Biomedical Engineering Reference
In-Depth Information
estimating kinetic parameters in activated sludge models, and the relation between floc
diffusion and kinetic parameters, in particular half-saturation coefficients, is discussed in the
literature [18-20]. Essentially it has been observed and suggested that mass transport
limitations are more important in larger flocs than in smaller flocs and that therefore the
observed half-saturation coefficients are bigger for larger flocs than for smaller flocs. Kinetic
parameters, and in particular half-saturation coefficients, do thus not represent the intrinsic
kinetics of activated sludge flocs in activated sludge modelling. Activated sludge models are
used to simulate the macro-kinetic behaviour of activated sludge systems, rather than the
intrinsic kinetics and micro-environment of the sludge floc. For this reason floc structure is
not taken into account in state of the art activated sludge models belonging to the ASM model
family.
The Role of Biofilm Structure in Biofilm Modelling
The picture of biofilm structure has changed over the past decades. Initially biofilms were
thought to have a homogeneous structure that covers the substratum as a film, i.e. a thin layer
(Fig. 2). This picture was supported by low resolution light microscopy and scanning electron
microscopy (SEM) [21]. The picture of homogenous biofilm structure has led to the so called
continuum approach in biofilm modelling.
Bulk Liquid
Biofilm
Substratum
Figure 2.
Schematic representation of homogenous biofilm (continuum approach).
In the continuum approach it is assumed that the biomass is evenly distributed in the
biofilm; concentration gradients are supposed to occur perpendicular to the substratum only.
Biofilm models based on the continuum approach are thus one-dimensional. Until the mid
nineties practically all biofilm models were based on the continuum approach [22]. The most
prominent example of a one-dimensional biofilm model based on continuum approach is the
multispecies biofilm model proposed by Wanner and Gujer [23]. This model was later
extended and implemented in the computer program AQUASIM [24].
The picture of a homogenous biofilm structure has changed since the mid-nineties,
because advanced microscopic examination tools, namely the confocal scanning laser
microscope (CSLM), have revealed that biofilms have a complex, heterogeneous three-
dimensional structure with pore channels and cell clusters (see [22] and references there in).
The picture of homogenous biofilm structure painted by 1D biofilm models in the continuum
approach appears therefore to be inadequate. It also has been shown that mass transport into
biofilms is not purely diffusional as assumed in 1D biofilm models; in contrast convective
mass transport can also take place through pores and channels of the biofilm [25].
