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Figure 3 Conceptual model of the biofilm in the reactor. Dotted arrows indicate transport of different substances. Reactions are marked with ellipses. The figure is redrawn from Hao et al. (2002). Oxygen and ammonium will diffuse from the boundary layer into the outer layer of biofilm where it will be used by the nitrifying bacteria to oxidise ammonium into nitrite. Not all of the ammonium will be used up by this process due to oxygen limitations. Some of the remaining ammonium will diffuse further into the anoxic layer of the biofilm together with some of the nitrite formed in the outer layer. These will act as substrates for the anammox bacteria which will oxidise the ammonium with the nitrite into dinitrogen gas. Some nitrate will be formed in the inner layer and this will diffuse out into the bulk-liquid. There will also be a net transport (not shown in the figure above) into the biofilm of alkalinity which is consumed for growth of the biomass (Hao, J. J Heijnen, and van Loosdrecht 2002). The transport rate of the different substances in and out of the biofilm depend on a number of factors. Some of the most important factors include concentrations in the bulk liquid, thickness, density and porosity of the different layers in the biofilm and temperature (Hao, J. J Heijnen, and van Loosdrecht 2002). 2.4.3

Process Parameters

The ASL (ammonium surface load), which is defined as the mass of ammonium-nitrogen each square meter of biofilm have to treat per day [ g NH +4 - N m−2 d −1 ] , can be used to express the current ammonium-load on the process. When the ASL increases the process’ oxygen demand will also increase. This is clear from the stoichiometry of equation 6. The DO level in the bulk liquid will be of crucial importance to the process efficiency since it affects the transport rate of oxygen to the biofilm. A too low level of DO will result in a too slow oxygen transferral rate into the nitrifying layer of the biofilm. This could create a deficit of nitrite within the anoxic layer of the biofilm. If this happens, some of the ammonium will remain unoxidised, which will lead to elevated levels of ammonium in the effluent (Hao, J. J Heijnen, and van Loosdrecht 2002). Figure 4 show the results of a simulation of how the ammonium in effluent varies with the DO (Van Hulle 2005). It is clear from the simulation made by Van Hulle that the system behaves as expected with high levels of ammonium in the effluent for low DO-levels. This 9