Influence of Biofilms on Porous Media Hydrodynamics - Porous Media: Applications in Biological Systems and Biotechnology - page 183

necessary for the analysis (Nam et al . 2000). For instance, in one study, the
mean biofilm thickness estimates based on SEM images were about 60%-
82% less than those obtained through optical microscopy (Jean et al . 2004).
Environmental scanning electron microscopy (ESEM) can avoid some of the
artifacts possibly introduced during sample preparation and imaging com-
pared to SEM. ESEM does not necessarily require coating of the sample with
a conductive material and much less stringent vacuum conditions; however, it
can still suffer artifacts associated with the destructive sampling of the porous
media.
X-ray tomography techniques for imaging porous media are available,
but microscale imaging of biofilms in porous media has not yet been estab-
lished completely. Some promising work has been published in the recent
years, which shows the principal feasibility of performing microtomography
on porous media to image the transport and deposition of fluids and colloids
in porous media (e.g., Wildenschild et al . 2005; Li et al . 2006; Gaillard et al .
2007). However, the lack of x-ray-detectable absorption properties that are
specific to the presence of biofilms or suitable biofilm-labeling techniques have
slowed the progress in this field. Furthermore, the applicability of high-energy
techniques to biological systems is limited because of the potential damage
that can occur during exposure.
Nonoptical techniques such as NMR (e.g., Hoskins et al . 1999; Paterson-
Beedle et al . 2001; Seymour et al . 2004a,b, 2007; Metzger et al . 2006; McLean
et al . 2007; Hornemann et al . 2009), ultrasound (e.g., Shemesh et al . 2007),
or complex conductivity (Davis et al . 2006) imaging of biofilms have immense
potential for imaging biofilms in opaque media. However, their application
is currently restricted because of their limited availability, resolution, and
because of the interference that natural substrates, such as natural soils, sand,
or stone cores, have on their signal.
5.3.2 Porous Media Biofilm Reactors
A large variety of bench- and pilot-scale porous media biofilm reactors have
been used to evaluate the effect of biofilm accumulation on porous media
hydrodynamics and mass transport.
Columns, ranging from a few millimeters to several meters in length and
millimeter to approximately 1 m diameter, are probably the most commonly
used reactor types. Unfortunately, natural porous media themselves as well
as most column materials are opaque and do not lend themselves to direct
optical interrogation. Hence, high-optical quality microscopic flowcells and
flat-plate reactors, which can represent natural porous media or fractures
more or less closely, have been used frequently to investigate fundamental
processes of biofilm development in porous media (e.g., Cunningham et al .
1991, 1995; Sharp et al . 1999a, 2005; Yarwood et al . 2002, 2006; Nambi et al .
2003; Knutson et al . 2005; Ross et al . 2007; Willingham et al . 2008). These
reactors can consist of materials that contain certain patterns or can be filled