N2O production in a single stage nitritation/anammox MBBR process

Figure 11. Calibration setup for microsensors.
Both sensors were calibrated by stepwise addition and signal reading at known
concentrations of N2O and NO2-N respectively. The resulting calibration curves are
illustrated in Figure 112. The linear regression shown in the figure is only based on one
single point registered by the computer software at each concentration. Both
concentration profile and linear regression obtained during calibration are drawn to
illustrate the procedure.
Figure 12. Examples of calibration points and concentration profiles for N 2O and NO 2-N
microsensors during calibration procedure. The first calibration point represents the signal
obtained in mV without any addition of N 2O or NO 2-N. As stepwise concentrations of N 2O and NO 2-N
were added during the calibration the sensor signal increased. A stable signal was awaited before
a voltage corresponding to the added concentration was registered.
Addition to known N2O concentrations was achieved by adding a defined volume of a
saturated N2O solution. The saturated N2O solution was prepared by bubbling N2O gas
through distilled water with a flow rate of 1 l/min for at least 30 minutes. For calibration
of the biosensor additions were made from a bulk solution with NaNO2 with a NO2-N
concentration of 5 mg/l. See Appendix A for calculated volume additions of the saturated
N2O and NO2-N solutions. After calibration both sensors were mounted directly into the
MMBR reactor, the N2O sensor was placed in a small metal-mesh basket for protection
from the moving carriers.
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