Biomedical Engineering Reference
In-Depth Information
biofilms ranging from biofilms grown directly on membrane surfaces (Siegrist
and Gujer 1985), on detached biofilm, or on activated sludge filtered by a mem-
brane (Matson and Characklis 1976) as well as to model systems such as yeast
cells entrapped in alginate (Cronenberg and van den Heuvel 1991). Beuling
and colleagues (1998) applied nuclear magnetic resonance (NMR) technique
to determine the self-diffusion of water in artificial and natural biofilms. f D
values for water within the range of 0.59-0.89 were measured depending on
biofilm density.
Several studies determined diffusion coe cients for biofilms (Cronenberg
and van den Heuvel 1991; Lewandowski et al. 1991; Bryers and Drummond
1998). For example, Yano and colleagues (Yano et al. 1961) evaluated biopel-
lets from Aspergillus niger, which were used for citric acid production, where
f D was calculated on the basis of overall conversion rates resulting in val-
ues ranging from 0.11 to 0.9 for aggregate densities of 170 and 30 kg/m 3 ,
respectively. The variability of these results was, however, high.
Detailed studies and accumulating data from traditional light microscopy
or from laser confocal scanning microscopy confirmed that biofilm struc-
ture determines the mass transport mechanism within the biofilm and mass
transport rates at the surface of the biofilm (de Beer et al. 1994b; Bishop
and Rittmann 1995; Yang and Lewandowski 1995; Bishop 1997). As biofilms
display different structures, it was assumed that the structural differences
reflect biofilm formation. Furthermore, environmental conditions, for exam-
ple, hydrodynamics, seem to have an impact on these structures as well as
on the chemical composition of the contacting medium and the chemical and
physical properties of the surface supporting the biofilm. The fundamental
processes, for example, attachment/detachment and growth occurring within
the biofilms are also indirectly affected. For example, it is known that biofilms
grown at high shear stress develop elongated microcolonies (Lewandowski
and Stoodley 1995). It is also known that dense biofilms develop either
as a result of high shear stress or as a result of starvation (Beyenal and
Lewandowski 2000).
Quantitative structural parameters of biofilms are currently solely available
from the data worked out from microscope micrographs. The best of these are
derived from confocal laser scanning microscopy as it allows the acquisition
of images of fully hydrated biofilms at high spatial three-dimensional resolu-
tion. Many researchers use image analysis to calculate values of parameters
characterizing biofilm structures by available technologies such as Bioquant
Meg-IV software (R&M Biometrics, USA), MOCHA Image Analysis Software
(Jandel Scientific, San Rafael, California), and IDL Interactive Data Language
(Research System Incorporated, Boulder, Colorado). Some researchers devel-
oped their own software, for example, COMSTAT (Heydorn et al. 2000) and
image structure analyzer (ISA) (Yang et al. 2000). These parameters include
fractal dimension characterization of the variability of biofilm structures as
well as porosity/voids array. Some researchers calculated more exotic param-
eters such as energy or textural entropy (Beyenal et al. 2004b).
Search WWH ::




Custom Search