N2O production in a single stage nitritation/anammox MBBR process
N2O production in a single stage nitritation/anammox MBBR process
N2O production in a single stage nitritation/anammox MBBR process
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The Canon <strong>process</strong> is often implemented with granular sludge and it is important that<br />
large biomass flocs with decreas<strong>in</strong>g oxygen gradient with<strong>in</strong> the floc are allowed to form.<br />
This <strong>in</strong> order to achieve an environment suitable for both aerobic ammonium oxidisers<br />
and <strong>anammox</strong> bacteria <strong>in</strong> the same reactor set up (Third et al., 2005).<br />
2.5. 3 Deammonification<br />
The deammonification <strong>process</strong> is also a one <strong>stage</strong> <strong>process</strong> for nitrogen removal through<br />
partial nitrification and <strong>anammox</strong>. The <strong>process</strong> scheme resembles that of the Canon<br />
<strong>process</strong> illustrated <strong>in</strong> Figure 6. The greatest difference between deammonification and<br />
the Canon <strong>process</strong> is that deammonification is a biofilm <strong>process</strong> utilis<strong>in</strong>g a carrier<br />
material to support biofilm growth.<br />
Conversion of ammonium <strong>in</strong>to d<strong>in</strong>itrogen gas takes place at different biofilm depths.<br />
Nitrification of ammonium <strong>in</strong>to nitrite is carried out by nitrifiers <strong>in</strong> the outer aerobic<br />
layers of the biofilm. This <strong>process</strong> together with diffusion provides the <strong>anammox</strong><br />
bacteria <strong>in</strong> the deeper anaerobic layers with their substrates (Egli et al., 2003), see<br />
Figure 7. The diffusion depth of oxygen is dependent on the DO concentration <strong>in</strong> the<br />
water bulk phase and it <strong>in</strong>fluences to which extent the conventional <strong>nitritation</strong> <strong>process</strong><br />
takes part <strong>in</strong> the biofilm (Helmer et al., 2001).<br />
Figure 7. Conversion of ammonium to d<strong>in</strong>itrogen gas takes place through two separate reactions at<br />
different biofilm depths. (Adapted from Rosenw<strong>in</strong>kel and Cornelius, 2005)<br />
To ensure anoxic conditions for the <strong>anammox</strong> bacteria the system must be operated at<br />
low oxygen concentrations or with alternat<strong>in</strong>g aeration. Low oxygen concentrations <strong>in</strong> a<br />
biofilm system also <strong>in</strong>hibits aerobic nitrite oxidisers which is needed to make sure that<br />
only the first step of nitrification is performed. The growth rate of aerobic ammonium<br />
oxidisers (AOB) is higher than for nitrite oxidisers at low oxygenation, AOB are also<br />
faster at recover<strong>in</strong>g <strong>in</strong> activity after the anoxic phase.Temperatures over 20°C are also<br />
favour<strong>in</strong>g the growth rate of AOB over nitrite oxidisers (Rosenw<strong>in</strong>kel and Cornelius,<br />
2005). Biofilm systems are suitable for the <strong>anammox</strong> <strong>process</strong> s<strong>in</strong>ce the bacteria are slow<br />
growers that require high biomass retention (Abma et al., 2006).<br />
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