N2O production in a single stage nitritation/anammox MBBR process

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2.6 Microsensors Microsensors measure changes in the chemical composition of complex and heterogeneous environments in a micrometer scale with a very short response time. The sensors can therefore be used in a broad range of scientific research, for example in cell and tissue analysis, microrespiration, marine ecology, biofilm analysis and wastewater treatment. Laboratory experiments in this master thesis are based on online measurements with one microsensor for nitrous oxide and one for nitrite. Both sensors that are developed by Unisense, Århus, Denmark rely on electrochemical detection of N2O. 2.6.1 Nitrous oxide sensor The N2O microsensor is a Clark-type microsensor constituted of a cathode shaft (tapered glass casing) equipped with silicone tip membrane. A N2O reducing cathode is positioned in an electrolyte behind the silicone membrane. The reference for the N2O reduction is a silver anode and the sensor is equipped with an oxygen front guard (with an silicone membrane in the tip. The front guard prevents oxygen from interfering with the nitrous oxide measurements. The guard is filled with an alkaline ascobate solution, an effective reducing agent, to prevent oxygen interference with nitrous oxide measurements (Andersen et al., 2001). The silicone membranes in the sensor tip only allows passage of gases and small uncharged molecules, shielding the electrolyte from the outer environment (Unisense b, 2007). Figure 8. Photo of the N 2O microsensor. 22
The microsensor is connected to a piccoameter that polarises the cathode surface where the nitrous oxide that diffuses through the silicone membrane is reduced to N2 gas. As nitrous oxide is reduced at the cathode surface two electrons from the silver anode is used for each reduced N2O molecule. The electron transport gives rise to a current proportional to the amount of reduced nitrous oxide. The current is registered and converted to an out signal by the piccoameter. The guard cathode is also polarised to deplete oxygen in the electrolyte which minimizes zero current (Unisense b, 2007). The nitrous oxide sensor has a measuring range of about 0-1 atmosphere pN2O with a response time less than 10 seconds. The stirring sensitivity is smaller than 2% and the out signal is temperature dependent with a temperature coefficient of about 2-3% per °C. Interference in the out signal might occur from electrical noise in the surrounding environment (Unisense b, 2007). 2.6.2 Nitrite biosensor The nitrite biosensor is a nitrous oxide sensor equipped with a replaceable biochamber (Figure 9a), (Unisense e, 2009). A plastic tube containing a carbon source and a bacterial culture constitutes the biochamber that is mounted in the front of the sensor tip (Figure 9b), (Unisense e, 2009). The biomass in the reaction chamber is positioned between the carbon source required for their growth and an ion-permeable membrane separating the microorganisms from the external environment (Unisense c, 2007). The denitrifying bacterial culture used in the biochamber is deficient in NO3 − and N2O reductase which means that it is only able to reduce NO2 − into N2O. As NO2 − diffuses into the biochamber it is reduced to N2O by the biomass (Nielsen et al., 2004). Since denitrifying bacteria are facultative aerobic they can use both oxygen and nitrite as oxidation agent for their respiration (Larsen et al., 1997). Oxygen is used preferential to nitrite as it results in a higher energy yield. This will create a NO2 − reducing gradient in the biochamber with the bacteria closest to the membrane respiring with oxygen. The NO2 − reducing capacity of the biosensor will depend on the length of the aerobic zone resulting in higher maximum detectable concentrations of NO2 − in anaerobic environments (Larsen et al., 1997). Produced N2O diffuses through the silicone membrane and is reduced at the cathode in the transducer part of the biosensor. A piccoameter measures the current arising from the electron transport just as in the case with the nitrous oxide sensor. The output signal is proportional to the amount of NO2 − that has been reduced after diffusion into the biochamber (Unisense c, 2007). 23
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: Table 3. N 2O emission from differe
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 and 58: % produced N₂O 12 10 8 6 4 2 Prod
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,
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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