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Strous, J. Gijs Kuenen, and Mike S.M. Jetten 1999). Other substances, such as phosphate, acetate, glucose, and pyruvate have also been shown to have negative effects on the process (van de Graaf et al. 1996). Most notable of the inhibitory substances is oxygen. Even concentrations as low as 0.01 mg l −1 can inhibit the process completely (van de Graaf et al. 1996). However, the inhibiting effect of oxygen has been shown to be reversible by experiments with intermittent aeration (M. Strous et al. 1997). pH and temperature have effects on the efficiency of the process as well. A range of suitable pHvalues have been found to be 6.3 – 8.3 with an optimum close to 8.0 (Van Hulle 2005). The optimum temperature depends on the specific type of anammox bacteria and optimum has been found to be 12 ⁰C, 15 ⁰C and 37 ⁰C in different studies (Rysgaard et al. 2004; M. Strous et al. 1997; Egli et al. 2001; Dalsgaard and Thamdrup 2002). Another factor of importance is the concentration of biomass. The anammox process have been shown to be active only in biomass concentrations of more than 1010 – 1011 cells per ml (M. Strous et al. 1999) A number of hypotheses of the reason for this has been put forward, but they still remain unproven. 2.4


By combining partial nitrification with the anammox process it is possible to convert most of the ammonium in a wastewater stream into dinitrogen gas, and this is the basis of the deammonification process. As a first step, partial nitrification is used to oxidise part of the incoming ammonium into nitrite. This provides the anammox bacteria with the nitrite they need in order to oxidise the remaining ammonium into dinitrogen gas. For an optimal performance of the deammonification process, only slightly more than half of the incoming ammonium should be oxidised into nitrite by the partial nitrification sub-process. It is clear from the stoichiometry of equation 5 that the anammox process needs a ratio of 1.32:1 of nitrite to ammonium. This means that the partial nitrification should ideally convert 57% of the incoming ammonium into nitrite. The overall reaction for the deammonification system is given by the combination of equation 5 and equation 1. This yields the following equation which can be used to describe the entire deammonification process (Khin and Annachhatre 2004). 1 NH +4 0.85O 2 0.435 N 2 0.13 NO -31.3 H 2 O1.4 H +


The deammonification process can convert most of the incoming ammonium into dinitrogen gas, but some of the ammonium will be converted into nitrate (equation 6). This formation of nitrate sets a theoretical cap of 87% as the maximum total nitrogen removal possible to achieve with the deammonification process with the described mechanisms(K. A. Third et al. 2005). However, in some systems denitrification will also occur. This could potentially increase the maximum possible total nitrogen removal above 87% (Trela et al. 2009). 2.4.1

Deammonification Process Configurations

The two sub-processes partial nitrification and anammox can be combined into a deammonification process in more than one way (Fig. 2). The first possibility is to have two reactors, one for each subprocess. In this configuration the first reactor is housing the partial nitrification sub-process. The Sharon process (Hellinga et al. 1998) is well suited for this task. The outflow from this reactor is fed into the next reactor, which contains the anammox process (van Dongen, M. S. M. Jetten, and Van Loosdrecht 2001). 7