N2O production in a single stage nitritation/anammox MBBR process

Typical profiles of how N2O and DO changes during the cycle are shown in Figure . The
N2O concentration measured in the water phase varies with the aeration of the MBBR.
When aeration starts at the beginning of the cycle the airflow strips N2O out of the water
phase and the concentration decreases to a constant minimum level. As soon as the
aeration is shut of the N2O starts to accumulate in the water phase until aeration is
switched on again and the procedure starts over as shown in Figure 2.
N₂O (µmol/l)
12
10
8
6
4
2
0
4.5
3.75
2.25
1.5
0.75
0
0 20 40 60 80
Time (min)
3
DO (mg/l)
N₂O
(µmol/l)
DO
(mg/l)
Figure 2. Concentration profiles of N 2O and O 2, reactor is operated with intermittent aeration at a
DO concentration of~3mg/l in the aerated phase. The cycle study starts at the beginning of the
aerated period. N 2O gas is stripped from the water phase at the same time as the oxygen
concentration rises.
Initial N2O production varies between 6-11% of influent nitrogen concentration that is
converted into dinitrogen gas (here after referred to as removed inorganic N-
concentration), while the maximum production ranges from 11-30% of removed
inorganic nitrogen.
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, results are shown in
Figure 3. The reactor was operated in the same manners as above and the same
sampling procedure was applied. After the anoxic period of 20 minutes when the
aeration usually went on during a normal cycle the mechanical mixing proceeded for
another two hours.
90