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© IWA Publishing 2014 Water Science & Technology
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N2O emissions from secondary clarifiers and their contribution to the total emissions of the WWTP Anna Mikola, Mari Heinonen, Heta Kosonen, Maarit Leppänen, Pirjo Rantanen and Riku Vahala
ABSTRACT Recent studies have indicated that the emissions of nitrous oxide, N2O, constitute a major part of the carbon footprint of wastewater treatment plants (WWTPs). Denitrification occurring in the secondary clarifier basins has been observed by many researchers, but until now N2O emissions from secondary clarifiers have not been widely reported. The objective of this study was to measure the N2O emissions from secondary clarifiers and weigh the portion they could represent of the overall emissions at WWTPs. Online measurements over several days were carried out at four different municipal WWTPs in Finland in cold weather conditions (March) and in warm weather conditions (June–July). An attempt was made to define the conditions in which N2O emissions from secondary clarifiers may occur. It was evidenced that large amounts of N2O can be emitted from the secondary clarifiers, and that the emissions have long-term variation. It was assumed that part of the N2O
Anna Mikola (corresponding author) Maarit Leppänen Pirjo Rantanen Riku Vahala School of Engineering, Aalto University, P.O. Box 14100, 00076 Aalto, Finland E-mail: anna.mikola@aalto.fi Mari Heinonen Heta Kosonen Helsinki Region Environmental Authority HSY, P.O. Box 100, FI-00066 Helsinki, Finland
released in secondary clarification was originally formed in the activated sludge basin. The emissions from secondary clarification thus seem to be dependent on conditions of the nitrification and denitrification accomplished in the denitrification–nitrification process and on the amount of sludge stored in the secondary clarifiers. Key words
| carbon footprint, N2O emissions, secondary clarifier
INTRODUCTION During the last decade, a growing concern about the greenhouse gas emissions from sewer systems and wastewater treatment processes has been expressed. Recent observations suggest that the emissions of nitrous oxide, N2O, could constitute a major part of the carbon footprint of wastewater treatment plants (WWTPs), being responsible for 35–65% of the global warming index of the total of wastewater treatment ( Johnson & Hiatt ). The global warming impact of N2O is considered to be approximately 298 times as strong as that of carbon dioxide (CO2) (US EPA ). N2O emissions are created during transformations of different nitrogen compounds. In WWTPs, during biological nitrogen removal, N2O emissions can be created both in nitrification and in denitrification with different mechanisms (Wunderlin et al. ). These emissions have been measured from aeration basins, both anoxic and aerobic zones, and to a lesser extent from other process steps, e.g., primary treatment, secondary clarifiers and doi: 10.2166/wst.2014.281
sludge storage tanks. Foley et al. () estimated that N2O emissions to the atmosphere occur predominantly in the aerated zones of the biological process, due to larger mass transfer coefficients. The mass transfer in quiescent process steps was calculated based on the power input for mixing. Recent studies have shown that the N2O emissions from full-scale WWTPs have a strong diurnal variability (Aboobakar et al. ; Daelman et al. ). The temporal changes in N2O production can be associated with the diurnal pattern of wastewater loading to the treatment plant. Daelman et al. (), Foley et al. (), Ahn et al. (), and Aboobakar et al. () suggested that N2O emissions are mainly produced during higher loading, when the nitrifying biomass of the activated sludge process is stressed due to the competition for available oxygen in the wastewater. Moreover, suboptimal process conditions related to dissolved oxygen concentrations, temperature, carbon limitation, pH or rapidly changing process conditions have been observed
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to lead to higher N2O emissions (Hwang et al. ; Law et al. ; Aboobakar et al. ). Daelman et al. () observed higher emission rates from February to June, although low wastewater temperature occurred from December to February and the emission rates seemed to be affected by the temperature with a time lag of 2 to 3 months. According to the authors, this observation may be related to the change of the sludge retention time. Generally, it has been assumed that the N2O emissions from secondary clarifiers are insignificant. Nevertheless, due to high solubility of N2O to water (Giraldo ), some of the N2O generated in aeration basins could be emitted in the secondary clarifiers. Foley et al. () measured relatively high dissolved N2O concentrations in some secondary clarifiers. Moreover, N2O could also be generated in secondary clarifiers during denitrification. Several researchers have reported denitrification occurring in the secondary clarifier basins. Siegrist et al. () observed in a full-scale WWTP that 30% of the total denitrification occurred in the secondary clarifiers. They proposed a model for denitrification in the secondary clarifiers based on the ratio of influent to return sludge flow and scraper interval. Also, Mikola et al. () reported that based on mass-balance calculations, denitrification occurring in secondary clarifiers played an important role in the overall denitrification process. In their study, the amount of sludge stored in secondary clarifiers was substantial, especially in wintertime. Batista et al. () observed very strong denitrification in secondary clarifiers, resulting in nitrogen bubbles lifting sludge solids to the surface. Typically, denitrification in secondary clarifiers is spontaneous and occasional. The conditions related to oxygen and carbon supply are uncontrolled. Thus, it seems that secondary clarifiers could be significant sources of N2O emissions in the wastewater treatment process.
METHODOLOGY This study aims to define the amount of N2O emissions from secondary clarifiers. Online measurements over several days were carried out at four different municipal WWTPs in Finland. N2O concentrations were measured from anoxic and aerated zones of aeration basins and from different parts of secondary clarifiers. The positions of the capture hood for the measurement in each WWTP are presented in Figure 1. The size of the plants varied between 10,000 and 800,000 population equivalent. All the plants consisted of primary clarification followed by a conventional activated
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sludge process with biological nitrogen removal with a denitrification–nitrification configuration and chemical phosphorus removal with simultaneous precipitation. All four plants had quite similar loading, approximately 0.15 kg N/m3d. The plants achieved 50–83% of total nitrogen removal in the conventional denitrification–nitrification process. At Plants 1, 2 and 3 the basins were covered, but at Plant 4 the basins were outdoors. In Plants 2 and 4, methanol was added as carbon source in the anoxic zone. Dissolved oxygen and pH were controlled at the plants with online measurements. The N2O emissions were first measured at the beginning of the snow melting period (March), when the wastewater temperatures were low and nitrification was incomplete. The measurements were repeated during the warm water period (June–July). The length of each measuring experiment is shown in Table 1. The analyser produced 5-minute-interval data in all the experiments. In addition, shorter measurements within 1 day were taken in order to determine the longitudinal profiles of the plants. The measurements were carried out with a Gasmet Dx4000 mobile gas analyser, which utilizes Fourier transform infrared spectroscopy, i e., Fourier transform infrared spectroscopy (FTIR) technology. The measuring equipment consisted of a portable sampling unit, an FTIR gas analyser and a laptop computer. The sample gases were collected from the surface of wastewater tanks with a capture hood (Figure 2) and pumped into the analyser through a heated sample line. In calibration and gas analysis, nitrogen gas was used as zero gas. The capture hood was connected to the sample line by a 5-metre long Teflon hose. In secondary clarifiers as well as in the unaerated zones of the aeration basin the capture hood was floating on the water surface, but in the aerated parts of the aeration basin the hood was lifted approximately 5 cm above the water surface to avoid the risk of sudden foaming. The generation of a partial vacuum and ensuing suction effect were prevented by equipping the hood with a gaspath. The gas flow rate of the analyser was 5 L/min. The gas transfer in aerated zones was estimated based on the introduced air flow per surface area. It was estimated that the gas flow rate in the secondary clarifiers was close to the flow rate of the analyser. The wastewater quality was analysed from grab samples taken from 20 to 50 cm depth close to the capture hood and with online analysers from the secondary effluent when available. Total nitrogen, nitrate and nitrite were analysed from the grab samples in a water laboratory. Ammonium, pH and dissolved oxygen were measured directly with probes. The sludge bed thickness was measured with a manual device. The methods are listed in Table 2.
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Figure 1
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Process schemes and positions of the capture hood (▴) for N2O emission measurement at all the studied WWTPs (ALF: ferrous aluminium sulfate).
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Table 1
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The lengths of different measuring experiments in different WWTPs
Winter conditions
Summer conditions
Date, length
Position
Date, length
Position
Plant 1
25–28/2, 68 h 28/2–1/3, 30 h 2–3/3, 30 h 3–7/3, 70 h 7–9/3, 48 h 9/3, 8 h 9–11/3, 48 h
SC, beginning SC, end SC, middle SC, middle AN AE AE
1–3/6, 48 h 3–6/6, 48 h 7–9/6, 48 h
SC, beginning AN AE
Plant 2
16–18/3, 48 h 18–21/3, 68 h 21/3, 8 h 21–23/3, 48 h
AE zone 2 SC, end SC, beginning and middle AE zone 2
10–13/6, 68 h 13–15/6, 48 h 16–20/6, 80 h
AE zone 2 AN SC end
Plant 3
28–29/3, 24 h
SC, beginning, middle and end
23–28/6, 120 h
SC, middle
30/6–4/7, 96 h 30/3, 2 h Plant 4
AE
No measurements
29/6, 1 h
AE
19/7, 1 h
AE
20/7, 3 h
AE þ AN
21/7, 1 h
AE
25/7, 4 h
SC
17–18/9, 6 h
SC
SC ¼ secondary clarifier; AE ¼ aeration, aerobic zone; AN ¼ aeration, anoxic zone.
Table 2
Figure 2
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The capture hood.
RESULTS
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Methods used for wastewater analyses
Standard method
Analyser/equipment
NO2
SFS-EN ISO 13395
Fiastar 5000
NO2 þ NO3
SFS-EN ISO 13395
Fiastar 5000
Total nitrogen
SFS-EN ISO 11905-1
UV spectrophotometric screening
The average values, standard deviations and minimum and maximum concentrations of N2O emissions measured from aerated zones of the aeration basins and from the secondary clarifiers at four plants during March and June–July are presented in Figure 3. The values are calculated with the 5-minute-interval analyser data for each corresponding measurement experiment (Table 1). In cases where several positions from the basin were measured, the average has been calculated with all positions included. In March, the wastewater temperature was between 8 and 13 C and in June–July, between 13 and 20 C. Snow melting period and minimum wastewater temperatures occurred in April and May. At Plant 1 the emissions were measured in the W
The nitrogen removal performance in the studied processes is presented in Table 3. The plants were fully nitrifying except for Plant 3 during wintertime.
W
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NH4-N and total nitrogen (Ntot) concentration in the effluent and removal rates during winter and summer periods in the WWTPs of the study Winter NH4-N (mg/L)
Plant 1
2
Plant 2
0.1
Plant 3
21
Plant 4
–
Figure 3
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Summer Removal (%)
Ntot (mg/L)
Removal (%)
NH4-N (mg/L)
Removal (%)
Ntot (mg/L)
Removal (%)
95
–
–
0.5
97
–
–
100
32
60
0.04
100
19
83
71
–
–
0.12
99
–
–
–
–
–
0.5
99
–
–
The N2O concentrations in gas emissions from aeration basins and secondary clarifiers during March and June–July (average, standard deviation (•), minimum and maximum values). Plant 1, PE 800,000; Plant 2, PE 40,000; Plant 3, PE 10,000; and Plant 4, PE 300,000.
anoxic zones as well. The concentration varied between 20 and 70 ppm with the average being 42 ppm in March and between 30 and 175 ppm with an average of 60 ppm in June–July. Measurements were not performed at Plant 4 during wintertime due to outdoor basins. Surprisingly, the concentrations of emissions from the secondary clarification were in the same range as the concentrations measured from aeration basins. Nevertheless, important differences between the plants were observed. The estimated and normalized N2O emissions from all four plants are presented in Table 3. Also, in normalized emissions high differences among the plants were observed. The emissions varied from 0.02 to 2.6% of the influent nitrogen load. Emissions from secondary clarifiers represented 0.5–44% of the emissions from aerated basins. A longitudinal profile of the N2O concentrations in the gas emissions is presented in Figure 4. The first measurement was carried out during wintertime and the second during summer, both between 10 am and 2 pm. It can be seen that the N2O emissions occur mostly towards the clarifier outflow, suggesting that the N2O is produced during denitrification in the secondary clarifiers. Nevertheless, the sludge bed in
Figure 4
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Longitudinal N2O profile from the secondary clarifiers at Plant 1. The daily average flow rate in June was 26,067 m3/d, whereas in March it was 35,089 m3/d.
Plant 1 was very thin. Due to the limitations of the measurement device, the thickness could not be determined between 0 and 20 cm. The sludge bed thickness was in this
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area throughout the study. Another longitudinal profile of N2O concentration was measured at Plant 4 with sludge bed thickness measurement. Plant 4 was chosen because more sludge was stored in the clarifiers, although at the time of the measurement the sludge bed thickness was also less than 50 cm. Two 3-hour measurements were performed in consecutive days between 10 am and 2 pm (Figure 5). The concentrations of N2O were relatively low, and a seemingly good correlation with the sludge bed thickness was observed. However, during all the measurements presented here, the amount of sludge in the clarifiers was small, less than 50 cm
Figure 5
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Figure 6
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of sludge bed thickness. It should be noted that the N2O dissolved in the activated sludge basin was probably also released at the beginning of the secondary clarifier. The released concentration diminished along the length of the clarifier when the N2O pool in the water became exhausted. Figure 6 shows the N2O emissions, flow rate and nitrate concentration in the aeration basin and secondary clarifier effluent from 27 July until 1 August. The diurnal variation of the N2O concentrations in the secondary clarification was perceptible. Highest N2O concentrations in the secondary clarifiers were measured in the early morning hours.
The correlation between the sludge bed thickness and N2O emissions at Plant 4. Water enters and sludge is removed at the beginning of the rectangular basin (0).
Diurnal variation of the measured N2O concentrations from one of the Plant 1 secondary clarifiers and NO3-N concentration in the secondary clarifier influent and effluent as well as the wastewater flow rate to the surveyed clarifier. Temperature varied between 17 and 21 C. W
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The nitrate concentration in the secondary effluent showed diurnal fluctuation as well, although it was not as prominent as in N2O concentrations. Sampling from deeper parts of the clarifier was unsuccessful. Temperature was quite stable. The flow rate showed normal diurnal variation, but during the second day at noon, the flow rate was more than doubled due to heavy rain. The flow rate stayed at a high level for approximately 18 h and after that returned to normal level. It seems that the peaks of N2O emissions increased due to the high flow rate, but, surprisingly, they stayed at a higher level also after the flow rate had decreased. The nitrate concentration in the aeration basin effluent was lower during the high flow rate and the following day, suggesting that nitrification suffered. The online ammonium analyser in the aeration basin influent was unfortunately not working during the time of the measurement, but in the aeration basin effluent ammonium concentration increased from 0 to 2 mg/L during the rain event and returned to normal after that. In Plant 1, the number of aerated zones is controlled by the effluent ammonium concentration. When effluent ammonium concentration increased, more zones were switched to aerated mode and back to anoxic when ammonium concentration decreased. This created an occasional transition in conditions.
DISCUSSION N2O emissions from both aeration basins and secondary clarifiers varied significantly between the studied plants. Also, important long-term variation was observed in the N2O concentrations measured from both aeration and secondary clarification. The N2O emissions from the secondary clarifiers of Plant 1 were more than two times higher in March than in July. The high emissions mainly occurred near the outflow of the clarifiers (Figure 4), which would indicate that N2O has not been generated in the aeration basin. Nevertheless, denitrification could not be confirmed, because nitrate concentration in the sludge blanket was not measured due to unsuccessful sampling. Moreover, the flow rate was significantly higher in March than in June, which could have affected the emissions. In all the plants some diurnal variation of emissions of N2O was observed. In Plant 1, this was studied in more detail (Figure 6). The flow conditions did not fully explain the diurnal variation in N2O concentrations. The variation can more probably be explained by the diurnal fluctuation of nitrogen load and transient conditions created by the switch zones in aeration. The sludge bed thickness usually
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has diurnal variation as well, as demonstrated by Mikola et al. (). This could also partly explain the variation in N2O concentrations. Nevertheless, in the case of Plant 1 the sludge bed was thinner than 20 cm throughout the measurement period and it seems improbable that it would have had an effect on N2O emissions observed. In Plants 2 and 3, the emissions were lower during colder months (March). In Plant 3, nitrification was not fully accomplished in winter and thus there was less potential for N2O formation during nitrification and denitrification. In Plant 2, nitrification was complete in winter, but low emissions were occurring. Also, N2O emissions from the aeration basin were relatively low and presumably N2O carried over to the secondary clarifier was low as well. Moreover, conditions for denitrification in the secondary clarifier might have been unfavourable. During summertime, on the other hand, both zones of Plant 2 that are usually anoxic were aerated and thus denitrification was only occurring in the secondary clarifiers. The plant achieved 83% total nitrogen removal. The emissions from both aeration basin and secondary clarifier were relatively high. The emissions from Plant 4 were low in both measurements, although nitrification was accomplished. The N2O concentrations were lower in the secondary clarifier than in the corresponding aeration basin at all times except during wintertime in Plant 1. Thus, in many cases it seems that N2O generated in the aeration basin was released in the secondary clarifier. Due to the high solubility of N2O, the N2O is not necessarily created where the gaseous emissions are occurring (Giraldo ). Nevertheless, denitrification, if taking place in a secondary clarifier would most probably occur in deeper parts of the clarifier (Mikola et al. ), where anoxic conditions could be created. In most cases during this study, denitrification would have been possible since nitrate was present in the sludge blanket. Only in Plant 3 could denitrification be confirmed by the observation of small bubbles on the surface of the secondary clarifier. In many cases, a decrease of the nitrate concentration in the water phase during the secondary clarification was observed. On the other hand, it was observed that denitrification did not always occur although nitrate was present, as in the case of Plant 2. Possible factors affecting denitrification and thus the N2O formation in the secondary clarifiers are the amount of sludge in the clarifiers, the sludge retention time (Batista et al. ), the process configuration and the available organic matter. Moreover, temperature indirectly affects the formation of nitrates by affecting the nitrification. Presumably, temperature also has an influence on the denitrification by affecting the rate of hydrolysis and thus the
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Table 4
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Normalised N2O emissions from the surveyed plants during winter and summer periods N2O-N kg/a from aerated zones
N2O-N kg/a from secondary clarifiers
N2O-N % of influent nitrogen load
g N2O-N/m3 of influent flow
% of emissions from secondary clarifiers
Plant 1 winter
29,185
12,897
1.80
0.82
31
Plant 1 summer
22,994
5,403
1.21
0.55
19
Plant 2 winter Plant 2 summer Plant 3 winter
386
19
0.86
0.90
5
1,082
128
2.57
2.70
11
9
2
0.04
0.02
16
Plant 3 summer
113
45
0.49
0.27
28
Plant 4 summer
142
69
0.02
0.19
33
The calculation is based on estimated gas flow rates from aeration basins and secondary clarifiers.
amount of suitable organic matter available for denitrifying bacteria. The dynamic conditions will also affect the sludge retention time and carbon to nitrogen ratio, which may change the availability of the organic matter in the clarifier. The results showed that the concentrations measured in secondary clarification were of the same level as the average concentrations measured in aeration basins with intermittent aeration. When the total emissions from aeration basins and secondary clarifiers were calculated, it was seen that a significant part of the total N2O emissions from the wastewater plants may be released in secondary clarification (Table 4). The normalized emissions from the two process units were in the range of recently reported values (Ahn et al. ; Daelman et al. ). In this study, emissions from secondary clarifiers were estimated to be higher than in the research reported by Foley et al. (). Nevertheless, it seems that mass transfer calculation based on mixing power might have underestimated the mass transfer in secondary clarifiers. Although the total volume of gas exhausted from secondary clarifiers is clearly smaller than the volume released from the aeration basins, it was unequivocal that the N2O emissions from secondary clarification played a significant role in some of the participating plants. Based on these results the origin of the N2O emitted in secondary clarifiers cannot be confirmed. Carry-over from aeration and denitrification in the secondary clarifier seem to be both contributing.
CONCLUSIONS N2O emissions were measured from the secondary clarifiers and from aeration basins in four different WWTPs. It was evidenced that large amounts of N2O can be emitted from the secondary clarifiers, and the emissions had significant
diurnal and long-term variation. Large differences between the plants were also observed. In most of the cases high emissions in aeration basins coincided with high emissions in secondary clarifiers. The normalized emissions for both process units together were in accordance with recently reported values. It was shown that the emissions in secondary clarifiers cannot straightforwardly be predicted with nitrate concentration in the secondary clarifier influent. Neither the season nor the temperature fully explained the differences in emissions. It was concluded that at least a part of the N2O released in secondary clarifiers originated from the N2O generated in the activated sludge basin. Nevertheless, indication that N2O was also generated in secondary clarifiers during denitrification was observed. Moreover, some evidence was found that the sludge bed thickness could correlate with emissions. Therefore, it can be estimated that higher emissions could occur when large amounts of sludge are stored in the secondary clarifiers. Nevertheless, more research would be needed to confirm this. As a conclusion, N2O emissions from or through secondary clarifiers may represent a significant part of the total N2O emissions of the wastewater treatment process. These emissions should be included when estimating the total emissions from the WWTPs.
REFERENCES Aboobakar, A., Cartmell, E., Stephenson, T., Jones, M., Vale, P. & Dotro, G. Nitrous oxide emissions and dissolved oxygen profiling in a full-scale nitrifying activated sludge treatment plant. Water Res. 47 (2), 524–534. 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. 44, 4505–4511.
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Batista, J. R., Becker, J. R., Unz, R. F. & Johnson, W. Phosphorus release in the secondary clarifier of a fullbiological phosphorus removal system. WEFTEC 2005 78th Annual Technical Exposition and Conference of the Water Environment Federation, New Orleans, LA, 2–6 October 2004. 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. 67 (10), 2350–2355. Foley, J., de Haas, D., Yuan, Z. & Lant, P. Nitrous oxide generation in full-scale biological nutrient removal wastewater treatment plants. Water Res. 44, 831–844. Giraldo, E. Nitrous oxide emissions from wastewater treatment plants. A balancing act. Proceedings of Nutrient Removal 2009 Conference, Washington, DC, 28 June–1 July. Hwang, S., Jang, K., Jang, H., Song, J. & Bae, W. Factors affecting nitrous oxide production: a comparison of biological nitrogen removal processes with partial and complete nitrification. Biodegradation 17, 19–29. Johnson, B. R. & Hiatt, W. C. Nitrogen removal system impacts on secondary treatment greenhouse gas production and whole plant carbon footprint. Proceedings of Nutrient Removal 2009 Conference, Washington DC, 28 June–1 July. Law, Y., Lant, P. & Yuan, Z. The effect of pH on N2O production under aerobic conditions in a partial nitritation system. Water Res. 45, 5934–5944.
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Mikola, A., Rautiainen, J. & Kiuru, H. The effect of flow equalisation and prefermentation on the sludge production and sludge characteristics in a BNR plant. Water Sci. Technol. 57 (12), 2023–2029. Mikola, A., Rautiainen, J. & Vahala, R. Secondary clarifier conditions conducting to secondary phosphorus release in a BNR plant. Water Sci. Technol. 60 (9), 2413–2418. SFS-EN ISO 11905-1 Veden laatu. Typen määritys. Osa 1: Peroksodisulfaattihapetus (Water quality. Determination of nitrogen. Part 1: Method using oxidative digestion with peroxodisulfate). Finnish Standards Association, Helsinki, Finland. SFS-EN ISO 13395 Veden laatu. Nitriitti- ja nitraattitypen desä niiden summan määritys spektrofotometrisesti CFA- ja FIA-tekniikalla (Water quality – Determination of nitrite nitrogen and nitrate nitrogen and the sum of both by flow analysis (CFA and FIA) and spectrometric detection). Finnish Standards Association, Helsinki, Finland. Siegrist, H., Krebs, P., Bühler, R., Putschert, I., Röck, C. & Rufer, R. Denitrification in secondary clarifiers. Water Sci. Technol. 31 (2), 205–214. US EPA (United States Environmental Protection Agency EPA) Methane and Nitrous oxide Emissions from Natural Sources. EPA 430-R-10–001. Office of Atmospheric Program, Washington, DC, 20460, 194 pp. 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. 46, 1027–1037.
First received 28 October 2013; accepted in revised form 9 June 2014. Available online 24 June 2014
© IWA Publishing 2014 Water Science & Technology
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N2O emissions from secondary clarifiers and their contribution to the total emissions of the WWTP Anna Mikola, Mari Heinonen, Heta Kosonen, Maarit Leppänen, Pirjo Rantanen and Riku Vahala
ABSTRACT Recent studies have indicated that the emissions of nitrous oxide, N2O, constitute a major part of the carbon footprint of wastewater treatment plants (WWTPs). Denitrification occurring in the secondary clarifier basins has been observed by many researchers, but until now N2O emissions from secondary clarifiers have not been widely reported. The objective of this study was to measure the N2O emissions from secondary clarifiers and weigh the portion they could represent of the overall emissions at WWTPs. Online measurements over several days were carried out at four different municipal WWTPs in Finland in cold weather conditions (March) and in warm weather conditions (June–July). An attempt was made to define the conditions in which N2O emissions from secondary clarifiers may occur. It was evidenced that large amounts of N2O can be emitted from the secondary clarifiers, and that the emissions have long-term variation. It was assumed that part of the N2O
Anna Mikola (corresponding author) Maarit Leppänen Pirjo Rantanen Riku Vahala School of Engineering, Aalto University, P.O. Box 14100, 00076 Aalto, Finland E-mail: anna.mikola@aalto.fi Mari Heinonen Heta Kosonen Helsinki Region Environmental Authority HSY, P.O. Box 100, FI-00066 Helsinki, Finland
released in secondary clarification was originally formed in the activated sludge basin. The emissions from secondary clarification thus seem to be dependent on conditions of the nitrification and denitrification accomplished in the denitrification–nitrification process and on the amount of sludge stored in the secondary clarifiers. Key words
| carbon footprint, N2O emissions, secondary clarifier
INTRODUCTION During the last decade, a growing concern about the greenhouse gas emissions from sewer systems and wastewater treatment processes has been expressed. Recent observations suggest that the emissions of nitrous oxide, N2O, could constitute a major part of the carbon footprint of wastewater treatment plants (WWTPs), being responsible for 35–65% of the global warming index of the total of wastewater treatment ( Johnson & Hiatt ). The global warming impact of N2O is considered to be approximately 298 times as strong as that of carbon dioxide (CO2) (US EPA ). N2O emissions are created during transformations of different nitrogen compounds. In WWTPs, during biological nitrogen removal, N2O emissions can be created both in nitrification and in denitrification with different mechanisms (Wunderlin et al. ). These emissions have been measured from aeration basins, both anoxic and aerobic zones, and to a lesser extent from other process steps, e.g., primary treatment, secondary clarifiers and doi: 10.2166/wst.2014.281
sludge storage tanks. Foley et al. () estimated that N2O emissions to the atmosphere occur predominantly in the aerated zones of the biological process, due to larger mass transfer coefficients. The mass transfer in quiescent process steps was calculated based on the power input for mixing. Recent studies have shown that the N2O emissions from full-scale WWTPs have a strong diurnal variability (Aboobakar et al. ; Daelman et al. ). The temporal changes in N2O production can be associated with the diurnal pattern of wastewater loading to the treatment plant. Daelman et al. (), Foley et al. (), Ahn et al. (), and Aboobakar et al. () suggested that N2O emissions are mainly produced during higher loading, when the nitrifying biomass of the activated sludge process is stressed due to the competition for available oxygen in the wastewater. Moreover, suboptimal process conditions related to dissolved oxygen concentrations, temperature, carbon limitation, pH or rapidly changing process conditions have been observed
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to lead to higher N2O emissions (Hwang et al. ; Law et al. ; Aboobakar et al. ). Daelman et al. () observed higher emission rates from February to June, although low wastewater temperature occurred from December to February and the emission rates seemed to be affected by the temperature with a time lag of 2 to 3 months. According to the authors, this observation may be related to the change of the sludge retention time. Generally, it has been assumed that the N2O emissions from secondary clarifiers are insignificant. Nevertheless, due to high solubility of N2O to water (Giraldo ), some of the N2O generated in aeration basins could be emitted in the secondary clarifiers. Foley et al. () measured relatively high dissolved N2O concentrations in some secondary clarifiers. Moreover, N2O could also be generated in secondary clarifiers during denitrification. Several researchers have reported denitrification occurring in the secondary clarifier basins. Siegrist et al. () observed in a full-scale WWTP that 30% of the total denitrification occurred in the secondary clarifiers. They proposed a model for denitrification in the secondary clarifiers based on the ratio of influent to return sludge flow and scraper interval. Also, Mikola et al. () reported that based on mass-balance calculations, denitrification occurring in secondary clarifiers played an important role in the overall denitrification process. In their study, the amount of sludge stored in secondary clarifiers was substantial, especially in wintertime. Batista et al. () observed very strong denitrification in secondary clarifiers, resulting in nitrogen bubbles lifting sludge solids to the surface. Typically, denitrification in secondary clarifiers is spontaneous and occasional. The conditions related to oxygen and carbon supply are uncontrolled. Thus, it seems that secondary clarifiers could be significant sources of N2O emissions in the wastewater treatment process.
METHODOLOGY This study aims to define the amount of N2O emissions from secondary clarifiers. Online measurements over several days were carried out at four different municipal WWTPs in Finland. N2O concentrations were measured from anoxic and aerated zones of aeration basins and from different parts of secondary clarifiers. The positions of the capture hood for the measurement in each WWTP are presented in Figure 1. The size of the plants varied between 10,000 and 800,000 population equivalent. All the plants consisted of primary clarification followed by a conventional activated
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sludge process with biological nitrogen removal with a denitrification–nitrification configuration and chemical phosphorus removal with simultaneous precipitation. All four plants had quite similar loading, approximately 0.15 kg N/m3d. The plants achieved 50–83% of total nitrogen removal in the conventional denitrification–nitrification process. At Plants 1, 2 and 3 the basins were covered, but at Plant 4 the basins were outdoors. In Plants 2 and 4, methanol was added as carbon source in the anoxic zone. Dissolved oxygen and pH were controlled at the plants with online measurements. The N2O emissions were first measured at the beginning of the snow melting period (March), when the wastewater temperatures were low and nitrification was incomplete. The measurements were repeated during the warm water period (June–July). The length of each measuring experiment is shown in Table 1. The analyser produced 5-minute-interval data in all the experiments. In addition, shorter measurements within 1 day were taken in order to determine the longitudinal profiles of the plants. The measurements were carried out with a Gasmet Dx4000 mobile gas analyser, which utilizes Fourier transform infrared spectroscopy, i e., Fourier transform infrared spectroscopy (FTIR) technology. The measuring equipment consisted of a portable sampling unit, an FTIR gas analyser and a laptop computer. The sample gases were collected from the surface of wastewater tanks with a capture hood (Figure 2) and pumped into the analyser through a heated sample line. In calibration and gas analysis, nitrogen gas was used as zero gas. The capture hood was connected to the sample line by a 5-metre long Teflon hose. In secondary clarifiers as well as in the unaerated zones of the aeration basin the capture hood was floating on the water surface, but in the aerated parts of the aeration basin the hood was lifted approximately 5 cm above the water surface to avoid the risk of sudden foaming. The generation of a partial vacuum and ensuing suction effect were prevented by equipping the hood with a gaspath. The gas flow rate of the analyser was 5 L/min. The gas transfer in aerated zones was estimated based on the introduced air flow per surface area. It was estimated that the gas flow rate in the secondary clarifiers was close to the flow rate of the analyser. The wastewater quality was analysed from grab samples taken from 20 to 50 cm depth close to the capture hood and with online analysers from the secondary effluent when available. Total nitrogen, nitrate and nitrite were analysed from the grab samples in a water laboratory. Ammonium, pH and dissolved oxygen were measured directly with probes. The sludge bed thickness was measured with a manual device. The methods are listed in Table 2.
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Figure 1
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Process schemes and positions of the capture hood (▴) for N2O emission measurement at all the studied WWTPs (ALF: ferrous aluminium sulfate).
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Table 1
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The lengths of different measuring experiments in different WWTPs
Winter conditions
Summer conditions
Date, length
Position
Date, length
Position
Plant 1
25–28/2, 68 h 28/2–1/3, 30 h 2–3/3, 30 h 3–7/3, 70 h 7–9/3, 48 h 9/3, 8 h 9–11/3, 48 h
SC, beginning SC, end SC, middle SC, middle AN AE AE
1–3/6, 48 h 3–6/6, 48 h 7–9/6, 48 h
SC, beginning AN AE
Plant 2
16–18/3, 48 h 18–21/3, 68 h 21/3, 8 h 21–23/3, 48 h
AE zone 2 SC, end SC, beginning and middle AE zone 2
10–13/6, 68 h 13–15/6, 48 h 16–20/6, 80 h
AE zone 2 AN SC end
Plant 3
28–29/3, 24 h
SC, beginning, middle and end
23–28/6, 120 h
SC, middle
30/6–4/7, 96 h 30/3, 2 h Plant 4
AE
No measurements
29/6, 1 h
AE
19/7, 1 h
AE
20/7, 3 h
AE þ AN
21/7, 1 h
AE
25/7, 4 h
SC
17–18/9, 6 h
SC
SC ¼ secondary clarifier; AE ¼ aeration, aerobic zone; AN ¼ aeration, anoxic zone.
Table 2
Figure 2
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The capture hood.
RESULTS
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Methods used for wastewater analyses
Standard method
Analyser/equipment
NO2
SFS-EN ISO 13395
Fiastar 5000
NO2 þ NO3
SFS-EN ISO 13395
Fiastar 5000
Total nitrogen
SFS-EN ISO 11905-1
UV spectrophotometric screening
The average values, standard deviations and minimum and maximum concentrations of N2O emissions measured from aerated zones of the aeration basins and from the secondary clarifiers at four plants during March and June–July are presented in Figure 3. The values are calculated with the 5-minute-interval analyser data for each corresponding measurement experiment (Table 1). In cases where several positions from the basin were measured, the average has been calculated with all positions included. In March, the wastewater temperature was between 8 and 13 C and in June–July, between 13 and 20 C. Snow melting period and minimum wastewater temperatures occurred in April and May. At Plant 1 the emissions were measured in the W
The nitrogen removal performance in the studied processes is presented in Table 3. The plants were fully nitrifying except for Plant 3 during wintertime.
W
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NH4-N and total nitrogen (Ntot) concentration in the effluent and removal rates during winter and summer periods in the WWTPs of the study Winter NH4-N (mg/L)
Plant 1
2
Plant 2
0.1
Plant 3
21
Plant 4
–
Figure 3
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Summer Removal (%)
Ntot (mg/L)
Removal (%)
NH4-N (mg/L)
Removal (%)
Ntot (mg/L)
Removal (%)
95
–
–
0.5
97
–
–
100
32
60
0.04
100
19
83
71
–
–
0.12
99
–
–
–
–
–
0.5
99
–
–
The N2O concentrations in gas emissions from aeration basins and secondary clarifiers during March and June–July (average, standard deviation (•), minimum and maximum values). Plant 1, PE 800,000; Plant 2, PE 40,000; Plant 3, PE 10,000; and Plant 4, PE 300,000.
anoxic zones as well. The concentration varied between 20 and 70 ppm with the average being 42 ppm in March and between 30 and 175 ppm with an average of 60 ppm in June–July. Measurements were not performed at Plant 4 during wintertime due to outdoor basins. Surprisingly, the concentrations of emissions from the secondary clarification were in the same range as the concentrations measured from aeration basins. Nevertheless, important differences between the plants were observed. The estimated and normalized N2O emissions from all four plants are presented in Table 3. Also, in normalized emissions high differences among the plants were observed. The emissions varied from 0.02 to 2.6% of the influent nitrogen load. Emissions from secondary clarifiers represented 0.5–44% of the emissions from aerated basins. A longitudinal profile of the N2O concentrations in the gas emissions is presented in Figure 4. The first measurement was carried out during wintertime and the second during summer, both between 10 am and 2 pm. It can be seen that the N2O emissions occur mostly towards the clarifier outflow, suggesting that the N2O is produced during denitrification in the secondary clarifiers. Nevertheless, the sludge bed in
Figure 4
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Longitudinal N2O profile from the secondary clarifiers at Plant 1. The daily average flow rate in June was 26,067 m3/d, whereas in March it was 35,089 m3/d.
Plant 1 was very thin. Due to the limitations of the measurement device, the thickness could not be determined between 0 and 20 cm. The sludge bed thickness was in this
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area throughout the study. Another longitudinal profile of N2O concentration was measured at Plant 4 with sludge bed thickness measurement. Plant 4 was chosen because more sludge was stored in the clarifiers, although at the time of the measurement the sludge bed thickness was also less than 50 cm. Two 3-hour measurements were performed in consecutive days between 10 am and 2 pm (Figure 5). The concentrations of N2O were relatively low, and a seemingly good correlation with the sludge bed thickness was observed. However, during all the measurements presented here, the amount of sludge in the clarifiers was small, less than 50 cm
Figure 5
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Figure 6
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of sludge bed thickness. It should be noted that the N2O dissolved in the activated sludge basin was probably also released at the beginning of the secondary clarifier. The released concentration diminished along the length of the clarifier when the N2O pool in the water became exhausted. Figure 6 shows the N2O emissions, flow rate and nitrate concentration in the aeration basin and secondary clarifier effluent from 27 July until 1 August. The diurnal variation of the N2O concentrations in the secondary clarification was perceptible. Highest N2O concentrations in the secondary clarifiers were measured in the early morning hours.
The correlation between the sludge bed thickness and N2O emissions at Plant 4. Water enters and sludge is removed at the beginning of the rectangular basin (0).
Diurnal variation of the measured N2O concentrations from one of the Plant 1 secondary clarifiers and NO3-N concentration in the secondary clarifier influent and effluent as well as the wastewater flow rate to the surveyed clarifier. Temperature varied between 17 and 21 C. W
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The nitrate concentration in the secondary effluent showed diurnal fluctuation as well, although it was not as prominent as in N2O concentrations. Sampling from deeper parts of the clarifier was unsuccessful. Temperature was quite stable. The flow rate showed normal diurnal variation, but during the second day at noon, the flow rate was more than doubled due to heavy rain. The flow rate stayed at a high level for approximately 18 h and after that returned to normal level. It seems that the peaks of N2O emissions increased due to the high flow rate, but, surprisingly, they stayed at a higher level also after the flow rate had decreased. The nitrate concentration in the aeration basin effluent was lower during the high flow rate and the following day, suggesting that nitrification suffered. The online ammonium analyser in the aeration basin influent was unfortunately not working during the time of the measurement, but in the aeration basin effluent ammonium concentration increased from 0 to 2 mg/L during the rain event and returned to normal after that. In Plant 1, the number of aerated zones is controlled by the effluent ammonium concentration. When effluent ammonium concentration increased, more zones were switched to aerated mode and back to anoxic when ammonium concentration decreased. This created an occasional transition in conditions.
DISCUSSION N2O emissions from both aeration basins and secondary clarifiers varied significantly between the studied plants. Also, important long-term variation was observed in the N2O concentrations measured from both aeration and secondary clarification. The N2O emissions from the secondary clarifiers of Plant 1 were more than two times higher in March than in July. The high emissions mainly occurred near the outflow of the clarifiers (Figure 4), which would indicate that N2O has not been generated in the aeration basin. Nevertheless, denitrification could not be confirmed, because nitrate concentration in the sludge blanket was not measured due to unsuccessful sampling. Moreover, the flow rate was significantly higher in March than in June, which could have affected the emissions. In all the plants some diurnal variation of emissions of N2O was observed. In Plant 1, this was studied in more detail (Figure 6). The flow conditions did not fully explain the diurnal variation in N2O concentrations. The variation can more probably be explained by the diurnal fluctuation of nitrogen load and transient conditions created by the switch zones in aeration. The sludge bed thickness usually
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has diurnal variation as well, as demonstrated by Mikola et al. (). This could also partly explain the variation in N2O concentrations. Nevertheless, in the case of Plant 1 the sludge bed was thinner than 20 cm throughout the measurement period and it seems improbable that it would have had an effect on N2O emissions observed. In Plants 2 and 3, the emissions were lower during colder months (March). In Plant 3, nitrification was not fully accomplished in winter and thus there was less potential for N2O formation during nitrification and denitrification. In Plant 2, nitrification was complete in winter, but low emissions were occurring. Also, N2O emissions from the aeration basin were relatively low and presumably N2O carried over to the secondary clarifier was low as well. Moreover, conditions for denitrification in the secondary clarifier might have been unfavourable. During summertime, on the other hand, both zones of Plant 2 that are usually anoxic were aerated and thus denitrification was only occurring in the secondary clarifiers. The plant achieved 83% total nitrogen removal. The emissions from both aeration basin and secondary clarifier were relatively high. The emissions from Plant 4 were low in both measurements, although nitrification was accomplished. The N2O concentrations were lower in the secondary clarifier than in the corresponding aeration basin at all times except during wintertime in Plant 1. Thus, in many cases it seems that N2O generated in the aeration basin was released in the secondary clarifier. Due to the high solubility of N2O, the N2O is not necessarily created where the gaseous emissions are occurring (Giraldo ). Nevertheless, denitrification, if taking place in a secondary clarifier would most probably occur in deeper parts of the clarifier (Mikola et al. ), where anoxic conditions could be created. In most cases during this study, denitrification would have been possible since nitrate was present in the sludge blanket. Only in Plant 3 could denitrification be confirmed by the observation of small bubbles on the surface of the secondary clarifier. In many cases, a decrease of the nitrate concentration in the water phase during the secondary clarification was observed. On the other hand, it was observed that denitrification did not always occur although nitrate was present, as in the case of Plant 2. Possible factors affecting denitrification and thus the N2O formation in the secondary clarifiers are the amount of sludge in the clarifiers, the sludge retention time (Batista et al. ), the process configuration and the available organic matter. Moreover, temperature indirectly affects the formation of nitrates by affecting the nitrification. Presumably, temperature also has an influence on the denitrification by affecting the rate of hydrolysis and thus the
727
Table 4
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Normalised N2O emissions from the surveyed plants during winter and summer periods N2O-N kg/a from aerated zones
N2O-N kg/a from secondary clarifiers
N2O-N % of influent nitrogen load
g N2O-N/m3 of influent flow
% of emissions from secondary clarifiers
Plant 1 winter
29,185
12,897
1.80
0.82
31
Plant 1 summer
22,994
5,403
1.21
0.55
19
Plant 2 winter Plant 2 summer Plant 3 winter
386
19
0.86
0.90
5
1,082
128
2.57
2.70
11
9
2
0.04
0.02
16
Plant 3 summer
113
45
0.49
0.27
28
Plant 4 summer
142
69
0.02
0.19
33
The calculation is based on estimated gas flow rates from aeration basins and secondary clarifiers.
amount of suitable organic matter available for denitrifying bacteria. The dynamic conditions will also affect the sludge retention time and carbon to nitrogen ratio, which may change the availability of the organic matter in the clarifier. The results showed that the concentrations measured in secondary clarification were of the same level as the average concentrations measured in aeration basins with intermittent aeration. When the total emissions from aeration basins and secondary clarifiers were calculated, it was seen that a significant part of the total N2O emissions from the wastewater plants may be released in secondary clarification (Table 4). The normalized emissions from the two process units were in the range of recently reported values (Ahn et al. ; Daelman et al. ). In this study, emissions from secondary clarifiers were estimated to be higher than in the research reported by Foley et al. (). Nevertheless, it seems that mass transfer calculation based on mixing power might have underestimated the mass transfer in secondary clarifiers. Although the total volume of gas exhausted from secondary clarifiers is clearly smaller than the volume released from the aeration basins, it was unequivocal that the N2O emissions from secondary clarification played a significant role in some of the participating plants. Based on these results the origin of the N2O emitted in secondary clarifiers cannot be confirmed. Carry-over from aeration and denitrification in the secondary clarifier seem to be both contributing.
CONCLUSIONS N2O emissions were measured from the secondary clarifiers and from aeration basins in four different WWTPs. It was evidenced that large amounts of N2O can be emitted from the secondary clarifiers, and the emissions had significant
diurnal and long-term variation. Large differences between the plants were also observed. In most of the cases high emissions in aeration basins coincided with high emissions in secondary clarifiers. The normalized emissions for both process units together were in accordance with recently reported values. It was shown that the emissions in secondary clarifiers cannot straightforwardly be predicted with nitrate concentration in the secondary clarifier influent. Neither the season nor the temperature fully explained the differences in emissions. It was concluded that at least a part of the N2O released in secondary clarifiers originated from the N2O generated in the activated sludge basin. Nevertheless, indication that N2O was also generated in secondary clarifiers during denitrification was observed. Moreover, some evidence was found that the sludge bed thickness could correlate with emissions. Therefore, it can be estimated that higher emissions could occur when large amounts of sludge are stored in the secondary clarifiers. Nevertheless, more research would be needed to confirm this. As a conclusion, N2O emissions from or through secondary clarifiers may represent a significant part of the total N2O emissions of the wastewater treatment process. These emissions should be included when estimating the total emissions from the WWTPs.
REFERENCES Aboobakar, A., Cartmell, E., Stephenson, T., Jones, M., Vale, P. & Dotro, G. Nitrous oxide emissions and dissolved oxygen profiling in a full-scale nitrifying activated sludge treatment plant. Water Res. 47 (2), 524–534. 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. 44, 4505–4511.
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Batista, J. R., Becker, J. R., Unz, R. F. & Johnson, W. Phosphorus release in the secondary clarifier of a fullbiological phosphorus removal system. WEFTEC 2005 78th Annual Technical Exposition and Conference of the Water Environment Federation, New Orleans, LA, 2–6 October 2004. 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. 67 (10), 2350–2355. Foley, J., de Haas, D., Yuan, Z. & Lant, P. Nitrous oxide generation in full-scale biological nutrient removal wastewater treatment plants. Water Res. 44, 831–844. Giraldo, E. Nitrous oxide emissions from wastewater treatment plants. A balancing act. Proceedings of Nutrient Removal 2009 Conference, Washington, DC, 28 June–1 July. Hwang, S., Jang, K., Jang, H., Song, J. & Bae, W. Factors affecting nitrous oxide production: a comparison of biological nitrogen removal processes with partial and complete nitrification. Biodegradation 17, 19–29. Johnson, B. R. & Hiatt, W. C. Nitrogen removal system impacts on secondary treatment greenhouse gas production and whole plant carbon footprint. Proceedings of Nutrient Removal 2009 Conference, Washington DC, 28 June–1 July. Law, Y., Lant, P. & Yuan, Z. The effect of pH on N2O production under aerobic conditions in a partial nitritation system. Water Res. 45, 5934–5944.
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Mikola, A., Rautiainen, J. & Kiuru, H. The effect of flow equalisation and prefermentation on the sludge production and sludge characteristics in a BNR plant. Water Sci. Technol. 57 (12), 2023–2029. Mikola, A., Rautiainen, J. & Vahala, R. Secondary clarifier conditions conducting to secondary phosphorus release in a BNR plant. Water Sci. Technol. 60 (9), 2413–2418. SFS-EN ISO 11905-1 Veden laatu. Typen määritys. Osa 1: Peroksodisulfaattihapetus (Water quality. Determination of nitrogen. Part 1: Method using oxidative digestion with peroxodisulfate). Finnish Standards Association, Helsinki, Finland. SFS-EN ISO 13395 Veden laatu. Nitriitti- ja nitraattitypen desä niiden summan määritys spektrofotometrisesti CFA- ja FIA-tekniikalla (Water quality – Determination of nitrite nitrogen and nitrate nitrogen and the sum of both by flow analysis (CFA and FIA) and spectrometric detection). Finnish Standards Association, Helsinki, Finland. Siegrist, H., Krebs, P., Bühler, R., Putschert, I., Röck, C. & Rufer, R. Denitrification in secondary clarifiers. Water Sci. Technol. 31 (2), 205–214. US EPA (United States Environmental Protection Agency EPA) Methane and Nitrous oxide Emissions from Natural Sources. EPA 430-R-10–001. Office of Atmospheric Program, Washington, DC, 20460, 194 pp. 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. 46, 1027–1037.
First received 28 October 2013; accepted in revised form 9 June 2014. Available online 24 June 2014
