That is to say, the culture medium must be periodically replaced by fresh medium,
according to the microbial growth rate (Teixeira and Oliveira, 1998). Otherwise, if
directly inserted in the reactor, a great number of carriers will remain bare. In the case of
methanogenic bacteria, one method to encourage their growth is to supply a substrate that
may be directly metabolised such as methanol (Balaguer et al., 1992). The start-up period
of anaerobic reactors can also be reduced with the adaptation of the inoculum to the
specific substrate properties. It is preferable therefore to use a mix of several sources of
active biomass instead of an inoculum from one source only.
When the biofilms are formed by heterotrophs producing large amounts of
exopolymers it is common for thick biofilms to develop in the quieter parts of the
reactors, as reported by Gjaltema et al. (1997). In extreme cases this can lead to the
clogging of the reactor. Those situations become worse when working with high C/N
ratios, which enhance the production of exopolymers. So, the C/N ratio must be
controlled to avoid such undesirable biofilm formation.
BIOFILM PROPERTIES, FORMATION AND PERFORMANCE
Composition and StructureThe following characteristics may be considered typical of microbial films:
− they are slimy layers with gel-like consistency and viscoelastic rheological behaviour;
− their colours vary from translucid white or yellowish to dark brown or black, depending
on the type of prevailing micro-organisms and substrates;
− they can have thicknesses that go from a few micra (films formed in liquids with low
substrate concentration) to some millimetres or even centimetres (e.g., in the so-called
“white waters” of pulp and paper mills);
− most of the wet biofilm mass is water (frequently, more than 90%);
− the mass of extracellular polymers, typically containing polysaccharides and
glicoproteins, may represent 70% or more of the dry weight of the biofilm;
− the fraction of micro-organisms in biofilms depends on the metabolic characteristics of
the microbial populations and on the substrate concentration, and it may correspond to
10% to 50% of the dry biomass.
The biofilm matrix is a natural structure built by the micro-organisms in what seems to be
a way of protecting themselves from external aggressions—either physical, chemical or
biological—and of finding suitable sites where nutrients can be more easily available.
The extracellular polymers contribute both to the initial adhesion of the microbial cells to
the surface and to the internal cohesion of the matrix due to the links established between
the polymeric chains. This network tends to offer some resistance to the diffusion of
components, be they toxic compounds or nutrients, and it can favour the adsorption of
molecules as well as the capture and accumulation of external particles within the
biofilm.
Biofilm structure has been a matter of considerable discussion in recent years (Lappin-
Scott and Costerton, 1995; de Beer and Stoodley, 1995; van Loosdrecht et al., 1995). The
surface of most biofilms is wavy, sometimes with protruding visco-elastic filaments that
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