CHAPTER TEN
BIOFILM REACTORS
LUIS F.MELO^1 AND ROSÁRIO OLIVEIRA^2
(^1) Department of Chemical Engineering, University of Porto, Rua
D.Roberto Frias, 4200–465 Porto, Portugal
(^2) Department of Biological Engineering, Universidade do Minho, 4710–
057 Braga, Portugal
ABSTRACT
After presenting the concept of biofilms, reference is made to their
importance in industry and health. Although biofilms are also well known
for their deleterious effects (biofouling), emphasis is here given to the
beneficial use of biofilms in wastewater treatment. The main types of
biofilm reactors are briefly described and the rôle of support material in
the adhesion and stability of biofilms is explained, taking into account the
mechanisms involved in biofilm attachment. Practical procedures for the
start-up of biofilm reactors are also mentioned.
Biofilm growth processes are described together with their properties,
structure and performance. The advantages and disadvantages of biofilm
reactors versus suspended biomass systems are discussed.
The main equations of the diffusion-reaction model are developed from
engineering science principles. Equations derived from the diffusion-
reaction model to calculate the reactor volume are presented, together
with experimental values of the kinetic parameters. Practical empirical
expressions or rules-of-thumb used in the design of fixed biomass reactors
are also given. An overall model to predict the growth rate of biofilms and
their final thickness or mass is established. The main problems concerning
biofilm reactor modelling are discussed and the “missing links” for an
optimised design are identified.
INTRODUCTION
Micro-organisms, like the vast majority of living creatures, tend to live in communities
and form their own specific habitats. It has been estimated that up to 90% of microbial
cells in nature grow within agglomerates (Costerton et al., 1987). Microbial films,
frequently designated as biofilms, are one of the types of biological agglomerates
(together with flocs and granules). They are communities of microorganisms attached to
surfaces, forming a porous matrix which contains the cells, the extracellular polymeric
substances (EPS) they produce, and a substantial amount of water (Characklis and
Marshall, 1990; Melo et al., 1992). In nature and industry, this biofilm concept is