N2O production in a single stage nitritation/anammox MBBR process

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NO₂-N (mg/l)
10
8
6
4
2
NO₂-N
biosensor
(mg/l)
NO₂-N LCK 342
(mg/l)
0
0 20 40 60 80
Time (min)
Figure 22. NO 2-N concentration profiles obtained with biosensor and with Dr Lange’s method, LCK
342, from measuring session when the reactor was operated at continuous aeration.
Changes in measured NO2-N concentration with LCK 342 varied between 4-6 mg/l,
while the NO2-N concentrations registered with the biosensor stayed in a slightly
narrower range of 4-5 mg/l. Figure 22 also shows that the biosensor seems to have a lag
phase, a phenomenon which can be seen in Figure 21 as well.
4.5 Diffusivity and stripping test of N2O
The diffusivity test of N2O performed in a reactor with carriers without biofilm during
mechanical mixing showed that N2O dissolved in the water phase left the system slowly.
Figure 23 shows how the initial N2O concentration decreased from ~11- 4 µmol N2O/l
during a period of about 8 hours, (500 minutes). Linear regression of the diffusion rate
showed that ~0.0132 µmol N2O left the reactor per minute. This molar concentration
corresponds to < 1% of N2O present in the water phase at all times during the
measurement. In order to validate the assumption that N2O diffusion can be neglected
during calculations of produced N2O the diffusivity rate/min has to be compared to the
production rate/min. The result of this comparison gives that diffusion corresponds to
about 10% of produced N2O during one minute.
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