Multiphase Bioreactor Design

Diffusing
component

Diffusion

coefficient×10^9 (m^2 /s)

Df (biofilm)/ /Df

(water) (%)

References

Oxygen 2.2 (20°C) 105% Kissel et al. (1984)

Oxygen 2.55 (20°C) 120% Williamson and
McCarty (1976)

Oxygen 1.75 (29°C) 66% Nogueira et al. (1998)

Lithium chloride 0.33 (35°C) 33% Nilsson and Karlsson
(1989)

Glucose 0.08–0.63 (20°C) 15%–117% Onuma and Omura
(1982)

reactor operating conditions are needed to predict diffusion coefficients for proper reactor
design.
The Monod saturation constant in biofilms is often considered similar to the one in
cell suspensions, but this may be quite erroneous on account of the microenvironment
surrounding the micro-organisms being quite different in a biological layer from that in a
dispersed cell culture, leading to distinct metabolic states (Fletcher 1992a, 1992b).

Mathematical Models for Multisubstrate and Multispecies Biofilms

The diffusion-reaction model presented above applies to simple situations where only one
microbial species and one substrate are present. Sometimes, for the sake of simplicity,
while incurring possible errors, the model has been applied to mixed cultures where one
or more substrates are involved in the biochemical reactions. Empirical parameters can be
obtained to describe the consumption of substrates in that specific situation, but it will not
be advisable to try to apply those values to any other case. More complex models were
published and tested for the cases where multisubstrate and/or multispecies are present,
as well as when inhibiting factors or particulate material affect the biological process. A
limited number of references is given below, for the reader interested in more specialised
models or in further developments of the above model: Wanner and Reichert, 1996;
Reichert, 1994, 1995; Coelhoso and Rodrigues, 1995; Bryers, 1993; van Ede et al., 1993;
San et al., 1993; Droste and Kennedy, 1986; Wanner and Gujer, 1986.
Comparison of the behaviour of plug flow and continuous stirred reactors may be
found in several published sources (Moser 1988; Rodrigues et al., 1983).
The recent investigations on the composition and spatial distribution of biomass and
void spaces within biofilms, using microelectrodes, confocal laser microscopy and
molecular probes, led to the development of new multidimensional modelling strategies
and techniques which may contribute to a better understanding and quantification of the
activity, population dynamics, stability, morphology of microbial films, as well as of the
dynamics of their transient growth processes (Wanner 1995; Ritmann et al., 1999;
Picioreanu et al., 1999; Noguera et al., 1999; Hermanowics, 1999).

Practical Design Procedures for Reactor Calculation

Biofilm reactors 315