N2O production in a single stage nitritation/anammox MBBR process

Summary
Wastewaters contain abundant nitrogen that causes eutrophication in the receiving
recipient if not removed before the water is released into nature. Common nitrogen
removal is performed through nitrification and denitrification which are biologic
processes. Microorganisms are utilised to convert inorganic nitrogen compounds into
dinitrogen gas through different chemical reactions in there metabolism. Nitrous oxide,
(N2O), can be an intermediate or end product in the metabolism of both nitrification and
denitrification. N2O is a greenhouse gas, 320 times stronger than carbon dioxide (CO2).
The gas is contributing to global warming and is also taking part in depletion of the
protecting ozone layer in the stratosphere. If large amounts of N2O are emitted from
wastewater treatment facilities the problem with abundant nitrogen in aquatic
environments is only transferred into the atmosphere and N2O emissions should
therefore be avoided.
The energy demand for biologic nitrogen removal is high since aeration is needed for the
aerobe nitrification process. Denitrification requires an organic carbon source that often
has to be added to the process, generally in the form of methanol. Burning of fossil fuels
for energy coverage of the wastewater treatment plant and during transportation of
carbon source is leading to CO2 emissions with negative effects on the climate. If an
additional organic carbon source is used in the denitrification process this will also
contribute to increased CO2 emissions from the wastewater treatment plant.
Biological nitrogen removal through anaerobic ammonium oxidation (anammox) is a
relatively new process solution in wastewater treatment. Anammox has the potential to
replace common nitrogen removal of recycled internal wastewater streams with high
strength of ammonium and low COD content. The bacteria responsible for the anammox
process are converting ammonium to dinitrogen gas with nitrite as electron acceptor
making a short cut in the nitrogen cycle. Only 50% of the influent nitrogen load in the
form of ammonium has to be converted to nitrite by the nitrifiers and no additional
carbon source is needed. This means that the process offers great saving possibilities,
economical as well as environmental.
Different system configurations are available for the anammox process, it can either be
operated as a two stage process where nitritation and anammox are performed in
separate reactors, or in a single stage process where both processes are taking part in
one reactor. A MBBR is suitable as a single stage nitritation/anammox process since a
biofilm with an outer aerobe layer for the nitrifiers and one inner anoxic layer for the
anammox bacteria can develop. To allow the build up of a biofilm structure with
different oxic layers the process has to be operated at low dissolved oxygen
concentrations. Insufficient oxygenation in the nitrification process is known to enhance
nitrous oxide emissions from ammonium oxidising bacteria. Since the single stage
anammox process involves nitritation at low dissolved oxygen concentrations the
process might lead to significant N2O emissions.
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