N2O production in a single stage nitritation/anammox MBBR process

to the liquid film, diffusion of substrates through the biofilm and substrate turnover in
the cellular mass (Ødegaard, 1993), see Figure 4. The diffusion dependent transport of
substrate from the liquid bulk and the diffusion within the biofilm is driven by a
concentration gradient. The substrate concentration profile decreases with biofilm
depth and has a downward curvature due to the substrate utilisation rate illustrated in
Figure 4, (Henze et al., 1997). Conversion rate on cellular level is dependent on
enzymatic processes. Available amount of enzymes handling the specific substrate will
decide how fast the conversion takes place. When substrate concentrations are low, the
accessible substrate S is the rate limiting factor and the rate equation is said to be of first
order with respect to S, see (2.4.3). At high substrate concentrations the substrate
turnover is limited by the amount of available enzymes. The reaction rate is now of zero
order since it is independent of the substrate concentration, see (2.4.4), (Warfvinge,
2008).
First and zero order approximations are given by the following equations:
,
·
(2.4.3)
,
(2.4.4)
where: rv,s describes the biological growth rate in a certain volume of biofilm at a certain
substrate concentration, (dimension M∙ L -3 ∙ T -1 ), µmax is the maximum specific growth
rate, (dimension T -1 ), XB is the concentration of biomass, (dimension M∙L -3 ), Ymax gives
the maximum yield constant, (dimension MXB∙Ms -1 ) and Ks is the saturation constant for
the substrate, (dimension M∙L -3 ), (Henze et al., 1997).
Figure 4. Schematic overview of substrate transport from liquid bulk phase to microorganisms
growing on carrier material. The concentration gradient profile in the biofilm depends on
transport into the biofilm and substrate utilisation rate in the film. (Adapted from Metcalf & Eddy,
2003, and Warfvinge, 2008).
14