N2O production in a single stage nitritation/anammox MBBR process

made the increase in N2O accumulation will then be equal to the N2O production rate,
(see appendix B for calculation example). The N2O production is calculated as % of
removed inorganic nitrogen. Two N2O production rates are estimated and referred to as
initial and maximum production rates. The initial production rate is calculated from the
increase of N2O that can be seen in the water phase immediately after switching of
aeration. Maximum N2O production is estimated between the two measuring points
where the increase in N2O has its maximum during the unaerated period. Mean N2O
concentration in the water phase when the MBBR is aerated, calculated initial and
maximum N2O production rates, mean O2 concentrations during the aerated period,
mean nitrogen concentrations, reduction and removal rates for all measurements are
summarised in Table 9 -Table 14. One figure of typical N2O and O2 profiles for each
operation mode is shown in Figure 15 -Figure 20, N2O and O2 profiles for all
measurements made are found in appendix C.
4.3.2 Intermittent aeration.
Measurements of produced N2O at intermittent aeration, (DO ~3.0 mg/l) were
performed at four different occasions. Typical profiles of how N2O and DO changed
during the cycle are shown in Figure 15. The N2O concentration measured in the water
phase varied with the aeration of the MBBR. When aeration started in the beginning of
the cycle the airflow striped N2O out of the water phase and the concentration decreased
to a constant minimum level. As soon as the aeration was shut off the N2O started to
accumulate in the water phase until aeration was switched on again and the procedure
started over as shown in Figure 15.
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 15. Concentration profiles of N 2O and O 2 during a cycle of intermittent aeration, DO ~3 mg/l
in the aerated phase. The cycle study started in the beginning of the aerated period. N 2O gas was
stripped from the water phase at the same time as the oxygen concentration rose.
Initial N2O production varied between 5.6-11% of influent nitrogen concentration that
was converted into dinitrogen gas (here after referred to as removed inorganic N-
34