Influence of Biofilms on Porous Media Hydrodynamics - Porous Media: Applications in Biological Systems and Biotechnology - page 197

the changes in permeability observed during studies in which biological growth
reduced the permeability of porous media or fractures.
The reduction of porous media permeability is generally believed to occur
through the accumulation of biomass and polysaccharides. Permeability reduc-
tions of three orders of magnitude or less are generally reported, but greater
reductions have been reported. The largest reductions reported have been
clearly associated with the coprecipitation of carbonate minerals (VanGulck
and Rowe 2004; Castegnier et al . 2006).
Like biofilm accumulation, permeability reduction is usually not homo-
geneous but rather spatially and temporally varied with higher produc-
tion of biomass, and thus greater permeability reduction, at the influent
(Rinck-Pfeiffer et al . 2000; VanGulck and Rowe 2004; Castegnier et al . 2006).
Permeability reduction also appears to be more pronounced in fine-textured
materials than in coarse-textured ones (Vandevivere 1995; Vandevivere et al .
1995).
Once achieved, decreased porous media permeability can often be main-
tained even during periods of environmental stress for the biofilm organisms,
such as starvation, toxic upset or similar, as demonstrated in one- and two-
dimensional flow fields in the laboratory (Cunningham et al . 1997; Kim and
Fogler 2000; Kim et al . 2006) as well as in a field-scale demonstration (Cun-
ningham et al . 2003). This persistence is usually attributed to the presence
of large amounts of EPS that do not degrade readily. These observations are
supported by Kim and Fogler who observed no EPS degradation in batch
experiments for a period of about 2 years (Kim and Fogler 1999).
5.4.4 Dispersion and Diffusion
Just like permeability and porosity, dispersivity (which relates the dispersion
coecient to velocity) is influenced by biofilm growth in porous media. Most
of the experiments reveal a two- to eight-fold increase in dispersivity (Sharp
et al . 1999b, 2005; Hill and Sleep 2002; Bielefeldt et al . 2002b; Arnon et al .
2005b), although order of magnitude changes in dispersivity have also been
observed (Taylor and Jaffe 1990a; Bielefeldt et al . 2002a; Seifert and Enges-
gaard 2007). In general, dispersivity increases over time in biofilm-affected
porous media but often reaches semistable values once the biofilm has reached
a pseudosteady state (Sharp et al . 1999a, 2005; Bielefeldt et al . 2002b). Hill
and Sleep stated that the dispersion coecient increased logarithmically with
hydraulic conductivity reduction in biofilm-affected fractures (Hill and Sleep
2002).
Increases in dispersivity have mostly been attributed to increases in tor-
tuosity of the porous medium, the development of no flow zones, or increased
influence of diffusive transport into and out of the developed biofilms (see for
example, Sharp et al . 1999a, 2005; Seymour et al . 2004a,b). Seymour et al .
demonstrated that diffusion out of no flow zones can clearly influence the
macroscale dispersion in biofilm-affected porous media (Seymour et al . 2004b).