PREFACE
The term bioreactor was not widey accepted for many years and initially, the term
fermentor was preferred. The science-based approach to studying fermentors, using all
the available knowledge from Chemical Engineering Science on the one hand, and the
widespread use of reactors to perform enzyme-catalysed reactions on the other, gradually
made the term bioreactor the most appropriate term to encompass every container where
a controlled biological reaction may take place.
A bioreactor is defined as “A vessel in which a biological reaction or change takes
place, usually a fermentation or a biotransformation, including tank bioreactors,
immobilised cell bioreactors, hollow fibre and membrane bioreactors and digestors”
(“Biotechnology—from A to Z”, William Bains, Oxford Univ. Press, 2000).
For a long time, bioreactors were able to accomplish a single biotransformation, and to
contain a single species of an organism. Even when a mixed microbial population was
present, the predominance of a certain species allowed for a model simplification, where
only the dominant species was taken into account.
An attempt to classify bioreactors in two main groups according to the source of
power and the degree of homogeneity was made by the Working Party of Bioreactor
Performance of the European Federation of Biotechnology (“Physical Aspects of
Bioreactor Performance”, Dechema, W.Crueger (Ed.), 1987). Even so, the authors noted
that “...Many designs of bioreactor attempt to keep the whole of their volume
homogeneous. Most stirred volumes are in this category although, as the scale increases,
total homogeneity becomes progressively more difficult. There is no such thing as a true
plug-flow bioreactor (except for immobilised bioconversions), since the characteristic
fermentation time is of a magnitude greater than the outflow rate. Nevertheless, some
bioreactors do aim to cycle the conditions; the deep shaft effluent treatment reactor is a
good example. Oxygen is absorbed at the high pressures in the bottom, while carbon
dioxide is desorbed best near the surface. In other configurations the designer may
consciously decide to let the conditions vary within acceptable limits (e.g. the
temperature in a reactor with an external cooling loop). These designs represent an
intermediate case.”
The omnipresence of mixed populations in natural systems has always been the rule
rather than the exception. Every bioprocess engineer working with single species cultures
is deeply conscious of how relentless his strife against external contamination has to be.
Today, there is a generalised recognition of wastewater treatment systems as mixed
population bioreactors, where a cohort of protozoa and many bacterial species cohabit
and co-operate to remove contaminants from water. Furthermore, with the enormous
liquid volumes dealt with by these systems—many billions of gallons of wastewater are
processed each day around the world—there is no more excuse for excluding mixed
species bioreactors as an important field of research.
Even when a single species is present, there is often the need for considering different
developmental stages or physiological states, of which only one is responsible for the