obviously too simple and should be extended to include organic debris and small
inorganic particles (clays, metallic oxides, etc.) captured by the polymeric network, as
well as adsorbed compounds. Microbial cells in biofilms are often bacteria, but living
algae and fungi often appear in significant quantity in such matrices. In some cases,
macro-organisms such as mussels and barnacles may attach to the microbial layer. Micro-
organisms in biofilms can survive in extreme conditions, with pH ranging from 0 to more
than 13, temperatures from –10°C to 120°C or higher, and even in ultra-pure water
(Flemming, 1991).
The immobilisation of cells on supports without the formation of an extracellular
polymeric matrix will not be considered here as a biofilm process.
Microbial films can be detrimental not only to health (infections on teeth, prosthetic
implants, urinary catheters, etc.), but also to engineered systems such as pipes, pumps,
valves, reverse osmosis membranes, heat exchangers and ship hulls (Characklis and
Marshall, 1990; Flemming and Geesey, 1991; Melo et al., 1992; Bott, 1995; Lapin-Scott
and Costerton, 1995). Furthermore, since they grow on almost any surface immersed in
aqueous environments offering a minimum availability of nutrients, they can
inconveniently develop on the walls of biological reactors as well as on other surfaces
existing inside fermenters, such as agitator blades, pH probes, etc. The purpose of the
present chapter is, however, to focus on the beneficial aspects of biofilms in reactors,
where the microbial layers can be used to degrade unwanted compounds or to obtain
desired products. The most common examples in the literature are wastewater treatment
facilities containing attached biomass (Harremöes, 1978), but there are also other
industrial processes using biofilm technology, such as the recovery of metals through
bacterial leaching of ores or the production of vinegar, ethanol (Dempsey, 1990) and
citric acid (Briffaud and Engasser, 1979).
BIOFILM REACTORS—TECHNOLOGICAL FEATURES
Types of Biofilm ReactorsA biofilm reactor is a biological reactor with fixed biomass. It is usually filled with
particles of a carrier material—the “support”—where the microbial film is attached. If the
particles are porous, the film forms not only on the surface but also within their pores. A
limiting case of a very simple biofilm reactor is a duct where the biomass develops on the
walls: such a situation actually occurs in sewers, where the adhered biomass may act as
an additional wastewater treatment biological reactor, although in some cases it also
contributes to the deterioration of the wall material. Another example is the case of
biofilms formed on rocks in contact with mountain streams that help to purify these
waters.
In industrial microbial film reactors, the fluid flows in contact with the biofilm
particles promoting the exchange of nutrients and metabolic products between the fixed
biomass and the surrounding fluid. There is also exchange of mechanical energy between
the two media: on one hand, the liquid exerts hydrodynamic forces on the biofilm
enhancing both the detachment of biomass and the compactness of biofilms (these effects
are particularly important in turbulent flow systems); on the other hand, the roughness
Biofilm reactors 289