscriptomic analysis suggested that the high PE_N2O values that occurred after stopping IGS result from the NO generated by chemodenitrification accumulating to levels that inactivate [4Fe:4S] clusters in the NosR protein essential for N2O reduction in the nosZI denitrifiers. This study provides an efficient and straightforward method for N2O recovery, widening the options for energy recovery from nitrogen-based wastes.

Nitrogen removal as nitrous oxide for energy recovery: Increased process stability and high nitrous yields at short hydraulic residence times

The Coupled Aerobic-anoxic Nitrous Decomposition Operation (CANDO) is a two-stage process for nitrogen removal and resource recovery: in the first, ammonia is oxidized to nitrite in an aerobic bioreactor; in the second, oxidation of polyhydroxyalkanoate (PHA) drives reduction of nitrite to nitrous oxide (N2O) which is stripped for use as a biogas oxidant. Because ammonia oxidation is well-studied, tests of CANDO to date have focused on N2O production in anaerobic/anoxic sequencing batch reactors. In these reactors, nitrogen is provided as nitrite; PHA is produced from acetate or other dissolved COD, and PHA oxidation is coupled to N2O production from nitrite. In a pilot-scale study, N2O recovery was affected by COD/N ratio, total cycle time, and relative time periods for PHA synthesis and N2O production. In follow-up bench-scale studies, different reactor cycle times were used to investigate these operational parameters. Increasing COD/N ratio improved nitrite removal and increased biosolids concentration. Shortening the anaerobic phase prevented fermentation of PHA and improved its utilization. Efficient PHA synthesis and utilization in the anaerobic phase correlated with high N2O production in the anoxic phase. Shortening the anoxic phase prevented reduction of N2O to N2. By shortening both phases, total cycle time was reduced from 24 to 12 h. This optimized operation enabled increased biomass concentrations, increased N2O yields (from 71 to 87%), increased N loading rates (from 0.1 to 0.25 kg N/m3-d), and shorter hydraulic residence times (from 10 to 2 days). Long-term changes in operational performance for the different bioreactor systems tested were generally similar despite significant differences in microbial community structure. Long-term operation at short anaerobic phases selected for a glycogen-accumulating community dominated by a Defluviicoccus-related strain.