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Nitrobacter, Nitrospina, and Nitrococcus belong to the second group, those who oxidise nitrite with oxygen to nitrate. The reaction catalysed by these bacteria could be described by equation 2 (Seviour 2010, 45). NO -2 0.5 O2  NO -3


The term partial nitrification refers to a process where only the first of the two steps in normal nitrification is performed. That is, the ammonium is oxidised to nitrite but the nitrite is never oxidised further into nitrate. During natural circumstances, in soils for example, this rarely happens (Eriksson et al. 2005, 236), but it has turned out to be possible to achieve this reaction by carefully controlling the process environment. 2.2.2

Environmental Factors

Certain parameters have been shown to affect the ammonium-oxidisers differently then they affect the nitrite-oxidisers. By carefully adjusting these parameters it is possible to create an environment where ammonium-oxidisers are favoured at the expense of nitrite-oxidisers, which is necessary for a partial nitrification process. One such parameter is temperature. It has been shown that the maximum growth-rate for the bacteria is dependent on the temperature, and this dependency differs between ammonium-oxidising bacteria and nitrite-oxidising bacteria. By operating the process at a temperature close to 35 ˚C the growth-rate of the ammonium-oxidisers are approximately twice that of the nitrite-oxidisers (Dongen, M. S. M. Jetten, and Loosdrecht 2001, 8). This could be used to wash out the nitriteoxidisers while retaining the ammonium-oxidisers by adjusting the retention time in the system. Another parameter affecting the two types of bacteria differently is pH. It has been shown that the actual substrates used by the bacteria are not actually the ions, but rather their uncharged counterparts, NH 3 and HNO 2. Since the pH will affect the equilibrium between the ions and their uncharged form, the pH will also affect the amount of substrate available for the bacteria. A higher pH will benefit the ammonium-oxidisers while it will put the nitrite-oxidisers at disadvantage (Van Hulle et al. 2010). The dissolved oxygen level will also affect the process. At low levels of DO, both the ammoniumoxidisers and the nitrite-oxidisers will suffer from oxygen deficiency, but the nitrite-oxidisers will be affected more strongly. This is believed to be due to the fact that ammonium-oxidisers get more energy for the same amount of consumed oxygen (Van Hulle et al. 2010). By combining these factors it is possible to create an environment within the reactor where the nitrite-oxidisers are out-competed. This will allow partial nitrification to occur within the reactor which is necessary for the deammonification process. 2.3


A lot of research have been devoted to the anammox process in recent years, not least since the process seems promising for treatment of wastewater. In this section the unique properties of the bacteria involved in the process are described, followed by the catalysed reactions and the environment necessary for a well functioning process. 2.3.1

Anammox Bacteria

The anammox group of bacteria was first discovered in the early 1990s (J. Gijs Kuenen 2008), much to the surprise of the scientific community. They have an unique ability to oxidise ammonium with nitrite to form dinitrogen gas. Their name, anammox, is an abbreviation of this ability, 5