reactors it is possible to use dense particles (e.g. basalt) with small diameters without
having high pumping power requirements because there is a drastic decrease in density
when the particles become covered by the biofilm. Another means to provide more area
for biomass attachment is to use rough and/or porous surfaces. Moreover, this is also a
way to provide niches to retain micro-colonies, shielding them from the effects of shear
forces (Bryers, 1987). For the purpose of colonisation, some authors consider surface
roughness as one of the most important parameters (Gjaltema et al., 1997), even more
important than internal surface area (Petrozzi et al., 1991). In order to accumulate large
quantities of biomass, the porosity must be suitably sized. According to Messing and
Opperman (1979 (a) and (b)), the adequacy of the pore size depends upon the cell
dimensions and its mode of reproduction: fission, budding or spores (with mycelial
growth). For instance, for microbes reproducing by fission at least 70% of the pores
should have diameters up to five times the largest major dimension of the microbial cell.
Other authors (Shimp and Pfaender, 1982) also confirmed that surface colonisation is
favoured when crevices are microbially sized.
However, micro-organisms retained in pores or niches can be subjected to diffusional
resistance to the flux of substrates and products. This is exemplified in a study already
mentioned (Pereira et al., 1999), where foam glass, pozzolana, clay and sepiolite were
compared as supports for an anaerobic consortium. Sepiolite, although having the highest
biomass retention, showed the lowest specific biological activity. This is a consequence
of the combined effect of the small pore size of sepiolite and the high amount of attached
biomass, promoting a deficient nutrient transport to the cells in the inner zones. Another
point to be considered is the accumulation of gaseous metabolites inside porous carriers
because this can induce the carriers washout, with a negative effect in the overall
performance. This problem is overcome if the carriers have large pores with large internal
porous volume because this enables the transport to be mediated also by convective flow.
The carrier concentration determines the available surface area for microbial
attachment, but at the same time acts as a controlling factor of biofilm formation. Biofilm
formation is balanced by biofilm detachment. In suspended bed reactors, where shear and
abrasion is considerable, the detachment is mainly caused by particle-particle collisions
(Heijnen et al., 1992). The collision frequency depends on the size, the relative velocity
and concentration of particles. In an airlift reactor, the rate of biomass detachment was
found to be linear with the concentration of particles up to a solids hold-up of 30% (v/v)
(Gjaltema, 1996).
Reactor Start-UpIn biofilm reactors there will always exist a competition between organisms growing in
suspension and organisms growing in the biofilm. Significant biofilm formation only
occurs under conditions where suspended cells are quickly washed-out (Heijnen et al.,
1992). Generally this is accomplished by starting the reactor in batch mode until a
significant amount of biomass is reached and then gradually increasing the dilution rate
(lowering the hydraulic retention time) until the maximum growth rate of the culture is
surpassed. From laboratory experience, in reactors with high shears (namely air-lift,
circulating bed and fluidised bed), it is advisable to pre-colonise the supports outside the
reactor in a sort of fed-batch mode, especially if the micro-organisms are slow growing.
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