N2O Production during Nitrogen Removal via Nitrite from Domestic

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Environ. Sci. Technol. 2009 43, 9400–9406

N2O Production during Nitrogen Removal via Nitrite from Domestic Wastewater: Main Sources and Control Method QING YANG,† XIUHONG LIU,‡ CHENGYAO PENG,† SHUYING WANG,† HONGWEI SUN,† AND Y O N G Z H E N P E N G * ,† Key Laboratory of Beijing for Water Quality Science and Water Environment Recovery Engineering, Beijing University of Technology, and School of Municipal and Environmental Engineering, Harbin Institute of Technology

Received June 29, 2009. Revised manuscript received October 24, 2009. Accepted October 26, 2009.

Nitrite has been commonly recognized as an important factor causing N2O production, which weakened the advantages of nitrogen removal via nitrite. To reduce and control N2O production from wastewater treatment plants, both long-term and batch tests were carried out to investigate main sources and pathways of N2O production during nitrogen removal via nitrite from real domestic wastewater. The obtained results showed that N2O production during nitrogen removal via nitrite was 1.5 times as much as that during nitrogen removal via nitrate. It was further demonstrated that ammonia oxidization were main source of N2O production during nitrogen removal from domestic wastewater; whereas, almost no N2O was produced during nitrite oxidization and anoxic denitrification. N2O production during nitrogen removal via nitrite decreased about 50% by applying the step-feed SBR, due to the effective control of nitrite and ammonia, the precursors of N2O production. Therefore, the step-feed system is recommended as an effective method to reduce N2O production during nitrogen removal via nitrite from domestic wastewater.

Introduction Nitrous oxide (N2O), which is one of the important greenhouse gases (CO2, CH4 and N2O) with a serious impact on the environment, not only contributes to the greenhouse effect, but also destructs the ozone layer. N2O concentration in the atmosphere is increasing globally at an alarming rate of 0.31% per year (1). A significant source of the global N2O emission can be attributed to microbial nitrification and denitrification occurring in terrestrial and aquatic systems (2), whereas these natural microbial nitrification and denitrification are enhanced artificially in biological nitrogen removal from wastewater. In biological nitrogen removal processes which provide aerobic and anoxic conditions, ammonia is oxidized to nitrite and further to nitrate, which is subsequently reduced to gaseous nitrogen through several steps in its biochemical pathway (denitrification). With the application of biological nitrogen removal processes world* Corresponding author phone: 86 10 67392627; fax: 86 10 67392627; e-mail: [email protected]. † Beijing University of Technology. ‡ Harbin Institute of Technology. 9400

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wide, water eutrophication will be reduced. However, substantial amounts of N2O emitted during nitrogen removal from wastewater will cause an increase of the N2O concentration in the atmosphere (3, 4). Many researchers have been carrying out significant and meaningful work on N2O production during biological nitrogen removal from wastewater. In these studies, N2O emission rate varied a lot because of differences in wastewater quality, operational parameters and environmental conditions. When treating artificial synthetic wastewater, N2O production during nitrification and denitrificaiton were 0.216% and 0.005-90% of the nitrogen load, respectively (5-9), whereas when real wastewater was applied, N2O production were only 0.1-4% and 0-6% of the nitrogen load, respectively (4, 10-12). N2O production during synthetic wastewater treatment is generally higher than that during real wastewater treatment due to the low microbial diversity performed in the former scenario. Whether treating artificial wastewater or real wastewater, nitrification and denitrification both appeared to be the origins of N2O production during wastewater treatment. N2O is produced by a wide variety of organisms, including autotrophic, heterotrophic nitrifies and heterotrophic denitrifiers through nitrification (5, 6, 13) and denitrification (7-9). To date, it remains unclear what is the main source and pathways of N2O production during nitrogen removal from wastewater, especially from real domestic wastewater. Identifying the main sources and pathways of N2O production benefits the study on key factors leading to N2O production. To control and reduce N2O production, key factors leading to N2O production should be well managed during the real operation of wastewater treatment plants (WWTPs) (3). To date, key factors leading to N2O emission during nitrogen removal from wastewater were reported ambiguously in the literature as low dissolved oxygen (DO) (4, 10, 11, 14), high ammonia concentration (10, 15), nitrite concentration (5, 10, 15, 16), low chemical oxygen demand (COD)/NOX--N ratio (7, 15), as well as short solid retention time (SRT) (5, 14). Therefore, only when identifying main sources and further determine key factors leading to N2O production, wastewater treatment processes with low N2O production could be developed. Recently, several novel processes, such as nitrogen removal via nitrite (nitritation-denitritation), simultaneous nitrification and denitrification, complete autotrophic nitrogen removal over nitrite (17), have been developed to enhance nitrification and denitrification more effectively and economically. In all these processes, nitrite, an intermediate product,wasaccumulated.Especially,duringnitritation-denitritation, ammonium is completely oxidized to nitrite, which would be directly reduced to gaseous nitrogen. This process offers considerable savings in both oxygen and carbon source demands because nitrite is an intermediary compound in both nitrification and denitrification (17). Nitrogen removal via nitrite can be obtained through appropriate control on low DO concentration, high free ammonia (FA) concentration, short SRT, as well as inhibitors. However, these methods for achieving nitritation are usually key factors leading to N2O production too. Moreover, several studies on N2O production during nitrogen removal have shown that it is usually accompanied by nitrite accumulation (5, 10, 15, 16). Therefore, N2O production during nitrogen removal via nitrite was possibly higher than that during nitrogen removal via nitrate. Until now, limited research has been conducted on N2O production during nitrogen removal via nitrite from real domestic wastewater. 10.1021/es9019113 CCC: $40.75

 2009 American Chemical Society

Published on Web 11/12/2009

FIGURE 1. Variations of N2O concentrations, RN2O, NH4+, NO2-, NO3-, and COD during nitrogen removal via nitrate (in SBR1; panels A-C) and nitrogen removal via nitrite (in SBR2; panels a-c) treating domestic wastewater. The vertical dashed line in panels A-C indicated ammonia was completely oxidized. The vertical solid line indicates the transition between the aerobic and the anoxic period. The horizontal dashed line in panels B and b indicated ammonia concentration was 10 mg/L.

Therefore, the study via long-term experiments and batch test experiments aimed to (a) determine N2O production during the typical nitrogen removal processes, nitrogen removal via nitrate and nitrogen removal via nitrite, from real domestic wastewater; (b) identify the main sources and pathways of N2O production during nitrogen removal from domestic wastewater; (c) reduce N2O production during nitrogen removal via nitrite from domestic wastewater.

Materials and Methods Wastewater. The fresh domestic wastewater used in this experiment was collected from residential area of Beijing University of Technology, Beijing, China. The mean concentrations of COD and NH4+-N in the wastewater were 223.0 mg/L and 59.97 mg/L, respectively.

Reactor Setup and Operation. Two SBRs with a working volume of 14 L and overhead space of 1 L were operated in parallel (see the Supporting Information (SI)). The biomass in SBR1 and SBR2 were originated from a municipal WWTP. SBR1 was operated in the aerobic-anoxic scheme with extensive aeration. Each cycle of the aerobic-anoxic SBR1 consisted of 3 min feeding, aeration, anoxic phase (adding methanol in the first minute of anoxic period), 1 h settling, 6 min decanting, and 1 min idling. Air was still provided for another 0.5 h after the appearance of the “ammonia valley” point on the pH curve (Figure 1C). The anoxic phase was stopped as soon as the pH started to decrease (Figure 1C). The operation of SBR2 was divided into three phases. During phase one (10 weeks), the aerobic-anoxic scheme with realtime control was applied (18). The durations of aeration and VOL. 43, NO. 24, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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a

(5)

(4)

(3)

(2)

(1)

NO3 -N COD COD NO3--N NO2--N N2O NO2--N DO pH NH4+-N DO pH NH4+-N DO pH NO2--N COD DOc pH

-

substrate

4 2.0-5.8 7.1-7.5 50 1.4-5.1 6.2-7.2 54 0.5-4.0 7.5-7.7 17 27 0.5 7.1-7.9

42 265 150 34

1

b

2 10 1.5-5.0 7.0-7.5 17 0.5-6.2 7-7.8 19 0.6-3.9 7.3-7.7 20 39 1.5 7.0-7.6

42 224 150

3

22 51 2.0 7.0 7.7

42 15b

4

SBR2

SBR2

SBR1

SBR1

SBR1

SBR1

sludge

to determine whether N2O was produced during denitrification in the absence of external carbon under different DO concentrations

to determine the effect of ammonia concentrations on N2O production during complete nitrification and nitritation

to determine whether N2O was produced during nitrite oxidization

to determine the possibility of N2O production in the absence of external carbon source during denitrification to determine N2O production and denitrifying rates using nitrate, nitrite and N2O as electron acceptors

aims

Means no external carbon source was added. c Means DO was controlled by adjusting air flow meter.

7 2.0-4.8 7.1-7.4 38 0.7-6.2 7.0-7.7 37 0.3-4.3 7.2-7.6 12 45 1.0 7.0-7.7

20

42 217 150

2

batch reactors/test phases

water characterization

DO and pH profiles during test (3)s(5) are shown in SI.

long-term test

aerobic batch tests

anoxic batch tests

tests

TABLE 1. Experimental Condition Applied in all Testsa

anoxic phase were exactly controlled at “ammonia valley” and “nitrate apex” points on the pH curve (Figure 1c). To determine whether N2O was produced during denitrification under different DO concentrations, during phase two (2 weeks), the aerobic-anoxic scheme with extensive aeration was applied in SBR2 (Table 1 long-term test).To reduce N2O production during nitrogen removal via nitrite, during phase three (8 weeks), the step-feed operational pattern with realtime control was applied (18). The step-feed SBR is a modification of the aerobic-anoxic SBR, in which wastewater is introduced twice with half of the total added volume. In both SBRs, a certain amount of sludge was removed before settling, to keep the mixed liquor suspended solids (MLSS) at approximately 2000-2300 mg/L. After settling, 10 L of clarified supernatant was decanted. Aeration was provided at a constant flow rate of 100 L/h by an air pump. Temperature was controlled at 28 °C by a temperature controller. Unless otherwise described, the DO concentration and pH in system was online monitored, but not controlled. The experiments reported below were carried out only after the SBRs indicated excellent and stable performance. N2O production and water quality was measured by weekly cycle studies. During N2O analysis throughout aerobic and anoxic reactions, SBR was completely airtight (see SI). During aerobic reaction, the emission gas was directly dehydrated, and then collected in gas sampling bags at the interval of 0.5-1 h. During anoxic reaction, to benefit the release of the emission gas and to simulate the airflow condition at the top of the reactor without being airtight, air was blew in at a constant flow rate of 40 mL/min from one side of the reactor’s top cover. The injected air transported the emitted gas into the dehydration unit and the gas sampling bags. According to N2O concentration and the volume of the collected gas, N2O production in the emission gas was calculated. At the end of each gas collection, N2O concentration dissolved in the mixture and water quality were also analyzed. Since the pressure in the headspace of the airtight SBR was almost equal to the atmospheric pressure, water quality and N2O transfer from the liquid to the gas phase were very similar to SBR without airtight condition. Thus, using this set of gas collection system and N2O analytical methods, the processes of N2O production and transfer during aerobic and anoxic phases in SBR can be investigated exactly. Additional Batch Test Experiments. To identify the main sources and pathways of producing N2O during nitrogen removal from domestic wastewater and to prepare for the study on reducing of N2O production, five batch test experiments were carried out. Table 1 shows the experimental conditions applied in these tests. All batch experiments were carried out in sealed batch reactors with a working volume of 1.4 L (see SI). All batch reactors had an overhead space of 0.1 L except for the third reactor in batch test (2). The magnetic stirrers provided adequate mixing and maintained at the constant temperature of 28 °C. The procedures of gas collection and N2O analyses were the same as for the two SBRs. The sludge used in batch tests was withdrawn at the end of settling phase from SBR1 or SBR2, and then, washed using distilled water three times to remove the background concentrations of COD, NH4+, NO2-, and NO3-. Unless otherwise described, the washed sludge was diluted with distilled water adjusting MLSS to about 2000 mg/L, and then transferred to the batch reactors, 1.4 L of mixture in each batch reactor. After decanting 100 mL of supernatant liquor, the batch reactors were sealed tightly with rubber stopples. In aerobic batch test (3), the mixture was preaerated to control of DO at about 2 mg/L. To avoid the occurrence of denitrification or nitrification before gas collection, after injecting 100 mL of solution which contained trace elements (19), NH4Cl, NaNO2, NaNO3 or

methanol depending on the test condition, the emission gases and liquid samples were collected immediately. In anoxic batch test (2), an N2O solution was prepared by sparging 1.2 L of Milli-Q water with 500 ppm N2O gas for 30 min in the third batch reactor. To prevent N2O emission, there was almost no overhead space in the third batch reactor. After preparing the N2O solution, the reactor was sealed and 0.2 L of the washed sludge with an MLSS concentration of 14 g/L, and 0.1 mL of methanol were injected immediately. In all batch tests, gas samples and water samples were taken at regular intervals to monitor N2O concentrations and water quality. Analysis. COD, NH4+, NO3-, NO2-, and MLSS were measured according to the standard methods (20). DO, pH, and ORP were measured online using oxygen, pH and ORP meters (WTW 340i, WTW Company, Germany) respectively. Total nitrogen (TN) were analyzed by Multi N/C 3000 (Analytik Jena AG, Germany). The total N2O production consists of the N2O production in the emission gas (the emission-gas N2O) and the N2O dissolved in the mixture (the dissolved N2O). N2O concentration in gas samples was analyzed in triplicate using a GC (Agilent 6890N, U.S.).The overhead space method was used to analyze the dissolved N2O (12). To compare N2O production with nitrogen concentrations in water samples, the emission-gas N2O-N concentration, the dissolved N2O-N concentration in a unit volume of the water sample, the total N2O-N production and the mean N2O-N production rates (RN2O) were calculated (see SI). FISH was carried out with EUBMix (EUB338, EUB338-II, and EUB338-III) specific for all Bacteria, NSO1225 specific for Ammonia-oxidizing β-Proteobacteria, NIT3 specific for Nitrobacter, and Ntspa662 specific for Nitrospira as described in Yang et al. (18).

Results and Discussion N2O Production during Nitrogen Removal via Nitrate/ Nitrite from Real Domestic Wastewater. Two SBRs were operated with nitrogen removal via nitrate and nitrite respectively. In SBR1, stable nitrogen removal via nitrate with TN lower than 3 mg/L in the effluent was achieved after 1.5 months. In SBR2, stable nitrogen removal via nitrite with nitrite accumulation (NO2--N/NOx--N) at the end of nitrification higher than 90% and TN in the effluent lower than 1.5 mg/L was achieved after 4 months. Figure 1 shows the variations of N2O concentrations, RN2O, NH4+, NO2-, NO3-, COD, pH, and DO during nitrogen removal via nitrate (in SBR1) and nitrogen removal via nitrite(in SBR2) in typical cycles. N2O was produced during both nitrogen removal schemes. A very small amount of N2O was produced during aeration reaction (0-60 min, in SBR1 and SBR2), whereas N2O production increased with the decrease of ammonia concentration and the increase of nitrite and nitrate concentrations. Sufficient carbon sources were provided for denitrification in SBR1 and SBR2. During nitrate or nitrite denitrification, no N2O was produced. N2O production during nitrogen removal via nitrite was 1.5 times as much as that during nitrogen removal via nitrate. The nitrite accumulation during complete nitrification and nitritation was 6.21 mg/L and 15.90 mg/L respectively, which implied that high nitrite accumulation, could possibly lead to high N2O production during nitrification (5, 10, 15, 16). Nitrite accumulated as soon as no more degradable organic matter was available. Aerobic reaction in SBR1 and SBR2 removed TN of 6.42 mg/L and 24.64 mg/L respectively, indicating that denitrification might still have been possible with organic products from endogenous respiration and lysis of other cells or with storage products. This is consistent with the literature, where N2O production during denitrifiVOL. 43, NO. 24, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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cation was only registered in the absence of dissolved organic matter and the presence of nitrite or low DO (7, 15). Hence, several biochemical reactions, including ammonia oxidization and nitrite oxidization and denitrification, occurred during aeration were likely to be the origins of N2O production during aerobic phase of nitrogen removal over nitrate/nitrite. Additional experimental tests were carried out to find out more about the main sources and pathways of N2O production (table 1). Identifying Main Sources and Pathways of N2O Production during Nitrogen Removal from Real Domestic Wastewater. By analyzing the obtained results from long-term experiments (Figure 1) and from the literature, it was concluded that N2O might be produced during aeration by heterotrophic denitrifiers and autotrophic nitrifiers via three pathways: (a) denitrification in the absence of dissolve organic carbon (7, 15), the presence of nitrite (5, 10, 15, 16) or low DO (10, 11); (b) the reduction of nitrite to N2O or N2 after ammonia was oxidized (nitrifier denitriifcation) (21, 22); (c) the reduction of nitrite to N2O or N2 with ammonia as the electron donor (23, 24). Denitrification at Low COD/NOx--N Ratio, With High Nitrite Concentrations or DO Provisions. N2O production under different COD/NO3--N ratios was investigated in batch test (1). The obtained results showed that when COD/NO3--N ratio was 4.7 and 3.8 (sufficient external carbon source), nitrate was completely denitrified with no N2O or nitrite produced, whereas when COD/NO3--N ratio was 1.5(insufficient external carbon source), the nitrate removal efficiency was about 59% with 0.01 mgN/(gMLVSS · h) of RN2O and 0.4 mgN/L of nitrite accumulated (see SI). When COD/NO3--N ratio was 0.4, without additional external carbon source but only utilizing endogenous carbon source, nitrate removal efficiency was only about 9% with 0.02 mgN/(g MLVSS · h) of RN2O produced and 0.6 mgN/L of nitrite accumulated. These results indicated that COD/NO3--N slightly influenced the N2O production during anoxic denitrification. Even without dosage of external carbon source did not result in producing large amount of N2O during anoxic denitrification. N2O production or consumption from denitrification and the respective denitrification rate were investigated in batch test (2) using NO3-, NO2-, and N2O as electron acceptors respectively. The results not only showed that no N2O was detected during denitrification no matter using nitrate or nitrite as electron acceptor, but also indicated that N2O further reduced to N2 rapidly, which further demonstrating anoxic denitrification had the potential to reduce N2O production when providing sufficient external carbon source (Figure 1). The specific nitrate and nitrite reduction rates were 0.21 mgNO3--N/(mgMLVSS · d)and0.54mgNO2--N/(mgMLVSS · d) respectively. The specific nitrite reduction rate was 2.6 times as much as the specific nitrate reduction rate, indicating that the nitrate reduction was the limitation step of denitrification. When using N2O as the only electron acceptor, the specific N2O reduction rate was about 0.62 mgN2O-N/ (mg MLVSS · d). As the specific N2O reduction rate was higher than the specific nitrate and nitrite reduction rate, no N2O was produced during denitrification with sufficient dissolved organic carbon. Since N2O reduction was the last step of denitrification, after nitrate or nitrite was reduced to N2O, if no carbon source can be available, N2O was possibly accumulated. In this study, as the specific N2O reduction rate was higher than the specific nitrate and nitrite reduction rate, even in the absence of dissolved organic carbon, only slight amount of N2O was accumulated during denitrification. Furthermore, the microbial communities, especially heterotrophic denitrifiers, were much diversified in the activated sludge cultivated in aerobic-anoxic SBR treating real domestic wastewater (14). Compared to the activated sludge cultivated by synthetic wastewater, there might be a lot more 9404

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heterotrophic denitrifiers producing N2 as the end product in the activated sludge cultivated by real domestic wastewater (9, 14). The difference in the rates of nitrate, nitrite, and N2O reduction found in the anoxic batch test (2) and the microbial diversity of the activated sludge cultivated in this study might explain that a little amount of N2O was produced during denitrification even at different COD/NO3--N ratios. N2O production during denitrification in the absence of carbon source and with high nitrite concentrations were tested under different DO concentrations, during phase two in SBR2. After long-term achieving nitrogen removal via nitrite in SBR2, FISH analysis showed that AOB became the dominant nitrifying bacteria, while no NOB was detected (see SI). Therefore, after ammonia was completely oxidized to nitrite, nitrification was finished concurrently. Hereafter, if aeration was maintained, the effect of ammonia oxidization and nitrite oxidization on N2O production could be avoided (see SI). With this, the possibility of N2O production during denitrification in the absence of carbon source and the presence of nitrite can be well tested under different DO concentrations, in SBR2. The obtained results showed that when DO was 1.0 mg/ L, RN2O was 0.09 mgN/(g MLVSS · h), whereas when DO was 0.5, 1.5, and 2.0 mg/L, RN2O was lower than 0.03 N mg/(g MLVSS · h) (see SI). In the absence of carbon source, RN2O during denitrification under DO provisions in SBR2 was a little higher than that during anoxic denitrification in batch test (1), indicating that DO inhibition could slightly accelerate N2O production because N2O reductase was very sensitive to oxygen stress (10, 11); whereas, compared to RN2O in SBR1 and SBR2(Figure 1), N2O was still slightly produced during denitrification under difference DO concentrations, even in the absence of carbon source and the presence of high nitrite concentrations. Nitrite Oxidization under Different Nitrite Concentrations. To avoid the occurrence of denitrification during the initial stage of aeration, in batch test (3), aeration was preprovided. After DO concentration increased to 1.5-2 mg/L (see SI), indicating that the microenvironment of activated sludge had turned from microaerobic condition to aerobic condition, synthetic solution contained nitrite was injected in the activated sludge taken from SBR1. Nitrite oxidization was mainly occurred in aerobic batch test (3). The obtained result showed that, no N2O was detected during aeration with different nitrite concentrations, indicating that nitrite oxidization was also not the main source of N2O production during nitrogen removal from domestic wastewater. Complete Nitrification and Nitritation with Different Feed Ammonia Concentrations. As shown in Figure 1, during longterm experiments on nitrogen removal via nitrate and nitrite, RN2O was found to increase with the increment of nitrite concentration; whereas, when ammonia concentration was lower than 10 mg/L, even if the accumulated nitrite concentration remained higher than 10 mg/L, RN2O still decreased sharply, which implied that N2O production not only related to nitrite concentration, but also ammonia concentration. More recently, ammonia was found to influence N2O production from soil-based and plug-flow scheme wastewater treatment system (10, 25). The aerobic batch test (4) and (5) was therefore conducted to identify the impact of ammonia concentrations on N2O production during complete nitrification and nitritation. Figure 2 shows N2O production during complete nitrification and nitritation with different feed ammonia concentrations. N2O production increased with the increase of the feed ammonia concentration, which further indicating that N2O production was directly related to the feed ammonia concentration. Meanwhile, N2O production during nitritation was higher than that during complete nitrification with the same feed ammonia concentration, further demonstrating

FIGURE 2. N2O production during the complete nitrification and nitritation at different ammonia concentrations. that higher nitrite accumulation resulted in higher N2O production. These results demonstrated N2O was mainly produced during ammonia oxidization. Nitrifier denitrification was reported to be one of the important sources of N2O production from soil and wastewater treatment (21, 22). If N2O was produced during nitrifier denitrification, then only presence of ammonia or nitrite, N2O was very possibly produced because nitrite was an intermediate product in this pathway of N2O production (21). However, this study suggests that only when nitrite and ammonia coexisted in the treatment system, N2O could be produced, which indicates that both ammonia and nitrite are substrates for N2O production. Previous studies also demonstrated AOB can reduce nitrite to N2O or N2 with ammonia or hydrogen as the electron donor (23, 24). We therefore propose that AOB can reduce nitrite to N2O with ammonia as the electron donor, but the mechanism of N2O production during aeration must be investigated in more detail before a final conclusion is possible. Controlling and Reducing of N2O Production during Nitrogen Removal via Nitrite from Domestic Wastewater Using a Step-Feed Scheme. As ammonia is partially converted to nitrite, nitrogen removal via nitrite offers a great potential to save capital and operating costs for domestic wastewater treatment (17). However, due to the same reason, ammonia and nitrite are unavoidable to coexist in the treatment systems, resulting in the production of high amounts of N2O. N2O production severely impedes the advantages of this process. Two methods could possibly control and reduce N2O production during nitrogen removal via nitrite from domestic wastewater: (a) reducing the substrates concentrations causing N2O production which were ammonia and/or nitrite; (b) enhancing anoxic denitrification using external carbon source to reduce nitrite and N2O during aerobic reaction. The highlight is that reducing N2O production shall not affect the stability of nitrogen removal via nitrite. Based on the above-mentioned, to reduce N2O production during nitrogen removal via nitrite, the operational pattern of SBR2 was shifted from the aerobic-anoxic scheme to the step-feed scheme. As shown in Figure 3, during the first 60 min of the first aeration stage, DO concentration was lower than 0.3 mg/L. Meanwhile, both COD and ammonia were greatly decreased with no N2O produced, indicating that anoxic denitrification using organic matter in the influent as carbon source was occurred simultaneously with nitrification under such low DO concentration (9). As the N2O reduction rate higher than nitrite reduction rate, even if N2O was produced in this stage, N2O was first reduced. However, low DO concentration also led to high N2O production (see

FIGURE 3. Variations of N2O concentrations, RN2O, NH4+, NO2-, NO3-, and COD during nitrogen removal via nitrite in step-feed SBR treating domestic wastewater. SI) (10, 11). Furthermore, after 60 min of the first aeration stage, anoxic denitrification might not occurred due to COD lower than 50 mg/L. Thus, N2O production during the first aeration stage was still 2.19 mgN/L. During the second aeration stage, ammonia concentration was only about 17.73 mg/L, half of that in the aerobic-anoxic SBR. During this stage, N2O production was only 0.29 mg/L. Applying the stepfeed scheme, therefore, ammonia and nitrite, the substrates of producing N2O, were successfully managed at the relatively low levels in the second aeration stage. Moreover, dissolved N2O at the end of two aeration stages was reduced immediately during the subsequent denitrification using the organic matters existed in the second feeding and ethanol as carbon sources respectively. There was a stark contrast in N2O production between nitrogen removal via nitrite in the step-feed SBR and nitrogen removal via nitrite in the aerobic-anoxic SBR. Compared to the aerobic-anoxic SBR, total N2O production during nitrogen removal via nitrite in the step-feed SBR was decreased about 50%. Therefore, the step-feed scheme was approved to be an effective approach to reduce N2O production during nitrogen removal via nitrite. Further beyond, the step-feed scheme not only saved carbon source dosage because it fully utilized the organic carbon in wastewater, but also benefited the achievement and stabilization of nitrogen removal via nitrite from domestic wastewater (18). Increase the feeding frequency of the step-feed SBR and control of the optimal DO concentration might minimize N2O production from nitrogen removal via nitrite further. The optimal DO concentration and feeding frequency of step-feed SBR to control N2O production from nitrogen removal via nitrite shall be investigated further. In wastewater treatment plants, especially municipal wastewater treatment plant, nitrite is generally believed to be less accumulative, thus avoiding N2O emission. However, VOL. 43, NO. 24, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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this study suggests that even low level nitrite accumulation can lead to N2O production. Figure 1(B) shows that N2O was produced at a nitrite concentration of approximately 2 mgN/ L. In municipal wastewater treatment plants, nitrite often accumulates at high level in summer. Kampschreur had found N2O emission from nitritation in a full-scale reject water treatment plant represented 1.7% of the nitrogen load (26). Nitrogen removal via nitrite was previous considered as an energy saving wastewater treatment process; however, nitrite accumulation threshold constitutes a serious concern for N2O production during biological nitrogen removal wastewater treatment plants, especially those designed for achieving nitrogen removal via nitrite. Reducing and controlling of N2O emission from biological nitrogen removal wastewater treatment plants should be investigated thoroughly. The accumulation of nitrite should be avoided or reduced in nitritation through step-feed operational pattern.

Acknowledgments This research was supported by National Key Science and Technology Special Projects (2008ZX07317-007-105), the projects of Beijing municipal education commission (PXM2008014204-050843; PHR20090502), and the State Key Laboratory ofUrbanWaterResourceandEnvironment(HIT)(QAK200802).

Supporting Information Available Schematic diagram of the present SBR and the batch reactor with gas collecting systems, detailed method for calculating N2O concentrations, FISH analyzing results in SBR2, and main results of test experiments. This material is available free of charge via the Internet at http://pubs.acs.org.

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