N2O production in a single stage nitritation/anammox MBBR process

2.5.1 Nitrification as a source of N2O emissions
Ammonium oxidising bacteria (AOB) are the organisms believed to be responsible for
N2O production during nitrification. N2O can be produced both through aerobic
oxidation of ammonium and through nitrifier denitrification of nitrite with ammonium
as an electron donor (Schmidt and Bock, 1997, Kampschreur et al., 2006).
In the presence of oxygen N2O is produced during oxidation of ammonium with oxygen.
2NH O 2HCO N O H O CO (2.5.1)
(Trela et al., 2005)
Hooper, (1968) detected hydroxylamine-nitrite reductase, an enzyme in Nitrosomonas
europaea that reduces nitrite in the presence of hydroxylamine with NO and N2O as
products. Nitrite is reduced anaerobically to N2O with hydroxylamine:
HN OH HNO N O 2H O (2.5.2)
The denitrifying activity of Nitrosomonas is only related to life supporting energy yield
and is probably a survival mechanism in anaerobic habitats (de Bruijn et al., 1995). Low
DO concentrations in the nitrification process has been shown to give higher N2O
emissions than a process operated under well aerated conditions (Magnaye et al., 2008).
High nitrite and ammonium concentrations, high organic loading, low temperature
together with short sludge age are other factors known to give rise to increased N2O
emissions in the nitrification process (Kampschreur et al., 2009).
2.5.2 Denitrification as a source of N2O emissions
Denitrifying organisms are producing N2O as an intermediate when nitrate or nitrite is
reduced to N2 (Kampschreur et al., 2007). Production of N2O takes place as the nitrate
reductase system for electron transport is induced to produce ATP under anoxic
conditions (Gray, 2004), the process occurs in the following sequence:
NO
(2.5.3)
A low pH (