N2O production in a single stage nitritation/anammox MBBR process

3.5 Diffusivity tests of N2O
The diffusivity of N2O was examined experimentally since no off-gas equipment was
available during N2O measurements and the calculations of produced N2O are based on
the assumption that the diffusivity of N2O can be neglected.
To get as close as possible to the real conditions in the MBBR process but without any
N2O producing bacteria a 7.5 l reactor of the same type as used for the MBBR process
was utilised during the diffusivity experiments. The influent synthetic wastewater was
selected to get a medium similar in salinity to that in the real process. The reactor was
heated to 30 ̊C with a thermostat bath and K1 heavy carriers (same type of carrier as K1
used in the MBBR, but with slightly higher density) without biomass was used. K1 heavy
was used to keep the carrier material without biofilm in the water phase and not floating
on top of the water surface.
To examine how fast N2O diffuses from the water phase during mechanical mixing N2O
saturated water was added to a concentration of ~11 µM. The decrease of N2O in the
water phase was registered with the N2O microsensor during a period of eight hours.
The stripping effect from aeration was also observed by registering the decrease of N2O
in the water phase during aeration at three different air flow rates. N2O was added to a
concentration of 12 µM.
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