N2O production in a single stage nitritation/anammox MBBR process

close to 0 mg/l. The N2O concentration is constant under a period of 20-50 minutes and
then slowly started to decrease as seen in Figure 16. Initial and maximum production
rates of N2O calculated during the prolonged cycles as are shown in Table 10. Initial N2O
production rates varied between 6.2-11% while maximum production varied between
10-30% of removed inorganic nitrogen.
Table 10. Prolonged measurement: Average N 2O concentration in the water phase during aeration.
Calculated initial and maximum N 2O production rates, mean O 2* concentrations during aerated
period, mean nitrogen concentration, reduction and removal rates.
Date
Average
N 2O
µmol/l
Produced N 2O in % of
removed inorganic N-
concentration O 2
mean N-concentration
mg/l
initial max mg/l NH 4-N NO 3-N NO 2-N
N-red.
%
Removal
gN/m 2 d
090925 0.9 6.2 30.3 3.0 234 - - 59 1.0
090926 2.5 10.7 10.7 2.8 237 - - 47 0.9
090927 2.2 9.5 9.5 3.1 228 - - 57 1.0
*Mean O2 concentration from the moment when DO concentration reached its maximum
level until aeration is shut off.
4.3.3 Continuous operation at DO ~1.5 mg/l
The MBBR was operated at continuous aeration which was switched off for twenty
minutes in order to estimate the N2O accumulation. Figure 17shows the concentration
profiles of N2O and O2. As seen in the figure they resemble the profiles obtained during
cycle studies of intermittent aeration. The N2O accumulation increased as O2 decreased
but not as fast as before.
12
10
4.5
3.75
N₂O (µmol/l)
8
6
4
2
3
2.25
1.5
0.75
DO (mg/l)
N₂O
(µmol/l)
DO
(mg/l)
0
0
0 20 40 60 80
Time (min)
Figure 17. Concentration profiles of N 2O and O 2 obtained from measurement during the period of
continuous reactor operation at a DO concentration of ~1.5 mg/l.
Twenty minutes of the anoxic period was enough to reach the maximum N2O
concentration and the period where N2O production seems to be in equilibrium with the
amount of N2O leaving the system. Figure 17 also illustrates that the mean concentration
36