N2O production in a single stage nitritation/anammox MBBR process

Table 13. Average N 2O concentration in the water phase during aeration. Calculated initial and
maximum N 2O production rates, mean O 2 concentrations during the aerated period, mean nitrogen
concentration, reduction and removal rates during measurements with N 2 gas in anoxic phase,
operation at DO ~1.0 mg/l.
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
091015 1.9 0.4 - 1.0 279 0.4 0.0 79 1.2
091016 1.1 0.8 - 1.3 278 0.3 0.0 77 1.2
When the MBBR was operated at a DO concentration of 1.5 mg/l instead of 1 mg/l, a
slightly higher N2O accumulation was observed right after the shift from aeration with
air to mixing with N2 gas. Accumulation proceeded for about 5 minutes and there after
the N2O concentration started to decrease. The rate with which N2O left the water phase
increased as aeration with air was switched on, see Figure 20
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 20. Concentration profiles of N 2O and O2 when N 2 gas is used for mixing during anoxic
period, reactor operated with continuous aeration at a DO level of ~1.5 mg/l.
Table 14 presents the results from the twomeasurements done when N2 gas was used
for mixing during the anoxic period.
Table 14. Average N 2O concentration in the water phase during aeration. Mean calculated initial
and maximum N 2O production rates, mean O 2 concentrations during aerated period, mean
nitrogen concentration, reduction and removal rates during measurements with N 2 gas in anoxic
phase, operation at DO ~1.5 mg/l.
Produced N 2O in % of
Average
N 2O
removed inorganic N-
concentration O 2
mean N-concentration
mg/l
N-red. Removal
Date µmol/l initial max mg/l NH 4-N NO 3-N NO 2-N % gN/m 2 d
091017 1.4 1.4 - 1.5 289 0.3 0.0 71 1.2
091018 1.1 2.1 - 1.8 289 0.3 0.0 73 1.3
39