This study measured greenhouse gas (GHG) emissions and concentrations at a wastewater treatment plant (WWTP) that was upgraded to use integrated fixed film activated sludge biological nitrogen removal (IFAS BNR). The study found: 1) Nitrous oxide (N2O) gas emissions were highest from the re-aeration zone but dissolved N2O concentrations were high in the aerated and post-anoxic zones. 2) Methane (CH4) gas emissions were greatest from the aerated and re-aeration zones, but dissolved CH4 concentrations were highest in inflow water, suggesting CH4 is produced upstream and stripped out in aerated zones. 3)

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Objective: To determine spatial and temporal variations in
Greenhouse Gas (GHG) emissions from the new Biological
Nitrogen Removal (BNR) process at the Field’s Point WWTP.
GHGs:
• Nitrous oxide (N2O)
• Methane (CH4)
• Carbon dioxide (CO2)
GHG Emissions from BNR at Field’s Point (FP) Wastewater
Treatment Plant (WWTP)
• FP WWTP (Figure 3) upgraded 10 tanks (Figure 5) for Integrated Fixed
Film Activated Sludge (IFAS) BNR
• Possible by-products of IFAS BNR:
• N2O from nitrification and denitrification (Figure 4,5)
• CH4 produced upstream or in anoxic zones of BNR can be
stripped in aerated zones of BNR (1,4)
The Grand Experiment:
Do smaller nitrogen loads into Narragansett Bay mean larger greenhouse gas emissions at Fields Point?
Elizabeth Brannon1, Serena Moseman-Valtierra Ph.D.1, Jim McCaughey P.E.2
1 University of Rhode Island, Department of Biological Sciences 2Narragansett Bay Commission
Acknowledgments: This research is part of a Master’s Thesis. I would like to thank my adviser, Dr. Serena Moseman-Valtierra, and my committee members: Dr. Jose Amador, Dr. Vinka Craver, and Dr. Bethany Jenkins. I would also like to thank my fellow Moseman-Valtierra Lab members who have been
extremely helpful in the field: Melanie Garate, Rose Martin, and Ryan Quinn. The following members of the Narragansett Bay Commission have also been helpful in data collection: Dave Aucoin, Brendan Cunha, Jim McCaughey, and Barry Wenskowicz. I would also like to thank Maria Briones and Jessica
Damicis, members of the Craver Lab who are collaborating on the project. Funding for this project is provided by the Narragansett Bay Commission and a USDA Hatch Grant (H-4002) to S. Moseman-Valtierra (URI Start up).
Literature Cited:
1Aboobakar, A et al. (2014).Water Air Soil Pollution, 225, 1814.
2Ahn, J. et al. (2010). Environ. Sci. Technol., 44, 4505-4511.
3Chalk, P.; Smith, C. (1983). Dev. Plant Soil Sci., 9, 65-89.
4Czepiel, P. et al. (1993). Environ. Sci. Technol., 27, 2472-2477.
5Forster, P. et al. (2007). Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
6Moseman-Valtierra et al. (2011). Atmospheric Environment, 45, 4390-4397.
7Narragansett Bay Commission (NBC). Facilities. Retrieved April 2, 2014 from: http://www.narrabay.com/About%20Us/Facilities.aspx.
8National Park Service (NPS). (2014). What is Climate Change? Available at: http://www.nps.gov/goga/naturescience/climate-change-causes.htm
9Thomas, et al. (2009) United States Global Change Research Program. Cambridge University Press, New York, NY, USA.
10Thomson, A. et al. (2012). The Royal Society Biological Sciences, 367, 1157-1168.
Figure 7. GHG analyzer that was
connected to the floating chamber via
tubing and a pump.
Table 1. Global Warming Potential (GWP)
of three major GHGs (5). The GWP is
based on how much a given mass of
each gas contributes to global warming
over a 100 year time frame compared to
the same mass of CO2.
GHG GWP
CO2 1
CH4 21
N2O 300
Figure 1. GHG concentrations have been
increasing since the Industrial Revolution (9).
Figure 2. GHGs are a necessary component of the
Greenhouse Effect which keeps the Earth at a livable
temperature. Increased GHG concentrations can lead
to excess warming and climate change (8).
Figure 3. Location of the FP WWTP
in Providence, RI (7).
Figure 6. Floating chamber placed on
water surface to measure GHG
concentrations.
Figure 8. GC used to
measure the dissolved GHG
concentrations.
Results and Discussion-N2O
Figure 10. Average N2O gaseous emission (mmol N2O m-2 hr-1) and dissolved N2O concentration (nmol N2O in water
sample) for each zone. Zones connected by the same letter are not significant. Standard deviation bars are shown.
Figure 9. Average gaseous N2O emission (mmol N2O m-2 hr-1) and average dissolved N2O concentration (nmol N2O in
water sample) for one tank on each measurement day. Dates connected by the same letter are not significant. Standard
deviation bars are shown.
Between 0.05-0.50% of the influent TKN was emitted as N2O gas. This falls
within the range (0.01-1.8% of influent TKN emitted as N2O) reported by a study
for 12 BNR processes throughout the U.S (2). There was significant temporal
variation of both N2O gas emissions and dissolved N2O (Figure 9).
Results and Discussion-CH4Introduction Methods
N2O gas emissions were greatest from the re-aeration zone but dissolved N2O
concentrations were high in the aerated and post-anoxic zones (Figure 10).
Between 0.004-0.026% of the influent COD was emitted as CH4 gas (Figure
11). This falls below values (0.07%) reported by previous studies (1). There
was significant temporal variation of both CH4 gas emissions and dissolved
CH4.
Figure 4. Simplified diagram of the nitrogen cycle showing possible
mechanisms of N2O production (3,10).
N2O
N2O
Denitrification
N2OAnaerobic
Nitrogen Fixation
N2
NH4
+
Nitrification
NO2
-
NO3
-
Aerobic
Figure 11. Average gaseous CH4 emission (mmol CH4 m-2 hr-1) and average dissolved CH4 concentration (µM CH4) for
one tank on each measurement day. Dates connected by the same letter are not significant. Standard deviation bars are
shown.
CH4 gas emissions were greatest from the Aerated IFAS and Re-Aeration
zones but dissolved CH4 concentrations were highest in the inflow water
(Figure 12). This suggests that CH4 is being produced upstream and stripped
out in the aerated zones.
Figure 12. Average CH4 gaseous emission (mmol CH4 m-2 hr-1) and dissolved CH4 concentration (µM CH4) for each
zone. Dates connected by the same letter are not significant. Standard deviation bars are shown.
Denitrification
Pre-Anoxic Zone Aerated IFAS Zone Post-Anoxic Zone Re-Aeration Zone
NO3
-
NH4
+
N2
NH4
+ NO3
-
NH4
+ N2NO3
-
N2O
N2ON2O
Figure 5. Aerial view of one of the upgraded IFAS tanks. Water flow is from left to right.
Idealized path of nitrogen removal and possible N2O and CH4 formation is shown.
NitrificationWater Flow
High dissolved CH4
in influent water
Dissolved CH4
stripped by
aeration
Dissolved
CH4 stripped
by aeration
•Measured GHG fluxes and dissolved GHG concentrations in water
twice each month from January – April 2014
•Used cavity ring down spectrometry GHG analyzer to measure GHG
concentrations in floating chambers (Figure 6,7)
•Calculated emissions based on Ideal Gas Law and Fick’s Law
•Two-factor ANOVAs used to test differences over time and zones, with
Tukey’s HSD for post hoc comparisons on means
Conclusions
• BNR will reduce anthropogenic nitrogen inputs into the bay by roughly 50% of historic levels
• N2O gas emissions (< 0.50% of influent TKN) fall within range of reported values from previous
studies (0.01-1.8% of influent TKN) (2)
• CH4 gas emissions (< 0.026% of influent COD) are lower than reported values from previous
studies (0.07% of influent COD) (1)
• Aerated and Re-Aeration zones are responsible for 95% of N2O gas emissions and 80% of CH4
gas emissions
• N2O gas emissions (200,000 µmol N2O m-2 d-1) are much larger than N2O gas emissions from salt
marshes that experience nitrogen loading (160 µmol N2O m-2 d-1) (6)
• After N removal, some dissolved N2O (~300% of influent dissolved N2O) and CH4 (~4% influent
dissolved CH4) is discharged to Narragansett Bay
A
C C
B
D
B
A
B
AB
B
B
AB
A
BC
C
A
B
C C CC
Zone Date Zone x Date
CH4 Gaseous Emissions F3,23=9.1586, P=0.0002 F5,23=4.5785, P=0.0034 F15,23=4.1829, P=0.0005
Dissolved CH4 F5,29=1724.003, P<0.0001 F4,29=16.1229 P<0.0001 F20,29=16.1229, P<0.0001
Zone Date Zone x Date
N2O Gaseous Emissions F3,27=779.0739, P<0.0001 F6,27=8.5817, P<0.0001 F18,27=8.1720, P<0.0001
Dissolved N2O F5,29=25.0346, P<0.0001 F4,29=42.6404, P<0.0001 F20,29=7.5605, P<0.0001
Table 2. Effects of zone and date on N2O gaseous emissions and dissolved N2O. F statistics, degrees of freedom and
significance values are reported for the two-factor ANOVA test.
Table 3. Effects of zone and date on CH4 gaseous emissions and dissolved CH4. F statistics, degrees of freedom and
significance values are reported for the two-factor ANOVA test.
Future Direction
• Examine daily variation of gaseous and dissolved emissions
• Examine GHG emissions and nutrient concentration correlation
• Lab isotope experiment to study mechanisms of N2O production
• Collaboration with Craver lab to examine possible correlation of DO and N2O emissions
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