the solid surface, providing them with the essential elements for their metabolism. As a
result, the biofilm mass increases by means of the growth and multiplication of microbes
together with the production of the polymeric matrix. Above a certain thickness, the
microbial metabolism inside the biological layer can become limited by the internal
resistance to mass transfer of substrates, and also by the fact that the micro-organisms
located near the biomass-liquid interface will tend to consume most of the substrate
before it reaches the inner zones of the biofilm. At the same time, products from
microbial metabolism are carried away from the biofilm. Apart from the initial period of
surface inoculation, the transport and adhesion of new micro-organisms to the biofilm
seems to have a minor role in the build up process, as compared to the growth processes
that depend on substrate availability (Bott, 1995).
The processes described above contribute to the growth of the biological layer.
Simultaneously, detachment processes occur, often promoted by the hydrodynamic forces
of the fluid flowing over the biofilm surface, resulting in its erosion or even in the
disruption (“sloughing off”) of portions of the attached biomass. Even in the absence of
fluid shear forces thicker biofilms may also slough off on account of their weaker internal
cohesion and lack of nutrients in the inner zones. The competition between the biological
growth and the detachment phenomena leads to a final balance where a maximum
average thickness (subject to fluctuations due to the periodic detachment) of the biofilm
is reached, which is sometimes considered a pseudo-steady state.
The structure and biological activity of microbial films depend on the history of their
formation, including not only the specificities of the microbial populations involved in
this process, but also the effects of environmental parameters such as the liquid velocity,
temperature and pH, the nature and concentration of the substrate(s) and the surface
properties (Bott, 1995; Characklis and Marshall, 1990; Messing and Oppermann, 1979a;
Mott and Bott 1991; Vieira et al., 1993). Biofilm development is favoured when the
temperature and pH approach the optimum values for microbial growth, although it
should be stressed that the conditions inside the biofilm are different from those in the
surrounding liquid. The pH is particularly affected by the metabolic products excreted by
the micro-organisms in the attached layer. A well documented example is the nitrogen
biological removal process, which includes the nitrification and denitrification steps.
While the pH inside nitrifying biofilms tends to decrease due to the production of H when
is oxidised to , the opposite occurs in biofilms containing denitrifying micro-
organisms (Harremöes, 1978).
The composition of the liquid, particularly the nature and concentration of nutrients
and substrates, has a direct influence on biofilm development. A higher carbon/nitrogen
ratio seems to favour bacterial attachment and the production of biopolymers (Veiga et
al., 1992), leading to an increase in biomass concentration in the reactor. Experiments
were carried out where the substrate was suppressed from the liquid (Bott, 1995; Vieira
and Melo, 1995) and, as a result, part of the microbial film detached from the surface
within a few hours or, at the most, one day. This period of time was remarkably increased
when small clay particles (around 10 μm) were incorporated in the biofilm during its
development (Vieira and Melo, 1995).
The liquid velocity in contact with the biofilm is a major parameter that affects the
dynamics of its development and its structure. In turbulent flow, higher velocities tend to
originate thinner biofilms: although nutrient mass transfer to the biofilm surface increases
Multiphase bioreactor design 298