Aeration Strategies To Mitigate Nitrous Oxide ... - ACS Publications

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Aeration Strategies To Mitigate Nitrous Oxide Emissions from SingleStage Nitritation/Anammox Reactors Carlos Domingo-Félez,† A. Gizem Mutlu,† Marlene M. Jensen, and Barth F. Smets* Department of Environmental Engineering, Technical University of Denmark, Miljøvej Building 113, 2800 Kongens Lyngby, Denmark S Supporting Information *

ABSTRACT: Autotrophic nitrogen removal is regarded as a resource efficient process to manage nitrogen-rich residual streams. However, nitrous oxide emissions of these processes are poorly documented and strategies to mitigate emissions unknown. In this study, two sequencing batch reactors performing single-stage nitritation/anammox were operated under different aeration strategies, gradually adjusted over six months. At constant but limiting oxygen loading, synthetic reject water was fed (0.75g-N/L·d) and high nitrogen removal efficiencies (83 ± 5 and 88 ± 2%) obtained. Dynamics of liquid phase nitrous (N2O) and nitric oxide (NO) concentrations were monitored and N2O emissions calculated. Significant decreases in N2O emissions were obtained when the frequency of aeration was increased while maintaining a constant air flow rate (from >6 to 1.7% ΔN2O/ΔTN). However, no significant effect on the emissions was noted when the duration of aeration was increased while decreasing air flow rate (10.9 ± 3.2% ΔN2O/ΔTN). The extant ammonium oxidation activity (mgNH4+-N/gVSS·min) positively correlated with the specific N2O production rate (mgN2O-N/gVSS·min) of the systems. Operating under conditions where anaerobic exceeds aerobic ammonium oxidation activity is proposed to minimize N2O emissions from single-stage nitritation/anammox reactors; increasing the frequency of aeration cycling is an efficient way of obtaining those conditions.

1. INTRODUCTION Recently, alternatives based on autotrophic nitrogen oxidation have been implemented as more energy and resource efficient processes compared to conventional nitrification/denitrification BNR (biological nitrogen removal) for nitrogen removal from residual waters, especially for nitrogen-rich streams with low content of biodegradable organic matter. Completely autotrophic systems have a low oxygen need and no external carbon source requirement, which reduces their sludge production, energy consumption, and carbon footprint.1 Single-stage nitritation/anammox systems rely on two central microbial functional guilds, aerobic and anaerobic ammonium oxidizers (AOB, AnAOB), but also contain aerobic nitrite oxidizing bacteria (NOB) and heterotrophic denitrifying bacteria (HB). Optimal conditions for autotrophic nitrogen removal in suspended single-stage systems, such as low and transient dissolved oxygen (DO) and the presence of nitrite, are the very conditions that may promote emissions of nitrogen oxides (NxO).2 Nitrous oxide (N2O) and nitric oxide (NO) are atmospheric trace gases, of which N2O is known as both a stratospheric ozone depleter and a greenhouse gas with 300 times higher radiative forcing than carbon dioxide.3 Contributions from wastewater treatment to anthropogenic N2O emissions are considered by the IPCC,3 but the emission factors used in this assessment4 likely underestimate true N2O emissions.2,5,6 Furthermore, the total greenhouse gas (GHG) footprint of a WWTP is most sensitive to its N2O emissions.7 Documented N2O emissions from lab and full-scale single-stage © 2014 American Chemical Society

nitritation/anammox systems have been higher than those measured from conventional BNR processes,2,8−10 potentially offsetting the benefit associated with their energy savings and corresponding CO2 emission reductions.11 How N2O emissions from nitritation/anammox treatment systems depend on design or operation has not been carefully examined. Such information is needed to propose and evaluate mitigation measures geared toward process optimization or alternative designs. The exact mechanisms and environmental controls of N2O production are subject to ongoing research.12−14 Reported pathways include nitrifier denitrification and hydroxylamine oxidation by AOB and incomplete denitrification by HB.15 Among environmental conditions, dissolved oxygen concentrations seem to play an important role; therefore, efficient aeration strategies are necessary to minimize N2O emissions while maintaining high process efficiencies.8,16 To ensure partial nitritation (conversion of a fraction of the influent ammoniumN to nitrite-N), autotrophic N removal technologies require limited aeration: such conditions can be set by adjusting air supply rates or aeration intervals.17 Continuous aeration can support simultaneous AOB and AnAOB activity as long as bulk DO levels remain low enough.18 Many systems use DO set Received: Revised: Accepted: Published: 8679

April 11, 2014 June 25, 2014 June 30, 2014 June 30, 2014 dx.doi.org/10.1021/es501819n | Environ. Sci. Technol. 2014, 48, 8679−8687

Environmental Science & Technology

Article

Table 1. Average Performance, Target Operational Conditions, and Average N2O Emissions of Nitritation/Anammox SBRs during the Measurement Campaign SBR_f f redox

ΔTN(mg-N/Ld)

TN Removal(%)

Qair (L/min)

LO2 (gO2/L·d)

ton/toff (min)

6 8 10 16 25

674 638 632 592 583 (641b)

88.9 86.2 82.8 78.3 74.9 (84.8b)

2.22 2.22 2.22 2.22 2.22 (2.63b)

0.81 0.81 0.81 0.81 0.81 (0.89b) SBR_t

22.4/52.2 16.7/39.0 13.4/31.2 8.4/19.5 5.4/12.5

ΔN2O/ΔNH4+ (%) 5.5 6.4 5.9 2.7 1.6

Ron

ΔTN(mg-N/Ld)

TN Removal(%)

Qair (L/min)

LO2 (gO2/L·d)

ton/toff (min)

53% 63% 72% 81% 93%

669 658 (678c) 641 693 632

88.6 87.7 (90.4c) 86.7 85.9 86.5

1.43 1.23 (0.79c) 0.58 0.51 0.42

1.17 1.25 (0.96c) 0.90 0.93 0.98

85/65 100/50 115/35 130/20 150/0

± ± ± ± ±

nN2O (−)

± ± ± ± ±

(3)a (9) (11) (17) (16)

0.5 1.0 0.8 0.8 0.4

ΔN2O/ΔNH4+ (%) 9.2 12.6 11.6 9.5 7.6

ΔN2O/ΔTN (%)

± ± ± ± ±

6.1 7.0 6.5 2.9 1.7

0.6 1.2 0.7 0.8 0.5

ΔN2O/ΔTN (%)

nN2O(−)

± ± ± ± ±

(15) (13) (12) (18) (21)

2.4 2.7 1.4 2.0 2.8

10.3 13.9 12.6 10.6 8.6

2.7 3.2 1.6 2.2 3.1

a

Cycles with lower N-load were not considered. bOxygen load was increased by increasing Qair from 2.22 to 2.63 L/min. cOxygen load was decreased by decreasing Qair from 1.23 to 0.78 L/min due to unbalanced AOB/AnAOB activity.

points and feedback from other parameters to operate onestage processes.18−21 Setting the air supply rate is an alternative to DO set points to control oxygen limitation.17 The effects of aeration regime on N 2 O emissions from single-stage nitritation/anammox systems are unclear: some have measured higher emissions under intermittent versus continuous aeration while others have reported no significant differences.9,18 Higher air flow rates, in continuous aeration mode, resulted in higher emissions from a full-scale nitritation/anammox reactor.8,16 In this study, N2O and NO dynamics and emissions were measured from two lab-scale sequencing batch reactors (SBR) performing stable and high-rate nitritation/anammox. The SBRs were operated under varying well-controlled aeration regimes. The objective was to evaluate whether aeration intermittency and intensity could be optimized to minimize N2O production, while retaining maximum liquid phase nitrogen removal efficiencies.

configuration and routine monitoring criteria are reported elsewhere.23 2.1.1. Study of Different Aeration Strategies. The total cycle time was 8 h and consisted of anoxic feeding, preliminary stirring, reaction, settling, discharge, and idle phases (Figure S1). The 446 min long reaction phase was divided in several redox cycling periods based on the following parameters: f redox being the number of aerated periods (also the number of redox cycling periods) within a react phase (eq 1), and Ron being the total aerated fraction of a cycle (eq 2)

2. MATERIALS AND METHODS 2.1. Sequencing Batch Reactor Configuration and Operation. During six months, a measurement campaign was conducted on two 4-L nitritation/anammox sequencing batch reactors (SBRs), SBR_f and SBR_t, subject to aeration regimes that were changed on a monthly basis. Both reactors were initiated more than one year before (SBR_f: 19 mo, SBR_t: 13.5 mo) and displayed stable performance. The vessels were mechanically mixed with Rushton impellers at 80 rpm. All equipment control was assembled in LabView (National Instruments, Austin, USA) by a tailor-made routine. Synthetic reject water of 0.5 gNH4+-N/L was fed at a constant volumetric N-loading of 0.75 gNH4+-N/L·d. The mineral medium composition was based on van de Graaf et al.22 The liquid volume exchange ratio was 50%, and the hydraulic retention time was 16 h. The solid retention time was set at 100 days and controlled by daily manual wasting of biomass. Compressed lab air was supplied via bubble diffusers and controlled by ELFLOW mass flow controllers (Bronkhorst, Ruurlo, The Netherlands). Dissolved oxygen (OxyFerm FDA, Hamilton, Bonaduz, Switzerland) and pH (EasyFerm plus K8, Hamilton, Bonaduz, Switzerland) were monitored continuously, and temperature was controlled at 30 °C with an electric heating jacket. Further details on substrate composition, reactor

where treact is the reaction phase length, tcycle is the cycle length, and ton and toff are the aerated and nonaerated periods of each redox cycling period. In SBR_f, f redox was gradually increased during the 6 month campaign, while the total aeration time (Ron = 28%) and air flow rate were kept constant. f redox was increased from 6 to 8, 10, 16, and 25 with concomitant shortening of each aerated period. In SBR_t, Ron was increased while the number of redox cycling periods was kept constant ( f redox = 3). Ron was increased from 53 to 63, 72, 81, and 93% of the cycle with a consequent decrease in the air flow rate (Qair) to maintain an equivalent overall oxygen load rate (LO2) per cycle (Table 1). The oxygen loading rate was calculated as

fredox ( −) =

R on(%) =

treact ton + toff

ton·fredox tcycle

(1)

·100

⎛ gO ⎞ LO2⎜ 2 ⎟ = kLaO2(SO*2 − SO2)R on ⎝ L·d ⎠

(2)

(3)

where kLaO2 is the volumetric mass transfer coefficient for oxygen, S*O2 is the oxygen saturation concentration at 30 °C, and SO2 is the oxygen concentration in the reactor liquid. The only actuator for DO control was Qair as f redox and Ron were fixed. To maintain DO limited conditions, the required oxygen loading was stoichiometrically estimated and corresponding Qair estimated17 from oxygen transfer tests. 8680

dx.doi.org/10.1021/es501819n | Environ. Sci. Technol. 2014, 48, 8679−8687


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2.2. Online N2O and NO Monitoring and Analytical Methods for Nitrogen Species. N2O liquid concentrations were measured during consecutive cycles in each reactor at least twice (2 ≤ n ≤ 6) with one set of cycles 1 week and another 4 weeks into a new aeration condition. Throughout the measuring campaign a total of 60 cycles were measured for SBR_f and 79 for SBR_t. Liquid N2O and NO concentrations were measured with Clark-type microsensors (N2O-R and NO-500, respectively, Unisense A/S, Aarhus, Denmark) and signals logged via a picoammeter every 30 s. The N2O microsensor was calibrated in effluent of each reactor at operating temperature before every experiment. The response in the working range (0−5 mgN2ON/L) was linear24 with similar slopes for each reactor effluent. A stock solution was prepared by sparging ≥99.998% N2O (Sigma-Aldrich) in deionized water. Different volumes from the stock solution were added to the calibration chamber. The concentration was calculated from room temperature N2O solubility25 and volume corrected for each point. The NO calibration procedure using an NO/N2 gas mixture is described elsewhere.26 Ammonium (NH4+-N) and nitrate (NO3−-N) concentrations in the effluent were logged for every cycle with ion selective electrodes (Varion, WTW, Wilheim, Germany), while nitrite (NO2−-N) concentration was measured daily with photometric test kits (Merck KGaA, Darmstadt, Germany). At the end of every aeration condition one cycle was thoroughly sampled for all N-species and analyzed photometrically (Figure S2). For each reactor cycle, a number of conversion ratios were calculated. These include the amount of ammonium consumed per total nitrogen removed (ΔNH4+/ ΔTN), the amount of nitrite produced per ammonium consumed (ΔNO2−/ΔNH4+), and the amount of nitrate produced per total nitrogen removed (ΔNO3−/ΔTN). Calculated values were compared to targets 1.15, 0, and 0.13, respectively, to verify balanced nitritation-anammox.17 2.3. Potential Activity Assays. Potential activity assays were conducted on bulk reactor biomass samples and biomass size fractions 90 μm from each reactor. An extant respirometric nitrifying assay was performed per Ellis et al.27 Biomass washed with mineral medium was added to a jacketed glass (100 mL) vessel maintained at 30 °C, mixed with a magnetic stir-bar, and flushed with pure oxygen until saturation. The vessel was then sealed with the Clark-type polarographic DO electrode (YSI Model 5331, Yellow Springs, OH) and its signal monitored after ammonium or nitrite spikes.28 Anammox activity assays were performed following Dapena-Mora et al.29 2.4. Nitrous Oxide and Oxygen Volumetric Mass Transfer Coefficients. To determine N2O and O2 mass transfer coefficients, stripping and reoxygenating experiments were performed separately for the range of used air flows (0− 2.6 L/min). The reactor was filled with mineral medium at 30 °C, with the same aeration control but without biomass. For each test, kLai was calculated by minimization of residual errors.30 For N2O, stripping tests were performed by spiking a volume of N2O solution to one of the reactors after which aeration was started. For O2 tests, mineral medium was sparged with N2 to strip any DO, followed by reoxygenation. At least duplicate tests at nine different flow rates were performed for each gas. Average kLai values at all flow rates were fitted to a power function5,31 kLa N2O or O2 = A ·Q air B + C

where A and B are the constants describing N2O stripping or reoxygenation, and C is the measured constant value at Qair = 0 L/min only due to mechanical mixing. 2.5. Calculation of N 2O Production Rates and Emissions. The liquid N2O concentration in the reactor is the consequence of the net rate at which it is produced and its net mass transfer rate. If stripping is not accounted for, interpretation of liquid profiles is not possible at different aeration rates. N2O mass transfer tests (kLaN2O) and liquid profiles (N2Oi) allow one to quantify both stripped and produced N2O. Net N2O production rates in the reactor were calculated as follows ri =

ΔN2Oi + kLa N2O·N2Oi Δt

(5)

where ri is the instantaneous net production rate (measured every 30 s), ΔN2Oi/Δt is the accumulation term estimated from linear regression (Δt = 1 min), and kLaN2O·N2Oi is the stripping rate, which also equals the emission rate. Instantaneous net rates were calculated to differentiate production from consumption phases within a cycle. The stripping rate depends on the mass transfer coefficient and the concentration gradient (kLaN2O·(N2Oi-N2O*)). Since the N2O equilibrium concentration in the air flow (N2O*) is much lower than the measured concentrations (N2O), the concentration gradient was set equal to the concentration in the reactor: (N2O-N2O*) ≈ N2O. The cycle integrated emission was calculated to compare the effect of aeration conditions. N2O emissions were expressed as a percentage of the removed NH4+ (%ΔN2O/ΔNH4+) and total nitrogen (%ΔN2O/ΔTN) (Table 1).

3. RESULTS 3.1. Reactor Performance. Both reactors displayed stable N removal prior to and during the N2O measuring campaign (Table 1). pH was not controlled and slightly decreased during a cycle (0.4−0.5 units), with average values of 7.4 ± 0.2 for SBR_f and 7.7 ± 0.2 for SBR_t. Biomass concentrations were 2.7 ± 0.5 and 2.8 ± 0.5 gVSS/L for SBR_f and SBR_t, respectively. At a nominal nitrogen load of 0.75 gN/L·d, the average removal efficiencies were 83 ± 5 and 88 ± 2% for SBR_f and SBR_t, respectively (Table 1). Changes in aeration regimes did not interfere with nitrogen removal in SBR_t. In the final days of SBR_f (f redox = 25), a 10% decline was noted, but efficiency could be recovered when a higher oxygen load (air flow rate) was applied. Increasing the number of aerated periods shortened the length of individual aerated periods, which lowered average NO2− concentrations at the end of these periods ( f redox = 6−25, 22.3−5.4 mgNO2−-N/L, R2 = 0.98). In SBR_t, nitritation and anammox activities occurred simultaneously at similar rates over the whole cycle, without NO2− accumulation during oxic phases ( 0.99) (Figure S3): kLaO2 (min−1) = 0.154·Q air (L/min)0.648 + 0.009

(7)

An alternative to directly measuring kLaN2O is to estimate its value from kLaO2 and the ratio of diffusivity coefficients.32 Here, this would underestimate kLaO2 for the range of Qair used (e.g., 8682



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Figure 2. Detailed and cycle-averaged N2O production profiles of SBRs operated under different aeration regimes. Top panels: Close-up of representative NxO concentration and production rate profiles of (A) SBR_f: f redox = 16, (C) SBR_t: Ron = 72%). Bottom panels: Cycle-integrated N2O production rates for SBRs under different aeration regimes (B) SBR_f and (D) SBR_t. * number of cycles included for SBR_f: n ≥ 3, SBR_t: n ≥ 6. ** during f redox = 6 under lower N-loads. *** during f redox = 25 recovered N-removal rates.

reflect a gradual increase to a consistent threshold concentration before an abrupt decline is noted. Maybe heterotrophic N2O consumption is temporarily activated as the N2O concentration attains a certain critical value, which might relate to its diffusive transport into aggregates. The first N2O peak of the react phase was common in SBR_t and decreased from 2.5 to 1.8−1.6 mgN2O-N/L as the air flow rate decreased. In the second and third aerated periods N2O concentration increased with ton potentially due to lower stripping. Different from SBR_f, N2O did not accumulate during nonaerated periods. No accumulation of NxO occurred in either reactor during the settling, decant and fill phases of the cycle (t = 446−480 min), which would be detected in the beginning of the next cycle during preliminary mixing (1 min) prior to the reaction phase (data not shown). 3.4. Production and Emission of N2O under Different Aeration Strategies. N2O Production Rates. N2O production rates decreased with higher aeration frequencies in SBR_f (Figure 2-B). Rates at lower aeration frequencies ( f redox = 6, 8, 10) were over 3 times larger than those at higher frequencies ( f redox = 16, 25) and values ranged from 0.17 to 0.71 mgN2ON/gVSS·h. No evidence of adaptation was observed at any aeration condition, as cycle-averaged values from all cycles of a condition (box plot median) were similar to those from the last cycles per condition (black squares, Figure 2-B). Values from days with nitrogen loads lower than design loading (f redox = 6;

corresponding with decreasing ammonium concentrations. Increasing the frequency of aeration (f redox = 6 to 25) lowered the peak N2O values during the nonaerated periods from approximately 1.1 to 0.2 mgN2O-N/L. The NO profiles followed similar trends every aerated/nonaerated period. Concentrations increased during aerated periods, corresponding to NO production, but decreased over a cycle (Figure 1-B, C). During nonaerated periods NO decreased toward the end of the cycle. As an N2O precursor, the decrease in NO production over a cycle matches the decrease in N2O concentrations. SBR_t, which was subject to progressively longer aerated periods and lower aeration rates during the react phase, displayed remarkably different and less seemingly consistent NxO profiles. However, similarities over the three aerated periods remained. Similar to SBR_f, N2O concentrations increased with onset of aeration after a nonaerated phase, but as the aeration period continued, the initial N2O peak was followed by a second and often third and fourth N2O peak. Simultaneous measurements showed no correlation between NO2− levels and N2O dynamics (Figures S2 and S4). Also, because the reactors were completely mixed with constant air flow and continuous stirring during aerated periods, settling and resuspension of biomass potentially causing spatially and temporally heterogeneous redox conditions were not plausible causes. On the other hand, the sequential N2O peaks appear to 8683



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4. DISCUSSION The effect of different aeration regimes on N2O emissions was studied in two nitritation/anammox reactors operating at high N-removal efficiencies. Simulation studies have suggested that optimum conditions for autotrophic ammonium removal favor N2O emissions.33 N2O formation was primarily observed during aerated periods in both systems. During aeration, oxygen limitation, which is known to increase N2O production in autotrophic nitrifying communities,34,35 remains possible inside the biomass aggregates. Chemical N2O production was unlikely as neither acidic conditions nor high free nitrous acid (FNA) concentrations occurred (lowest pH = 6.9, FNAmax = 0.0026 mg-N/L).36,37 AOB and denitrifying HB are considered the main contributors to biological N2O production.38 The known N2O production pathways from these groups are nitrifier denitrification and hydroxylamine oxidation by AOB and incomplete denitrification by HB. N2O emissions from pilot and full-scale single-stage N-removing systems are in the same range as those measured in SBR_f (0.4%, 6.7−7.3% ΔN2O/ΔNH4+), while emissions from SBR_t are higher.8,9,16,18 Similarly, lab-scale reactors have reported emissions from less than 0.01% (corresponding with very low nitrogen removal) to 6.4% N2O/N-load.39,40 The majority of emissions in both reactors occurred during aerated periods (>95%),16,41,42 and N2O discharged in the liquid effluent represented a minor part of the total produced ( 120). Hence, neither NO2− nor FNA appear good indicators of N2O formation in nitritation/anammox systems. Net N2O production rates followed the same pattern for all the studied aeration frequencies: NO and N2O production increased with the onset of aeration, while the imposition of anoxic periods decreased NO and N2O production rapidly until net N2O consumption was observed (Figure 2-A). Shortening the aerated periods from 22.4 to 5.4 min (f redox = 6 to 25) significantly lowered N2O production over a cycle (Figure S5). Because N2O production rates increased with time, longer aerated periods result in progressively higher N2O production. Moreover, initial N2O production rates (first 5 min after start of aeration, Figure S5) decreased with increasing frequencies. Clearly, the frequency of aeration switching affected the initial N2O production under aerated conditions.

days 24−27) were not considered in the calculation of average cycle rates. Profiles were very similar within an aeration condition, differing only slightly at their baseline levels, and every aerated period had the same pattern with a minor decline toward the end of the react phase (Figure 2-A). The N2O production rate increased almost linearly from the onset until the end of aeration. Stopping aeration caused rapid decreases in N2O production rates until net consumption was observed. The N2O consumption, observed solely during nonaerated periods, was equivalent to 6.0 ± 4.1% of the produced N2O during aerated periods throughout the measurement campaign. The duration of aeration in SBR_t did not affect the overall N 2 O production of each R on condition (R 2 < 0.01, pseudosteady cycles, n ≥ 6); cycle averaged N2O production rates ranged from 0.80 to 1.56 mgN2O-N/gVSS·h. Production profiles differed within an aeration condition, and every aerated period (3 per cycle) showed varying patterns. All the production occurred during aerated periods, and production was 2 orders of magnitude higher than consumption during nonaerated periods. Recovery from nonaerated conditions caused initial peak N2O production, followed by a lower but sustained rate which rapidly decreased by the reimposition of nonaerated conditions (Figure 2-C). The presence of NxO peaks upon reaching each aerated period indicates that imposition of aeration, and not the nonreactive part of the cycle, was the main contributor of NxO peaks (Figure 1, D-F). N2O production during a react phase did not follow any obvious trend with increasing Ron: while most of the N2O was produced during the first aerated period for Ron values of 53, 63, and 93%, the second aerated period emitted most N2O for Ron of 72 and 81%. Common to all measured cycles was lower production rates during the last aerated period, where substrate concentrations were lowest. NO levels are, like N2O, determined by biological and physicochemical processes. During aerated periods, the calculated N2O production profiles mimicked the NO concentration profiles, which agrees with the notion that NO serves as precursor in N2O production. For example, the sharp decrease in NO concentration after the start of aeration is also evident in the N2O production profile but not in the N2O concentration profile (Figure 2-C, t = 150−200 min). Consistently, N2O production rates, but not N2O concentrations, correlated strongly with measured NO concentrations (R2SBR_f_prod_rate = 0.76, R2SBR_f_conc = 0.61, nSBR_f > 2400; R2SBR_t_prod_rate = 0.85, R2SBR_t_conc = 0.40, nSBR_t > 6700; Figure S4). N2O Emissions. From the mass transfer experiments and N2O measurements, N2O emissions were calculated for each cycle. Emissions via the discharged effluent accounted for only 0.1 and 0.6% of the N2O produced for SBR_f and SBR_t respectively and are not considered here. Emissions followed the same trends as cycle production rates (Table 1). Gas stripping during aerated periods was responsible for the majority of emissions (95.2 ± 0.8% in SBR_f and 98.5 ± 1.3% in SBR_t) regardless of aeration conditions. Average emissions from SBR_f ranged from 1.7 to 7.0% ΔN2O/ΔTN, and decreased with increased f redox (R2 = 0.87). Emissions at lower frequencies ( f redox = 6−8−10) were significantly higher than at higher frequencies (f redox = 16−25 p < 0.01). On the other hand, in SBR_t no significant differences in emissions were observed between the lowest and highest studied aeration duration (p > 0.05): emissions ranged between 8.6 and 13.9% ΔN2O/ΔTN. 8684



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similar emissions for an intermittently (Ron = 75%) and continuously aerated full-scale nitritation/anammox system.9 4.3. Potential Mechanisms. Several microbial groups present in nitritation/anammox systems have been related to N2O production. Many studies have examined the influence of operational conditions (N-load, aeration rate, feeding strategies, etc.) on N2O production but not on individual mechanisms.8,9,11,16,18,40 In 2-stage nitritation/anammox systems, most emissions derive from the nitritating stage, where AOB are dominant.10,46,54 The reaction conditions in our study, such as aerated/nonaerated periods, carbon-limited feeding, nonlimiting FA and NO3−, support possible N2O production by all known pathways: incomplete denitrification, nitrifier denitrification, and hydroxylamine oxidation.38,55,56 Incomplete heterotrophic denitrification has been speculated as the main N2O production pathway in a full-scale system and in anammox granules under low COD/N.18,46 However, in SBR_f and SBR_t ΔNO3−/ΔTN ratios indicated little heterotrophic denitrification with even less contribution to the overall N2O production. On the other hand, N2O consumption during nonaerated periods must be attributed to heterotrophic denitrification. Recovery from anoxic periods and presence of FA would stimulate the hydroxylamine oxidation pathway, yielding N2O peaks in SBR_t and a fast increase in N2O production in SBR_f. Nitrifier denitrification, in the presence of NO2− would account for the sustained N2O production throughout aerated periods in both reactors. Extant AOB activity assays carried with bulk biomass from each reactor and corresponding N2O production rates from the closest cycles are shown in Figure 3.

At higher frequencies (f redox = 16, 25) the performance of SBR_f slightly decreased, which easily recovered by adjusting oxygen loading (air flow rate). As a consequence N2O production decreased even further (Figure 2-B), indicating that the lower N2O production at higher aeration frequencies was not due to a lower nitrogen removal but due to the imposed aeration regime. N2O was also produced during nonaerated periods yet at lower rates. Imposition of anoxia quickly decreased but did not cease NO production. Transition to anoxia was not sufficient to cause N2O production in an autotrophic AOB culture if only NH4+ was present.12 However, NO production has been linked to higher NO2− concentrations in the presence of NH4+ and to increases in the relative transcription of autotrophic denitrification genes by N. europaea 19718.48 It is in agreement with ammonium oxidation being the electron source to the expected increase in NO2− reduction activity and nirK expression of AOB,11,49 which followed by NO reduction would result in liquid N2O accumulation. Therefore, the presence of both NO2− and NH4+ in the bulk liquid from the previous aerated period after transition to anoxia, as occurred in SBR_f, was necessary for N2O production.9 Before the restart of aeration, net N2O consumption phases were detected, likely due to heterotrophic denitrifying activity. Anoxic conditions favor N2O reductase activity, and FNA values in SBR_f were not inhibitory.50−52 4.2. Effect of Aeration Duration and N2O Production. N2O production in SBR_t, as in SBR_f, was triggered upon recovery from rather than entry into anoxia (Figure 2-C). N2O production in SBR_t could be the result of either AOB pathway. The combination of low NO2−, low DO, and high FA can accommodate both the NH2OH oxidation pathway as well as the nitrifier denitrification pathway,12,13,53 while the N2O peaks observed after recovery from anoxic conditions (Figure 1) can be attributed to the oxidative pathway as a short-term response to changing conditions.37 Low NO concentrations before detecting initial NO peaks indicated that NO was produced at the beginning of aerated periods as a metabolic response and not during anoxic phases and subsequently stripped upon aeration.37 A ΔNO3−/ΔTN ratio lower than the stoichiometric ratio for anammox indicated the presence of heterotrophs, yet no periods of N2O consumption were detected; the longer periods of aeration might have prevented onset of heterotrophic N2O consumption or even caused N2O production from incomplete heterotrophic denitrification. Higher N2O production rates were measured at higher aeration rates (Qair) during aerated periods. However, because higher Qair corresponded with shorter aerated periods, overall N2O production over a cycle was not significantly different. Higher emissions were also measured with intense aeration in a full-scale nitritation-anammox reactor.8,16 The effect of Qair on the cumulative N2O production upon recovery from anoxia is illustrated in Figure S6. N2O production occurred at a nearly constant rate throughout the aerated periods at lower Qair (higher Ron). On the other hand, as Qair increased, N2O production occurred mainly at the onset of aeration. After only 20% of total aeration time, at the highest air flow rate (Ron = 53%) almost four times more N2O was produced than at the lowest air flow rate (Ron = 93%). Higher aeration rates increased N2O production but only transiently. No obvious effects in N2O emissions were seen upon transition from intermittent to continuous aeration (Figure 2-D), in agreement with the observations by Yang et al., who measured

Figure 3. Cycle averaged N2O production rate versus off-line measured extant ammonium oxidation activity for SBR_f (black, n = 27) and SBR_t (blue, n = 33) (R2 = 0.81).

Even though these tests cannot identify the pathway of the emissions, N2O production and extant ammonium oxidation rates are clearly correlated. Hence, AOB activity may provide a simple estimator of N2O emission rates in single-stage nitritation/anammox systems. A similar relation was noted for an enriched nitritating AOB culture.41 Other biological guilds present in the system do not directly, but indirectly, affect N2O production: e.g. NO has been postulated as an intermediate (and a potential sink) in the anammox reaction but N2O is not part of AnAOB metabolism;57 NO2− is a substrate for AnAOB 8685



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(7) Daelman, M. R. J.; van Voorthuizen, E. M.; van Dongen, L. G. J. M.; Volcke, E. I. P.; van Loosdrecht, M. C. M. Methane and nitrous oxide emissions from municipal wastewater treatment - results from a long-term study. Water Sci. Technol. 2013, 67, 2350−2355. (8) Kampschreur, M. J.; Poldermans, R.; Kleerebezem, R.; van der Star, W. R. L.; Haarhuis, R.; Abma, W. R.; Jetten, M. S. M.; van Loosdrecht, M. C. M. Emission of nitrous oxide and nitric oxide from a full-scale single-stage nitritation-anammox reactor. Water Sci. Technol. 2009, 60, 3211−3217. (9) Yang, J.; Trela, J.; Plaza, E.; Tjus, K. N2O emissions from a one stage partial nitrification/anammox process in moving bed biofilm reactors. Water Sci. Technol. 2013, 68, 144−152. (10) Desloover, J.; De Clippeleir, H.; Boeckx, P.; Du Laing, G.; Colsen, J.; Verstraete, W.; Vlaeminck, S. E. Floc-based sequential partial nitritation and anammox at full scale with contrasting N2O emissions. Water Res. 2011, 45, 2811−2821. (11) Schneider, Y.; Beier, M.; Rosenwinkel, K.-H. Effect of substrate availability on nitrous oxide production by deammonification processes under anoxic conditions. Microb. Biotechnol. 2012, 5, 415− 424. (12) Yu, R.; Kampschreur, M. J.; van Loosdrecht, M. C. M.; Chandran, K. Mechanisms and specific directionality of autotrophic nitrous oxide and nitric oxide generation during transient anoxia. Environ. Sci. Technol. 2010, 44, 1313−1319. (13) Chandran, K.; Stein, L. Y.; Klotz, M. G.; van Loosdrecht, M. C. M. Nitrous oxide production by lithotrophic ammonia-oxidizing bacteria and implications for engineered nitrogen-removal systems. Biochem. Soc. Trans. 2011, 39, 1832−1837. (14) Schreiber, F.; Loeffler, B.; Polerecky, L.; Kuypers, M. M. M.; de Beer, D. Mechanisms of transient nitric oxide and nitrous oxide production in a complex biofilm. ISME J. 2009, 3, 1301−1313. (15) Law, Y.; Ye, L.; Pan, Y.; Yuan, Z. Nitrous oxide emissions from wastewater treatment processes. Philos. Trans. R. Soc. London, Ser. B 2012, 367, 1265−1277. (16) Castro-Barros, C.; Daelman, M.; Mampaey, K. E.; van Loosdrecht, M. C.; Volcke, E. I. P. Dynamics of N2O emission from partial nitritation-anammox in a full-scale granular sludge reactor. Proc. from 9th Int. Conf. Biofilm React. 532 28−31th May, Paris, 2013. (17) Mutlu, A. G.; Vangsgaard, A. K.; Sin, G.; Smets, B. F. An operational protocol for facilitating start-up of single-stage autotrophic nitrogen-removing reactors based on process stoichiometry. Water Sci. Technol. 2013, 68, 514−521. (18) Joss, A.; Salzgeber, D.; Eugster, J.; König, R.; Rottermann, K.; Burger, S.; Fabijan, P.; Leumann, S.; Mohn, J.; Siegrist, H. Full-scale nitrogen removal from digester liquid with partial nitritation and anammox in one SBR. Environ. Sci. Technol. 2009, 43, 5301−5306. (19) Vázquez-Padín, J. R.; Pozo, M. J.; Jarpa, M.; Figueroa, M.; Franco, A.; Mosquera-Corral, A.; Campos, J. L.; Méndez, R. Treatment of anaerobic sludge digester effluents by the CANON process in an air pulsing SBR. J. Hazard. Mater. 2009, 166, 336−341. (20) Wett, B. Development and implementation of a robust deammonification process. Water Sci. Technol. 2007, 56, 81−88. (21) Christensson, M.; Ekström, S.; Andersson Chan, A.; Le Vaillant, E.; Lemaire, R. Experience from start-ups of the first ANITA Mox plants. Water Sci. Technol. 2013, 67, 2677−2684. (22) van de Graaf, A. A.; de Bruijn, P.; Robertson, L. A.; Jetten, M. S. M.; Kuenen, J. G. Autotrophic growth of anaerobic am moniumoxidizing microorganisms in a fluidized bed reactor. Microbiology 1996, 142, 2187−2196. (23) Vangsgaard, A. K.; Mutlu, A. G.; Gernaey, K. V.; Smets, B. F.; Sin, G. Calibration and validation of a model describing complete autotrophic nitrogen removal in a granular SBR system. J. Chem. Technol. Biotechnol. 2013, 8, 2007−2015. (24) Andersen, K.; Kjær, T.; Revsbech, N. P. An oxygen insensitive microsensor for nitrous oxide. Sens. Actuators, B 2001, 81, 42−48. (25) CRC Handbook of Chemistry and Physics, 89th ed.; Lide, D. R., Ed.; CRC Press/Taylor and Francis: Boca Raton, FL, 2009. (26) Hogg, N.; Kalyanaraman, B. Nitric Oxide Protocols; Thitheradge, M. A., Ed.; Humana Press: NJ, 1997; Vol. 100, p 232.

and NOB; therefore, both can potentially remove unwanted substrates for N2O production. AOB-driven N2O production in nitritation/anammox systems was also suggested by Wunderlin et al.,58 although heterotrophic contribution cannot be excluded. 4.4. Implications for the Operation of Single-Stage Nitritation/Anammox Reactors. The potential impact of N2O emissions on carbon footprint calculations in the wastewater industry underlines the need for minimization strategies. Our findings highlight the possibility of adequate aeration control as a tool to reduce N2O emissions from nitritation/anammox systems.8 The in-depth study of two aeration strategies under stable performance both relying only on Qair control allowed us to propose high frequency of aeration switching as a potential strategy to minimize N2O emissions. Frequently switching aeration shortens aeration times, limiting the increasing N2O production rates while maintaining the overall system ammonium removal capacity. The lowest N2O emissions were achieved when the system was operated at low potential AOB activities. Performance would then be AOB-limited with AnAOB in excess.

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ASSOCIATED CONTENT

S Supporting Information *

List of acronyms and abbreviations and Figures S1−S6. This material is available free of charge via the Internet at http:// pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +45 4525 1600. Fax: +45 4593 2850. E-mail: bfsm@ env.dtu.dk. Author Contributions †

C.D.F and A.G.M. contributed equally to this manuscript.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was funded, in part, by the Danish Agency for Science, Technology and Innovation through the Research Project LaGas (12-132633) and Research Centre EcoDesign (09-067230). We would like to thank Ms. Chen Lv for her support in reactor operation and routine measurements.

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REFERENCES

(1) Vlaeminck, S. E.; De Clippeleir, H.; Verstraete, W. Microbial resource management of one-stage partial nitritation/anammox. Microb. Biotechnol. 2012, 5, 433−448. (2) Kampschreur, M. J.; Temmink, H.; Kleerebezem, R.; Jetten, M. S. M.; van Loosdrecht, M. C. M. Nitrous oxide emission during wastewater treatment. Water Res. 2009, 43, 4093−4103. (3) IPCC Fifth Assessment Report - The physical science basis; Cambridge University Press: Cambridge, United Kingdom and New York, NY, USA, 2013; p 1535. (4) Czepiel, P.; Crill, P.; Harriss, R. Nitrous oxide emissions from municipal wastewater treatment. Environ. Sci. Technol. 1995, 29, 2352− 2356. (5) Foley, J.; de Haas, D.; Yuan, Z.; Lant, P. Nitrous oxide generation in full-scale biological nutrient removal wastewater treatment plants. Water Res. 2010, 44, 831−844. (6) Ahn, J. H.; Kim, S.; Park, H.; Rahm, B.; Pagilla, K.; Chandran, K. N2O emissions from activated sludge processes, 2008−2009: results of a national monitoring survey in the United States. Environ. Sci. Technol. 2010, 44, 4505−4511. 8686



Article

biological process of N2O emission from anammox granules. Water Res. 2011, 45, 6461−6470. (47) Shiskowski, D.; Mavinic, D. The influence of nitrite and pH (nitrous acid) on aerobic-phase, autotrophic N2O generation in a wastewater treatment bioreactor. J. Environ. Eng. Sci. 2006, 283, 273− 283. (48) Yu, R.; Chandran, K. Strategies of Nitrosomonas europaea 19718 to counter low dissolved oxygen and high nitrite concentrations. BMC Microbiol. 2010, 10, 70. (49) Cua, L. S.; Stein, L. Y. Effects of nitrite on ammonia-oxidizing activity and gene regulation in three ammonia-oxidizing bacteria. FEMS Microbiol. Lett. 2011, 319, 169−175. (50) Zhou, Y.; Pijuan, M.; Zeng, R. J.; Yuan, Z. Free nitrous acid inhibition on nitrous oxide reduction by a denitrifying-enhanced biological phosphorus removal sludge. Environ. Sci. Technol. 2008, 42, 8260−8265. (51) von Schulthess, R.; Wild, D.; Gujer, W. Nitric and nitrous oxides from denitrifying activated sludge at low oxygen concentration. Water Sci. Technol. 1994, 30, 123−132. (52) Zumft, W. G. Cell biology and molecular basis of denitrification. Microbiol. Mol. Biol. Rev. 1997, 61, 533−616. (53) Wunderlin, P.; Mohn, J.; Joss, A.; Emmenegger, L.; Siegrist, H. Mechanisms of N2O production in biological wastewater treatment under nitrifying and denitrifying conditions. Water Res. 2012, 46, 1027−1037. (54) Kampschreur, M. J.; van der Star, W. R. L.; Wielders, H. A.; Mulder, J. W.; Jetten, M. S. M.; van Loosdrecht, M. C. M. Dynamics of nitric oxide and nitrous oxide emission during full-scale reject water treatment. Water Res. 2008, 42, 812−826. (55) Park, K. Y.; Inamori, Y.; Mizuochi, M.; Ahn, K. H. Emission and control of nitrous oxide from a biological wastewater treatment system with intermittent aeration. J. Biosci. Bioeng. 2000, 90, 247−252. (56) Itokawa, H.; Hanaki, K.; Matsuo, T. Nitrous oxide production in high-loading biological nitrogen removal process under low COD/N ratio condition. Water Res. 2001, 35, 657−664. (57) Kartal, B.; Tan, N. C. G.; Van de Biezen, E.; Kampschreur, M. J.; Van Loosdrecht, M. C. M.; Jetten, M. S. M. Effect of nitric oxide on anammox bacteria. Appl. Environ. Microbiol. 2010, 76, 6304−6306. (58) Wunderlin, P.; Siegrist, H.; Joss, A. Online N2O measurement: the next standard for controlling biological ammonia oxidation? Environ. Sci. Technol. 2013, 47, 9567−9568.

(27) Ellis, T. G.; Barbeau, D. S.; Smets, B. F.; Grady, C. P. L. Respirometric technique for determination of extant kinetic parameters describing biodegradation. Water Environ. Res. 1996, 68, 917−926. (28) Chandran, K.; Smets, B. F. Single-step nitrification models erroneously describe batch ammonia oxidation profiles when nitrite oxidation becomes rate limiting. Biotechnol. Bioeng. 2000, 68, 396−406. (29) Dapena-Mora, A.; Fernández, I.; Campos, J. L.; MosqueraCorral, A.; Méndez, R.; Jetten, M. S. M. Evaluation of activity and inhibition effects on Anammox process by batch tests based on the nitrogen gas production. Enzyme Microb. Technol. 2007, 40, 859−865. (30) Stenstrom, M. Measurement of Oxygen Transfer In Clean Water; ASCE Publications: 2007. (31) Garcia-Ochoa, F.; Gomez, E. Bioreactor scale-up and oxygen transfer rate in microbial processes: an overview. Biotechnol. Adv. 2009, 27, 153−176. (32) De Heyder, B.; Vanrolleghem, P.; Van Langenhove, H.; Verstraete, W. Kinetic characterization of mass transfer limited biodegradation of a low water soluble gas in batch experiments– necessity for multiresponse fitting. Biotechnol. Bioeng. 1997, 55, 511− 519. (33) Van Hulle, S. W. H.; Callens, J.; Mampaey, K. E.; van Loosdrecht, M. C. M.; Volcke, E. I. P. N2O and NO emissions during autotrophic nitrogen removal in a granular sludge reactor − a simulation study. Environ. Technol. 2012, 33, 2281−2290. (34) Colliver, B. B.; Stephenson, T. Production of nitrogen oxide and dinitrogen oxide by autotrophic nitrifiers. Biotechnol. Adv. 2000, 18, 219−232. (35) Sliekers, A.; Haaijer, S. C. M.; Stafsnes, M. H.; Kuenen, J. G.; Jetten, M. S. M. Competition and coexistence of aerobic ammoniumand nitrite-oxidizing bacteria at low oxygen concentrations. Appl. Microbiol. Biotechnol. 2005, 68, 808−817. (36) Udert, K. M.; Larsen, T. A.; Gujer, W. Chemical nitrite oxidation in acid solutions as a consequence of microbial ammonium oxidation. Environ. Sci. Technol. 2005, 39, 4066−4075. (37) Kampschreur, M. J.; Picioreanu, C.; Tan, N.; Kleerebezem, R.; Jetten, M. S. M.; van Loosdrecht, M. C. Unraveling the source of nitric oxide emission during nitrification. Water Environ. Res. 2007, 79, 2499−2509. (38) Schreiber, F.; Wunderlin, P.; Udert, K. M.; Wells, G. F. Nitric oxide and nitrous oxide turnover in natural and engineered microbial communities: biological pathways, chemical reactions, and novel technologies. Front. Microbiol. 2012, 3, 372. (39) Sliekers, A. Completely autotrophic nitrogen removal over nitrite in one single reactor. Water Res. 2002, 36, 2475−2482. (40) De Clippeleir, H.; Vlaeminck, S. E.; De Wilde, F.; Daeninck, K.; Mosquera, M.; Boeckx, P.; Verstraete, W.; Boon, N. One-stage partial nitritation/anammox at 15 °C on pretreated sewage: feasibility demonstration at lab-scale. Appl. Microbiol. Biotechnol. 2013, 97, 10199−10210. (41) Law, Y.; Lant, P.; Yuan, Z. The effect of pH on N2O production under aerobic conditions in a partial nitritation system. Water Res. 2011, 45, 5934−5944. (42) Hu, Z.; Zhang, J.; Li, S.; Xie, H.; Wang, J.; Zhang, T.; Li, Y.; Zhang, H. Effect of aeration rate on the emission of N2O in anoxicaerobic sequencing batch reactors (A/O SBRs). J. Biosci. Bioeng. 2010, 109, 487−491. (43) Schneider, Y.; Beier, M.; Rosenwinkel, K.-H. Nitrous oxide formation during nitritation and nitrification of high-strength wastewater. Water Sci. Technol. 2013, 67, 2494−2502. (44) Law, Y.; Ni, B.-J.; Lant, P.; Yuan, Z. N2O production rate of an enriched ammonia-oxidising bacteria culture exponentially correlates to its ammonia oxidation rate. Water Res. 2012, 46, 3409−3419. (45) Law, Y.; Lant, P.; Yuan, Z. The confounding effect of nitrite on N2O production by an enriched ammonia-oxidizing culture. Environ. Sci. Technol. 2013, 47, 7186−7194. (46) Okabe, S.; Oshiki, M.; Takahashi, Y.; Satoh, H. N2O emission from a partial nitrification-anammox process and identification of a key 8687


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