N2O production in a single stage nitritation/anammox MBBR process

The main objective with this master thesis work was to study the production of nitrous
oxide from a laboratory single stage nitritation/ anammox MBBR. The N2O
measurements were performed online in the water phase with a Clark–type
microsensor developed by Unisense, Århus, Denmark. The reactor was operated at both
intermittent and continuous aeration. The results from the experiments are summarised
below:
At intermittent aeration % reduction and removal rate in gN/m 2 d were in the range of
47-59% and 0.9-1.1 gN/m 2 d respectively. As the reactor mode was shifted into
continuous aeration at a lower DO concentration both % reduction and removal rate in
gN/m 2 d was more stable and higher than during intermittent aeration. % nitrogen
reduction was between64-65% and the removal rate in the interval of 1.3-1.6 gN/m 2 d.
The MBBR system produced N2O regardless of operation mode. The nN2O production
was determined through measurements of initial accumulation of nitrous oxide in the
water phase when aeration was turned off Intermittent aeration at high dissolved
oxygen concentrations 3 mg/l was resulting in significant nitrous oxide production
ranging from 6-11% of removed inorganic nitrogen. Operation at continuous aeration
yielded nitrous oxide emissions corresponding to about 2-3% of removed inorganic
nitrogen. Higher process performance may be an explanation to smaller amounts of
emitted N2O.
Conclusions that can be made from the experiments are summarised below:
• The single stage nitritation/anammox system produced significant amounts of
N2O with a minimum production of 2% of removed inorganic nitrogen.
• Operating the MBBR at intermittent aeration with a DO of ~3 mg/l gave the
highest N2O production with initial and maximum productions of 6-11% and 10-
30% respectively.
• Smaller amounts of N2O were produced by the partial/nitritation anammox
system during continuous operation at DO in the interval 1-1.5 mg/l. The initial
N2O production was found to be 2-3% and the maximum N2O production
corresponded to 2-6%.
• When the MBBR was exposed to a longer period of anoxic conditions both
ammonium oxidation and N2O production ceased.
• From results of mixing with N2 gas during the anoxic period it cannot be said
with certainty that the N2O production is the same during aeration and anoxic
phase. The absolute number on overall N2O production for an operation mode
(based on the measurements of N2O accumulating during the anoxic phase) could
be both overestimated or underestimated and should therefore be used as a
comparative tool.
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