N2O production in a single stage nitritation/anammox MBBR process

Chapter 2
2. Background
2.1 Biological nitrogen removal in wastewater treatment
Nitrogen removal is one of the major tasks in wastewater treatment. Biologic nitrogen
removal is the most efficient way to eliminate nitrogen from the wastewater and a
variety of system configurations like activated sludge plants with suspended growth,
biofilters designed for attached growth and combinations of the two have been
developed. These systems are built on the knowledge of microbial nitrogen cycling
where nitrogen compounds like NH4+, NO2 − and NO3 − are removed by conversion into
elemental N2 gas released to the atmosphere, see Figure 1. The well known nitrification
and denitrification processes are commonly used to achieve satisfactory nitrogen
removal. Today anaerobic ammonium oxidation, a relatively new technology for
nitrogen removal is also in use at several places.
Figure 1. Major biological transformations of nitrogen in wastewater treatment. (Kampschreur et
al., 2009).
2.1.1 Nitrification
Nitrification is oxidation of ammonium into nitrate under aerobic conditions, the
process occurs in two separated reaction steps, each involving different species of
bacteria. The nitrifiers are chemo-lithoautotrophs which means that they use carbon
dioxide or carbonate as carbon source and inorganic nitrogen is used for both energy
supply and cellular growth (Gray, 2004). In the first nitritation step ammonium
oxidising species like Nitrosomonas and Nitrosospira oxidises ammonium into nitrite:
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