of biofilms attached to a solid surface. Granules have a lower polymeric content, are
usually more compact than microbial films and their formation follows specific distinct
steps (Brito and Melo, 1997b).
A great variety (in shape, dimensions and materials) of supports is used in biofilm
reactors, including: rough and rather large pieces of stones and gravel as the ones
employed in the first generation of trickling filters (Metcalf and Eddy, Inc. 1987); small
particles of sand, basalt or clay in fluidised bed and airlift reactors; particles with
complex artificial geometries made of light plastic material used in circulating bed
systems and agitated batch reactors; simple flat surfaces such as plastic disks in rotating
biological
Figure 10.2 Illustrative example of
plastic supports used in trickling
filters.
contactors. The specific area of the supports has been increasing over the last decades
(Figure 10.2) in order to improve the efficiency and compactness of the reactors: initially,
low specific surface areas of 100 m^2 /m^3 were available in trickling filters with rock
supports and in biological disks; nowadays, fine granular and porous supports with 500
m^2 /m^3 or more are in use in many submerged, fluidised or moving bed reactors.
The Importance of the SupportThe prerequisite for the formation of a biofilm is the adhesion of the microbial cells to the
support surface. Therefore, much effort has been put into investigating adhesion
mechanisms (Busscher et al., 1995, Teixeira and Oliveira, 1999, Azeredo et al., 1999).
However, in the more recent generation of biofilm-on-carrier reactors other processes
might have a greater influence on biofilm formation. The carriers are subjected not only
to higher turbulence and liquid shear than most fixed-support biofilms but also to the
erosion and abrasion promoted by particle collisions (Gjaltema et al., 1997). This means
that the selection of the supports for biomass immobilisation is of great importance to
obtain a stable biofilm leading to high reactor efficiency. In this way, the support must
favour bacterial adhesion, must have a high mechanical resistance, a low cost and a great
availability. To accomplish the first requirement, parameters like surface charge,
Biofilm reactors 293