N2O production in a single stage nitritation/anammox MBBR process

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% produced N₂O 12 10 8 6 4 2 Produced N₂O in relation to % N- reduction R² = 0.7029 0 0 20 40 60 80 100 % N-reduction Figure 28. Correlation between % N 2O production and % N-removal. NO2-N concentrations during continuous aeration decreased as the aeration was switched off, at the same time N2O production was not as high as during intermittent aeration. This result can be partly explained with better control of influent nitrogen fractions during these measurements. A different microbial composition in the MBBR during continuous aeration or that conditions are not favouring N2O production to the same extent as during intermittent aeration are other possible explanations to lower N2 O emissions during continuous operation of the reactor. In this study it is not possible to determine whether better process performance was the reason or if lower N2O production might be a result of other reasons such as different composition of the microbial community during continuous aeration. Increased NH4-N concentrations and decreasing NO2-N concentrations recorded during prolonged unaerated studies showed that the nitrifying activity decreased as the MBBR was left without oxygen supply for a longer period. N2O production within the system ceased at the same time indicating that nitrifier denitrification of ammonium with nitrite performed by AOB was the reason to N2O emissions. Why N2O production was not taking part as long as there was NO2-N available for nitrifier denitrification in the water phase is unknown. One explanation might be that the NO2-N concentration in the biofilm was below concentrations that the bacteria can utilise. Stripping tests of N2O and mixing with pure N2 gas during the anoxic phase indicates that the N2O accumulation registered by the microsensor is due to the microbial activity producing N2O and to termination in stripping N2O out of the water. It is not possible to say if the production rate is the same during aeration and the anoxic phase. Uncertainties and sources of errors can be many during laboratory work some are shortly discussed here. During these experiments a synthetic wastewater was used, this might influence the N2O production from the system, a real waste water is more complex and might give other emission results, both higher and lower. The fact that diffusion corresponded to 10% of produced N2O in the MBBR indicates that emissions from the 46
laboratory system might be underestimated. If calibrations have been performed with an unsaturated N2O solution this will give rise to overestimated N2O productions from the partial nitritation/anammox MBBR. As pointed out by Kampschreur et al., (2009) changing environmental conditions might lead to higher N2O emissions and short term laboratory scale measurements might therefore give over estimated N2O emissions. Since the anammox process needs less resources and produces less CO2 than common nitrogen removal, anammox has been pointed out as a more environmental friendly alternative (Fux and Siegrist, 2004). Kuenen and Robertson, (1994) are calling attention to that wastewaters in the Netherlands generally have a nitrogen content between 40-60 mg/l, each person produces about 150 l/d which gives a nitrogen production of 2.2 kg nitrogen per person and year and that even a small N2O production corresponding to 0.1% of the nitrogen concentration will result in significant N2O emissions. Considering that the MBBR process has been found to produce N2O corresponding to a minimum of 2% of removed inorganic nitrogen it has to be further examined whether this single stage anammox process is more environmental friendly than common nitrogen removal processes. Even if rather high N2O production was obtained in this study, experiences in pilot scale trials with similar operation modes has given N2O production as low as 0.2 % of removed inorganic nitrogen (Christensson, 2010). Additional research is needed to determine if the N2O production from a full scale process would be as high as the production found from the laboratory MBBR system. It also has to be determined which bacteria that are responsible for producing N2O, whether the relatively high N2O emissions found from the laboratory MBBR are due to biofilm structure with oxygen pore conditions. Amounts of N2O emissions have to be further evaluated in correlation to process operation and performance. A single stage biofilm system might not be the best solution for the partial nitritation anammox process if this process design always gives rise to high N2O emissions. 5.3 Measurements with NO2-N biosensor The biosensor gave results that correlated very well with concentrations obtained with LCK 342 at some occasions and the fluctuations in NO2-N concentration measured with the biosensor always showed the same trends as achieved with LCK 342. However the NO2-N biosensor did not give reliable results at all times in use and could never replace LCK 342 for determination of NO2-N concentrations during this master thesis work. Some measurements performed with the biosensor recorded much higher NO2-N concentrations than obtained with the Dr. Lange kit. This was probably due to electric disturbances that caused electric migration which is the transport of a charged body in an electric field. Kjær et al., (1999), have shown that this phenomenon can be used to force negatively charged NO3 − ions over the semi permeable membrane of the biochamber increasing the ion sensitivity by a factor of more than 10.000. The electric disturbances can have been caused by other laboratory equipment or since the ground channel on the backside of the piccoameter was used. This ground port has another 47
Page 1:
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 and 56: Aeration 1.6 (l/min) with carriers
Page 57: % produced N₂O 12 10 8 6 4 2 Prod
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,
Page 106: 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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