hydrophobicity, porosity, roughness, particle diameter, density and concentration can be
of great importance.
An irreversible initial adhesion is the key factor for the development of a stable
biofilm, but the support ability for microbial colonisation is also of crucial importance.
This determines the colonisation velocity and ultimately the start-up rate of the reactor.
For many years the adhesion mechanism was tentatively interpreted in terms of DLVO
theory, with microbial cells considered as colloidal particles (Marshall et al., 1971, van
Loosdrecht et al., 1988). Accordingly, the net force of interaction arises from the balance
between the Lifshitz-van der Waals forces of attraction and the generally repulsive forces
generated during the approach of the electrical double-layers of the interacting species
(Oliveira, 1992). This repulsive character is due to the fact that most of the existing solid
materials display a net negative charge when immersed in aqueous solutions with pH
near neutrality. Bacterial cells are an example of negatively charged surfaces, especially
because in most cases they are only able to survive in mild pH conditions. Therefore, the
possibility for generating an electrostatic attraction is to utilise a positively charged
support. However, as was said before, in nature only very few materials, like some
metallic hydroxides, are able to display such behaviour. Other materials can be
engineered in order to be positively charged; this is feasible for laboratory purposes, but
it is not economically compatible with large-scale operation.
More recently it was demonstrated that the wettability, or in a reverse sense the
hydrophobicity, of solid surfaces strongly influences adhesion either of bacteria,
eukariotic cells or proteins (Margel et al., 1993, Prime and Whitsides, 1993, Wiencek and
Fletcher, 1997, Taylor et al., 1997, Teixeira and Oliveira, 1999). According to van Oss
(1997), hydrophobic interactions are usually the strongest of all long-range non-covalent
interactions in biological systems and can be defined as the attraction between apolar or
slightly polar entities (molecules, particles or cells) when immersed in water. It must be
noted that hydrophobic attraction can prevail between one hydrophobic and one
hydrophilic entity immersed in water, as well as between two hydrophobic surfaces (van
Oss, 1995). The interaction between two hydrophobic surfaces is favoured in aqueous
medium because they can establish a closer contact by squeezing the water in between. In
other words, an increasing degree of hydrophobicity enhances adhesion. This has been
confirmed by recent studies on the selection of supports for different types of biofilm
reactors and using different bacterial strains. One example is the relation found between
the higher degree of hydrophobicity of some polymeric materials and the increased
biofilm activity of a consortium of autotrophic nitrifying bacteria (Sousa et al., 1997). A
more direct relation was established between the number of initially adhered cells of
Alcaligenes denitrificans and the hydrophobicity of polymeric supports: the number of
adhered cells increased linearly with the increase in hydrophobicity (Teixeira and
Oliveira, 1999). A linear correlation was also obtained between the amount of attached
biomass of an anaerobic consortium and the hydrophobicity of the supports: foam glass,
pozzolana, clay and sepiolite (Pereira et al., 1999).
The newest generation of suspended carriers biofilm reactors were designed to have a
high biofilm area, which allows for higher loading rates and smaller space requirements.
The smaller the diameter of the carrier particles the higher the surface area available for
biofilm development. Particles with diameters as small as 0.2 mm have been used in
airlift reactors (Heijnen et al., 1992). It is interesting to note that even in fluidised bed
Multiphase bioreactor design 294