Six years of experience with N 2 O emission from wastewater treatment

Transcription

1 Six years of experience with N 2 O emission from wastewater treatment Eveline I.P. Volcke 1, Matthijs R.J. Daelman 1,2, Kris M. Mampaey 1, Celia M. Castro-Barros 1, Mark C.M. van Loosdrecht 2 1 Department of Biosystems Engineering, Ghent University, Belgium 2 Department of Biotechnology, Delft University of Technology ecostp16, June 2016, Cambridge, UK

2 Outline N 2 O emission from wastewater treatment Introduction - conventional versus innovative N removal N 2 O emission - N 2 O formation routes and influencing factors - N 2 O emission factors How to measure? - N 2 O sampling strategies - N 2 O monitoring method Results from full-scale measurement campaigns - municipal WWT - one-stage partial nitritation-anammox - stand-alone partial nitritation reactor Conclusions 2

3 Biological nitrogen removal from wastewater More sustainable way: partial nitritation Het SHARON-Ana : pmmox-proces Up to 63% savings in aeration energy (O 2 supply) Up to 100% savings in carbon source addition Almost no CO 2 emission Less sludge production compared to conventional N removal from wastewater: nitrification-denitrification + NH 4 but how about other greenhouse gases e.g.n 2 O? N 2 nitrogen gas ammonium anammox - anammox O 2 nitrification by ammonium oxidizers = nitritation organic C source denitrification denitrification organic C source NO 2 nitrite NO 3 nitrate O 2 nitrification by nitrite oxidizers partial nitritation 3

4 Wastewater treatment 3.2% of anthropogenic N 2 O N 2 O Global Warming Potential of N 2 O = 298 CO 2 -equivalents

5 Routes for N 2 O formation by AOB: forward N 2 O production pathway = NH 2 OH high high metabolic activity (q close to q max ) by AOB: autotrophic denitrification = nitrifier low high nitrite heterotrophic low low high nitrite

6 N 2 O emission factors FACT 1. Large variation in reported N 2 O emission factors over time as well as between different WWTPs different ways of expression - % of nitrogen load, % of ammonium converted, % of N denitrified 0 10% of nitrogen load (Kampschreur et al., 2009; Ahn et al., 2010; Foley et al., 2010) Given the enormous variability of the emission, sampling strategy will affect the estimate FACT 2. Large variation in applied N 2 O sampling stragies duration: from 1 day to 1.5 years sampling frequency: from a single grab sample to online sampling 6

7 Research questions RQ 1. How to adequately quantify emission factor? RQ 2. Which are the main N 2 O formation mechanisms? RQ 3. Conventional versus innovative nitrogen removal? RQ 4. How to mitigate N 2 O emission? 7

8 Introduction - conventional versus innovative N removal N 2 O emission - N 2 O formation routes and influencing factors - N 2 O emission factors How to measure? - N 2 O sampling strategies - N 2 O monitoring method Results from full-scale measurement campaigns - municipal WWT Outline N 2 O formation and emission during biological nitrogen removal from wastewater - one-stage partial nitritation-anammox - stand-alone partial nitritation reactor Conclusions 8

9 N 2 O sampling strategies N 2 O sampling strategies applied in literature were tested on long-term (416 days) online (every 25 min) dataset Daelman et al., Water Res. (2013) Long-term sampling required to quantify emission factor - be it online or grab sampling - short-term sampling ignores long term variation - night-time and weekend samples contribute significantly to more accurate estimate Online sampling required to identify N 2 O formation mechanisms - indispensable to correlate emissions with operational variables - low frequency grab sampling ignores diurnal pattern Precision as function of # samples / length of sampling campaign => Guideline for planning sampling campaign 9

10 N 2 O monitoring method Gas stripping method for monitoring dissolved N 2 O N 2 in to gas analyzer liquid sampling from reactor measured stripping gas N 2 O stripping flask scum trap flask calculated reactor dissolved N 2 O Mampaey et al., Environ. Technol. (2015)

11 N 2 O monitoring method Gas stripping method for monitoring dissolved N 2 O Monitoring dissolved N 2 O concentration based on gas phase measurements - under aerated and non-aerated conditions - follow variations in time and in space Calculation of N 2 O formation rate Gas stripping device characteristics - maximum sampling frequency - detection limit can be adjusted according to process needs Not limited to dissolved N 2 O measurements Mampaey et al., Environ. Technol. (2015)

12 Outline N 2 O emission from wastewater treatment Introduction - conventional versus innovative N removal N 2 O emission - N 2 O formation routes and influencing factors - N 2 O emission factors How to measure? - N 2 O sampling strategies - N 2 O monitoring method Results from full-scale measurement campaigns - municipal WWT - one-stage partial nitritation-anammox - stand-alone partial nitritation reactor Conclusions 12

13 N 2 O emission from municipal WWT WWTP Kralingseveer (NL) 16 month monitoring campaign on fully covered municipal WWTP CO 2 CH 4 N 2 O N 2 O emission factor kg N 2 O-N.(kg TKN influent ) -1 this study: 2.8% IPCC (2006) 0.035% Nitrous oxide 78.4 % 56 kg CO 2 PE -1 y -1 Carbon dioxide 8.1 % 5.8 kg CO 2 PE -1 y -1 Methane 13.5 % 9.6 kg CO 2 PE -1 y -1 Both CH 4 and N 2 O more important than CO 2! Daelman et al., Water Sci. Technol.,

14 N 2 O emission from municipal WWT WWTP Kralingseveer (NL) Clear seasonal variability! Correlation with temperature? - Not significant - Lag-time of 2-3 months Daelman et al., Water Sci. Technol.,

15 WWTP Kralingseveer (NL) Diurnal variability N 2 O emission from municipal WWT coincides with nitrite concentration nitrate concentration Nitrifier denitrification? ammonia loading rate Heterotrophic denitrification? Daelman et al., Sci. Tot. Environ. (2015) 15

16 N 2 O emission from municipal WWT WWTP Kralingseveer (NL) Different time scales: seasonal + diurnal variation Occurrence of daily nitrite peaks as diagnostic for N 2 O emission - detection of N 2 O indicates occurrence of nitrite concentration peaks Data suggests that N 2 O is mainly produced as a result of sub-optimal oxygen concentrations denitrification by nitrifying bacteria hampered heterotrophic denitrification process. Mitigation: separate zones nitrification - denitrification Dataset available for use by other researchers! 16

17 Outline N 2 O emission from wastewater treatment Introduction - conventional versus innovative N removal N 2 O emission - N 2 O formation routes and influencing factors - N 2 O emission factors How to measure? - N 2 O sampling strategies - N 2 O monitoring method Results from full-scale measurement campaigns - municipal WWT - one-stage partial nitritation-anammox - stand-alone partial nitritation reactor Conclusions 17

18 N 2 O emission from one-stage PNA Granules for partial nitritation-anammox High settleability Low footprint Olburgen (NL) 18

19 N 2 O emission from one-stage PNA Granular sludge reactor Olburgen (NL) 1-week monitoring campaign Q L,in = 129 m 3.h -1 Air flow rate = 340 gn.m -3 Fresh air (O 2 ) Total gas flow through reactor = fresh air + recirculated gas = constant N 2 O emission factor = 2% kg N 2 O-N.(kg TKN influent ) -1 - calculated on 80,5 hours of normal operation Castro-Barros et al., Water Res. (2015) 19

20 N 2 O emission from one-stage PNA Plant operation Olburgen (NL) Intermittent operation: air flow rate varies between high and low levels Gas Air flow rate 5 hours Castro-Barros et al., Water Res. (2015) 20

21 N 2 O emission from one-stage PNA Effect of aeration regime Olburgen (NL) Castro-Barros et al., Water Res. (2015) Air flow rate N 2 O emission N 2 O formation rate Dissolved N 2 O High aeration: Higher N 2 O emission Higher N 2 O formation Dissolved N 2 O emission > formation stripping Low aeration: Lower N 2 O emission Lower N 2 O formation Dissolved N 2 O emission < formation accumulation24

22 N 2 O emission from one-stage PNA Experiment: no fresh air only fresh air Olburgen (NL) Castro-Barros et al., Water Res. (2015) Under anoxic conditions, N 2 O was consumed High N 2 O emission (=formation) upon recovery from anoxic conditions N 2 O formation 25

23 N 2 O emission from one-stage PNA Influencing factors and formation mechanisms N 2 O formation favoured by sudden changes from relatively low O 2 (or anoxic conditions) to relatively high O 2 - during transient low -> high aeration (normal operation) - upon recovery from anoxic conditions Characterized by - accumulation of ammonium - higher ammonium oxidation rate - no accumulation of nitrite - transient to higher oxygen content Olburgen (NL) NH 2 OH pathway suggested as important contributor to N 2 O formation Heterotrophic denitrification cannot be discarded to cause N 2 O formation N 2 O consumption under anoxic conditions probably heterotrophic denitrification Castro-Barros et al., Water Res. (2015) 26

24 N 2 O emission from one-stage PNA Mitigation measures Olburgen (NL) Smoother shifts in aeration patterns Shorten aeration periods Castro-Barros et al., Water Res. (2015) 27

25 Outline N 2 O emission from wastewater treatment Introduction - conventional versus innovative N removal N 2 O emission - N 2 O formation routes and influencing factors - N 2 O emission factors How to measure? - N 2 O sampling strategies - N 2 O monitoring method Results from full-scale measurement campaigns - municipal WWT - one-stage partial nitritation-anammox - stand-alone partial nitritation reactor Conclusions 28

26 N 2 O emission from stand-alone partial nitritation SHARON partial nitritation reactor Rotterdam (NL) Ammonium NH 4 + NH 4 + nitrite NO 2 - nitrate NO 3 - Continuous stirred tank reactor (CSTR) High temperature, short HRT=SRT Effluent NO 2 - : NH 4 + of 1:1 suitable for Anammox SHARON Operating cycle for fixed aerobic HRT non-aerated aerated Time [min]

27 N 2 O emission from stand-alone partial nitritation SHARON reactor Rotterdam (NL) 3 weeks monitoring campaign Emission factor: 3.7% of incoming N load emitted as N 2 O Typical profile during normal operation (57% NH 4+ converted to NO 2- ) N2O [ppm] aerated non-aerated off-gas data aerated data fit non-aerated data fit aerobic N 2 O formation stripping anaerobically accumulated N 2 O QG,aer [m 3. h -1 ] DO [go2.m -3 ] Anoxic N 2 O formation was responsible for 66% of the N 2 O emission, even though the anoxic periods only accounted for 36% of the time Mampaey et al., Water Res., 2016 ph [ - ] QL,in m 3. h -1 ] 30

28 N2O [ppm] N 2 O emission from stand-alone partial nitritation SHARON reactor Rotterdam (NL) Prolonged aeration experiment (~ 11 cycles) aerated Mampaey et al., Water Res., 2016 off-gas data aerated data fit aerobic N 2 O formation stripping anaerobically accumulated N 2 O non-aerated Time [hours] N 2 O formation rate was constant during aerobic periods 31

29 N 2 O emission from stand-alone partial nitritation N2O [ppm] SHARON reactor Rotterdam (NL) Prolonged non-aerated period aerated non-aerated Mampaey et al., Water Res., 2016 off-gas data aerated data fit aerobic N 2 O formation stripping anaerobically accumulated N 2 O N2O [ppmv] liquid N2O [gn.m -3 ] N 2 O formation rate increased during anoxic periods higher mean anoxic N 2 O formation rate for longer anoxic periods 32

30 N 2 O emission from stand-alone partial nitritation SHARON reactor Rotterdam (NL) More experiments Mampaey et al., Water Res., Lowered DO experiments increased aerobic N 2 O formation - Shorter cycles reduced emission N 2 O formation mechanisms - aerobic: likely heterotrophic denitrification (low DO, high nitrite) - anoxic: heterotrophic denitrification could explain anoxic EF of at most 1.1% instead of 2.6% - accumulated NH 2 OH remaining in the anoxic phase as a trigger for subsequent N 2 O formation by nitrifier denitrification? N 2 O mitigation measures - applying short cycles drawback: nitrate formation - eliminate anoxic conditons: smaller reactor or variable volume 33

31 Outline N 2 O emission from wastewater treatment Introduction - conventional versus innovative N removal N 2 O emission - N 2 O formation routes and influencing factors - N 2 O emission factors How to measure? - N 2 O sampling strategies - N 2 O monitoring method Results from full-scale measurement campaigns - municipal WWT - one-stage partial nitritation-anammox - stand-alone partial nitritation reactor Conclusions 34

32 Conclusions Biological nitrogen removal processes emit N 2 O! Adequate N 2 O sampling strategy depends on goal Gas stripping device method - for dissolved N 2 O and N 2 O formation rate N 2 O emissions can be significant - contribution to carbon footprint - also significant variations in time/space Innovative (1 or 2-reactor) versus conventional N removal Formation mechanisms depend on conditions Options for mitigation - depends on formation mechanism (e.g. anoxic periods: sink or source) - in general: avoid / smoothen transitions 35

33 Acknowledgements Matthijs Daelman Kris Mampaey Celia Castro-Barros Mark van Loosdrecht Thank you for your attention! 36

34 Thank you for your attention! twitter: EvelineVolcke 37