N2O production in a single stage nitritation/anammox MBBR process

in one cycle ( 0, 36 and 66 minutes) considered that the variation of the influent medium
during one hour should not be significant.
3.3.2 Prolonged study, intermittent aeration
A study of the effect of prolonged, intermittent aeration was made in order to observe
how the N2O production was influenced by a longer anoxic period. During the first hour
the reactor was operated in the same manner as above and the same sampling
procedure was applied. The anoxic period of 20 minutes during a normal cycle was
prolonged with 2 hours.
Grab samples were taken in the effluent with 6 minute intervals and every 36 minutes in
the feeding medium.
3.3.3 Continuous aeration
Measurements during continuous aeration were performed at DO concentrations of
~1.5 mg/l and ~1 mg/l. To determine the production of N2O gas during these operation
conditions the aeration was turned off and the unaerated period was determined to 20
minutes to be comparable to the measurements done during intermittent aeration.
Measurements proceeded 20 minutes after the aeration was switched on again.
To examine if the accumulation of N2O gas in the water phase during the anoxic period
depended on increased production or was an effect of stripping during aeration, mixing
with pure N2 gas was used during the anoxic period. The N2 gas flow was equal to the
airflow during the aerated period.
Grab samples were taken in the effluent every 6 th minute and every 36 th minute in the
influent medium.
3.4 Calibration of microsensors
The N2O production and NO2 − concentration profile were measured online with Clarktype
microelectrodes described in chapter 2.6. Before usage the microsensors were
calibrated separately in a jacketed, temperature controlled beaker with 300 ml of pH
regulated synthetic medium to assure the same salinity, temperature and pH of
calibration solution and reactor, see Figure 11.
The sensor to be calibrated was mounted in the calibration beaker, a stable sensor signal
was awaited and the zero value was read and registered with Unisense’s software
SensorTrace BASIC. A top mounted stirrer was used for fast mixing and uniform
concentration of the calibration solution.
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