N2O production in a single stage nitritation/anammox MBBR process

Chapter 5
5. Discussion
5.1 Process performance
Theoretical maximum nitrogen removal by the anammox process is 88%, (Strous et al.,
1998), highest achieved performance in the MBBR during the period of this master
thesis work was about 80% with fluctuations down to a reduction corresponding to only
20%, the mean nitrogen conversion was 58%. Non stable process performance is
probably due to operation disturbances like; stop in the influent flow, power failure,
fluctuations in influent nitrogen compounds, (caused by microbial conversion of
nitrogen compounds in the synthetic wastewater).
As the reactor operation 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 the average during intermittent aeration shown in Figure 13 and Figure 14.
This is consistent with results obtained by Szatkowska et al., (2003) who showed that
higher DO concentrations impact a MBBR anammox process negatively with decreased
nitrogen conversion rates as a result. Since the oxygen penetration depth within the
biofilm increases with increasing DO (Henze et al., 1997) the anaerobic layer where
anammox activity is taking part will be thinner which is causing a lower inorganic
nitrogen conversion rate.
One drawback with the anammox process is that NO3-N is produced during cell
synthesis of anammox bacteria. However this problem is not significant since the
effluent from the anammox process can be re-circulated with the influent water to the
wastewater treatment plant.
5.2 N2O production
Compared to N2O emissions from nitrogen removal processes found in literature (Table
3) the emissions from the single stage nitritation/anammox system examined in this
work can be regarded as relatively high.
During this study N2O production have been higher at intermittent aeration where the
dissolved oxygen concentration averaged around 3 mg/l in the aerated period. Lowest
N2O production recorded was during continuous aeration at DO concentrations
corresponding to 1.0-1.5 mg/l. The emissions during continuous aeration are in the
same range as emissions from a nitrifying SBR reactor (2.8%) and a Sharon reactor
(1.7%) reported by Kampschreur et al., (2008 b and a). R 2 value obtained when
comparing % N2O production of removed inorganic nitrogen with DO concentrations
shows that there is a correlation between the higher N2O emissions and DO in the, see
Figure 27.
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