N2O production in a single stage nitritation/anammox MBBR process

Figure 9 A) Nitrite biosensor with removable biochamber(Unisense e, 2009). B) Enlargement of
biochamber (Unisense e, 2009).
At 20 °C the biosensor has a measuring range in the interval 0-1000 µM NO2-N, (0-14
mg/l), and gives about 1.25-4 nA in output signal per 100 µM NO2-N (1.4 mg/l) added.
The sensor signal depends on both ionic composition and temperature of the sample. It
might vary up to 30% due to salinity and it has a temperature coefficient of about 2-4%
per °C. Sensitivity to stirring of the sample depends both on temperature and salinity.
The 90% response time in a stirred sample is less than 90 seconds (Unisense c, 2007).
Nitrous oxide diffusing into the biochamber from the external environment is detected
by the N2O transducer and is therefore interfering with the NO2 − signal. The theoretical
sensitivity to N2O should be a signal 2.5 times higher than for equal concentrations of
NO2 − . This since it takes two NO2 − molecules to form one N2O molecule and the diffusion
coefficient for NO2 − is 0.8 times that of N2O (Nielsen et al., 2004).
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