Anoxic, denitrifying biomass
Methanol
Rotating drum 0.10–0.20 Jansen and
Kristensen (1980)
Jansen (1982)Anoxic, denitrifying biomass
Nitrate
Rotating drum 0.02–0.14 Jansen and
Kristensen (1980)
Jansen (1982)Anoxic, denitrifying biomass
Nitrate
Rotating disk 0.11 Watanabe (1978)engineers do not seem yet to rely on such values for the efficient design of biofilm
reactors.
So far, there are no widely acceptable values for other model parameters, such as the
effective diffusion coefficient (Harremöes 1978; Henze and Harremöes 1995) and the
Monod saturation constant, which may be quite different from the values obtained in
suspended cell cultures, both of them having strong effects on the results yielded by the
model.
Some authors advocate that the diffusion coefficient in biofilms should be taken
approximately as 80% of the value in water, but there is much disagreement on the
experimental values published in different sources (for example, some values are even
larger than the coefficient in water). Table 10.3 presents only a few illustrative values of
the diffusion coefficients in different biofilm systems.
Such large differences in diffusivities may be due to a variety of factors (Fan et al.,
1990, de Beer et al., 1997) related to the nature of diffusing substance, the microbial
species present and the physical structure of the biofilms. The latter is in turn very much
dependent on the conditions under which the microbial layer was developed, among
others the nature and concentration of the substrate, the hydrodynamic parameters (fluid
velocity, turbulence), the presence of toxic substances, etc. Information about the precise
chemical, microbiological and hydrodynamic parameters that affected the history of
formation of the biofilm are missing in many of the publications reporting diffusivity
values. Some authors have tried, with a certain degree of success, to correlate the
diffusivity with biofilm properties such as the cell or biomass density in the attached
layer. Fan et al. (1990) presented the following correlation based on the experimental
data of a significant number of authors:
(44)which shows that the diffusion coefficient decreases with the increase in the cell density
(Xv, kg.m−^3 ) within the biofilm. Dw is the diffusion coefficient in water (variable Xv
seems to represent biomass dry density and not only cell density, in spite of the authors
having used this last name). Although this type of correlation is a step forward in the
estimation of diffusivities, relationships between the biomass density in biofilms and the
Table 10.3 Measured or estimated effective
diffusion coefficients in biofilms
Multiphase bioreactor design 314