N2O production in a single stage nitritation/anammox MBBR process

Table 12. 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 for studies at continuous operation mode 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
091013 2.5 1.7 4.3 1.0 285 0.0 0.0 85 1.6
091014 2.3 2.0 2.0 1.0 284 0.0 0.3 73 1.3
4.3.5 Effect of mixing with N2 gas during unaerated phase, continuous
operation at DO ~1.0 mg/l and ~1.5 mg/l
Pure N2 gas was used during the anoxic period instead of mechanical mixing, in order to
evaluate the stripping effect from the gas (the same gas flow rate was used as during
aeration with air). A small accumulation of N2O was observed right after the switch from
aeration with air to N2 gas, see Figure 19 The increase was followed by a sharp decrease
in the N2O concentration profile. When aeration was switched on again, the N2O
concentration increased faster than during previous measurements.
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 19. Concentration profiles of N 2O and O 2 when N 2 gas was used for mixing during anoxic
period, reactor was operated with continuous aeration at a DO level of ~1.0 mg/l.
The accumulation of N2O that can be seen was converted to a corresponding % N2O
production calculated to be