processes, the disk is (almost) completely submerged at any time, in order to avoid
contact of the biofilm with the air. Trickling filter reactors were also adapted to the
degradation of volatile organic compounds (VOC) in gaseous effluents (Pederson and
Arvin, 1996, 1999; Peixoto, 1998), the biofilm being slightly humidified by water or
another liquid.
Membrane biofilm reactors, where the microbial layer is attached to a porous gas
permeable membrane, are a promising technology in some situations, including in VOC
removal, since they provide a more efficient method of supplying gas to the base of the
biofilm (Suzuki et al., 1993; Wilderer, 1995; Freitas dos Santos and Livingston, 1995).
Detailed descriptions and comparative analysis of the advantages and disadvantages of
the various types of biofilm reactors, as well as their performances vis a vis the
conventional activated sludge systems can be found in a number of text books and
research papers (e.g., Metcalf and Eddy, Inc. 1987; Eckenfelder et al., 1989; Lazarova
and Manem, 1994; Cabral and Tramper, 1994; Willaert et al., 1996).
Table 10.1 summarises the characteristic types of biofilm reactors and some are
schematically presented in Figure 10.1.
Most biofilm reactors operate in a continuous mode. An exception is the Sequencing
Biofilm Batch Reactor (SBBR), where the tank containing biofilm particles is
periodically filled with the feed liquid and discharged (Wilderer, 1995). The latter
remains in the reactor during the “reaction period”, after which it is drained out. This
operation mode is particularly favourable when consecutive processes are involved: for
example, in a nitrogen removal process, the biological nitrification (with aeration) and
denitrification (without aeration) steps can be carried out in the same unit provided it
contains biomass with nitrifying and denitrifying abilities. The operational flexibility of
such reactors has been demonstrated to be advantageous in certain processes.
Sometimes, reactors containing microbial granules without support, such as the Up-
flow Anaerobic Sludge Blanket reactor (UASB) are also treated as biofilm reactors,
mainly as regards the kinetics of substrate consumption (Lettinga and Hulshoff-Pol,
1992; Alphenaar et al., 1993; Brito and Melo, 1997a); aerobic granules have also been
developed and tested in nitrification processes (Tijhuis et al., 1995). The granules can be
considered to be dense flocs composed by microbes and extracellular polymers, and in
fact the problems they pose to the modelling of substrate diffusion and reaction are
similar to those
Table 10.1 Characteristic types of biofilm reactors
Fixed bed biofilm reactors Expanded bed biofilm reactors
–Trickling filter –Fluidised bed reactor (& inverse fluidis.)
–Submerged filter (downflow or upflow) –Moving bed reactors:
–Rotating disk reactor –air lift reactor
–circulating bed reactor–Membrane biofilm reactor (^)
–Sequencing batch biofilm reactor (SBBR)
Reactors containing dense microbial granules not attached to solid supports:
Biofilm reactors 291