N2O production in a single stage nitritation/anammox MBBR process

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Chapter 5 5. Discussion 5.1 Process performance Theoretical maximum nitrogen removal by the anammox process is 88%, (Strous et al., 1998), highest achieved performance in the MBBR during the period of this master thesis work was about 80% with fluctuations down to a reduction corresponding to only 20%, the mean nitrogen conversion was 58%. Non stable process performance is probably due to operation disturbances like; stop in the influent flow, power failure, fluctuations in influent nitrogen compounds, (caused by microbial conversion of nitrogen compounds in the synthetic wastewater). As the reactor operation mode was shifted into continuous aeration at a lower DO concentration both % reduction and removal rate in gN/m 2 d was more stable and higher than the average during intermittent aeration shown in Figure 13 and Figure 14. This is consistent with results obtained by Szatkowska et al., (2003) who showed that higher DO concentrations impact a MBBR anammox process negatively with decreased nitrogen conversion rates as a result. Since the oxygen penetration depth within the biofilm increases with increasing DO (Henze et al., 1997) the anaerobic layer where anammox activity is taking part will be thinner which is causing a lower inorganic nitrogen conversion rate. One drawback with the anammox process is that NO3-N is produced during cell synthesis of anammox bacteria. However this problem is not significant since the effluent from the anammox process can be re-circulated with the influent water to the wastewater treatment plant. 5.2 N2O production Compared to N2O emissions from nitrogen removal processes found in literature (Table 3) the emissions from the single stage nitritation/anammox system examined in this work can be regarded as relatively high. During this study N2O production have been higher at intermittent aeration where the dissolved oxygen concentration averaged around 3 mg/l in the aerated period. Lowest N2O production recorded was during continuous aeration at DO concentrations corresponding to 1.0-1.5 mg/l. The emissions during continuous aeration are in the same range as emissions from a nitrifying SBR reactor (2.8%) and a Sharon reactor (1.7%) reported by Kampschreur et al., (2008 b and a). R 2 value obtained when comparing % N2O production of removed inorganic nitrogen with DO concentrations shows that there is a correlation between the higher N2O emissions and DO in the, see Figure 27. 44
% produced N₂O 12 10 8 6 4 2 Produced N₂O in relation to DO concetration R² = 0.6859 0 0 1 2 3 4 DO (mg/l) Figure 27. Correlation between the % N 2O production and DO concentration. It has been observed that changing environmental conditions can give rise to higher N2O emissions (Kampschreur et al., 2008, b) and the shifting oxygen conditions during intermittent aeration can be an explanation to higher N2O emissions during this operation mode. If the concentration profiles registered with the biosensor during intermittent aeration are considered it is shown that nitrite concentrations are actually increasing during the anoxic phase which is the opposite situation to what could be expected. Since nitritation is inhibited by low oxygen concentrations the conversion of ammonium into nitrite should decrease and anammox activity should consume nitrite leading to a total decrease in nitrite concentrations. High influent nitrite concentrations and the possible presence of nitrite oxidisers are two likely explanations to increasing nitrite concentrations during the anoxic period. Since increasing NO2-N concentrations have been observed to give higher N2O emissions (Tallec et al., 2006,a) rising nitrite concentrations during the anoxic period observed in this study can also be a reason for higher emissions during intermittent operation of the MBBR. Process performance seems to influence the extent of N2O emitted from the MBBR since less N2O was produced when higher nutrient removal was achieved during periods of continuous aeration. Figure 28 which shows the correlation between % N2O production and % N-reduction indicates that process performance might influence the N2O emissions from the system (R 2 =0.70). 45
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Water and Environmental Engineering
Page 5 and 6: Summary Wastewaters contain abundan
Page 7: Acknowledgements I would like to ex
Page 11 and 12: Table of content Chapter 1 ........
Page 13 and 14: Chapter 1 1. Introduction Nitrogen
Page 15 and 16: 1.3 Objectives The aim with this ma
Page 17 and 18: Chapter 2 2. Background 2.1 Biologi
Page 19 and 20: are intermediates in the chemical r
Page 21 and 22: minimum concentration of 2 mg/l sho
Page 23 and 24: 2.3 Biofilm reactors Biofilm reacto
Page 25 and 26: 2.3.4 Moving bed reactor Another wa
Page 27 and 28: To enable efficient nutrient remova
Page 29 and 30: The Canon process is often implemen
Page 31 and 32: 2.5.1 Nitrification as a source of
Page 33 and 34: Table 3. N 2O emission from differe
Page 35 and 36: The microsensor is connected to a p
Page 37 and 38: Chapter 3 3. Material and Methods 3
Page 39 and 40: 3.3 Analytical methods Concentratio
Page 41 and 42: Figure 11. Calibration setup for mi
Page 43 and 44: Chapter 4 4. Results 4.1 Process pe
Page 45 and 46: 10 8 DO concentration, pH & tempera
Page 47 and 48: concentration), while the maximum p
Page 49 and 50: of N2O during aeration was slightly
Page 51 and 52: Table 13. Average N 2O concentratio
Page 53 and 54: 12 NO₂-N (mg/l) 10 8 6 4 2 NO₂-
Page 55: Aeration 1.6 (l/min) with carriers
Page 59 and 60: laboratory system might be underest
Page 63: 6. Conclusions The following conclu
Page 67 and 68: 8. References Abma W.R., Schultz C.
Page 69 and 70: Hippen A., Rosenwinkel K-H., Baumga
Page 71 and 72: Metcalf & Eddy I. (2003). Wastewate
Page 73: van Niftrik L.A., Fuerst J.A., Sinn
Page 79 and 80: Appendix B Calculations of N2O emis
Page 81 and 82: The rN value is calculated with the
Page 83 and 84: Appendix C Microsensor measurements
Page 85 and 86: 12 090927 Prolonged unaerated perio
Page 87: 12 091016 Continuously operation, D
Page 90 and 91: 350 090921 NH₄-N 350 090922 NH₄
Page 92 and 93: 300 090926 Prolonged unaerated peri
Page 94 and 95: 091010 NOx-N 091013 NOx-N NOx-N (mg
Page 96 and 97: 35 091015 NO₃-N 70 091016 NOx-N 3
Page 98 and 99: 350 091018 NH₄-N 35 091018NO₃-N
Page 100 and 101: The bacteria performing the microbi
Page 102 and 103: Typical profiles of how N2O and DO
Page 104 and 105: 12 10 091014 Continously operation,
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Mulder A.V.A, Robertson L.A., Kuene
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N2O production in a single stage nitritation/anammox MBBR process

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