Biomedical Engineering Reference
In-Depth Information
In most cases, the dispersivity in biofilm-affected porous media has been
estimated on the basis of tracer tests and subsequent parameter fitting in
mathematical models (e.g., Taylor and Jaffe 1990a; Sharp et al . 1999b;
Kildsgaard and Engesgaard 2001; Hill and Sleep 2002; Bielefeldt et al . 2002a,b;
Sharp et al . 2005; Arnon et al . 2005b). However, recently more advanced tech-
niques have been developed.
Magnetic resonance microscopy (MRM) techniques are ideally suited to
provide nondestructive, spatially, and temporally resolved measurements of
hydrodynamic dispersion and velocity in porous media while allowing limited
biofilm imaging (10-100 sec of
m resolution). The recent work by Seymour
et al . clearly demonstrates the strength of this technique and already indicates
the development of anomalous hydrodynamic dispersion in biofilm-affected
porous media (Seymour et al . 2004b; Seymour et al . 2007).
In the future, this technique will allow for the correlation of local and bulk
dispersivity with the distribution of biofilms in porous media environments.
This capability will ultimately allow for the development of improved mod-
els for the description of reactive transport in biofilm-affected porous media.
MRM techniques as well as other techniques capable of spatially resolving
dispersion and diffusion phenomena will also allow for the evaluation of the
importance of diffusive transport processes in biofilms.
The importance of diffusion on biofilm processes in general has been
reported widely (Williamson and McCarty 1976a,b; Debeer et al . 1994;
Stewart 1996, 1998; Xu et al . 1998, 2003; Stewart and Costerton 2001) and
its influence on reactive transport in porous media affected by biofilms should
be considered to explain micro- and macroscale processes (Seymour et al .
2004a,b, 2007).
µ
5.4.5 Constant Head versus Constant Flow
Most experimental systems for the investigation of the influence of biofilm on
porous media hydrodynamics have been operated under constant flow con-
ditions by using pumps. Although there are situations in industry and the
environment where constant flow conditions are encountered, constant head
conditions are more commonly encountered in subsurface environments. For
instance, most aquifers are subject to constant head conditions. This section
will give an example that clearly demonstrates differences in biofilm develop-
ment and its influence on porous media hydrodynamics under constant flow
and constant head conditions.
Significant biofilm growth in porous media will reduce the pore space avail-
able for advective flow. Figures 5.7 and 5.8 provide a comparison of tracer
studies and images, which compare solute transport through two-dimensional
mesoscale (3.8 cm
8.5 cm) porous media reactors. Similar studies demon-
strating the utility of these reactors for monitoring reactive and nonreactive
transport in porous media were conducted with Vibrio fischeri (Sharp et al .
2005). It became apparent in the studies by Sharp et al . (2005) that electron
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