Influence of Biofilms on Porous Media Hydrodynamics - Porous Media: Applications in Biological Systems and Biotechnology - page 189

biofilm-affected porous media, where electron acceptor and donor supply is
the greatest (see case study below as well as Hosokawa et al . 1992; Chen
et al . 1994; Jennings et al . 1995; Ince et al . 2000). Decreasing availability
of electron donors, acceptors, or other important nutrients (e.g., phospho-
rous or nitrogen) can be as responsible for a decrease in biofilm formation
as the presence of inhibitory compounds. There are definite reports that the
availability of carbon can influence the formation of biofilms in porous media
(Hand et al . 2008). However, there are also reports that indicate increased
biofilm growth in the euent regions as well as where carbon might be lim-
ited (Kim et al . 2006; Wheeler 2009). This might be partially explained by the
increased availability of another limiting growth factor, for example, the avail-
ability of oxygen. In column systems, which are repeatedly opened or which
are connected to gas-permeable tubing, such as silicon tubing, the entry of
oxygen into the systems can result in increased microbial growth in those
regions.
Changes in hydrodynamics in biofilm-affected porous media are not always
solely due to biofilm growth itself but also often due to the biofilm induced
formation of minerals, such as di- or tri-valent (e.g., Fe, Ca, Mg) carbonate,
sulfate, sulfide, and phosphate minerals (Mclean et al . 1997; Benner et al . 1999;
Rinck-Pfeiffer et al . 2000; VanGulck and Rowe 2004). In at least one case of
investigating the effect of biofilm growth in porous media, the majority of the
“clog material” was identified as calcium carbonate (Rowe et al . 2000).
5.4.2 Porosity
When describing the porosity of biofilm-affected porous media one has to
consider at least three types of porosity: (1) the overall porosity of the porous
medium, (2) the effective porosity of the porous medium, and (3) the internal
porosity of the biofilm itself.
The internal porosity of biofilms was initially believed to be negligible.
Biofilms were treated as hydrogels with little relevant internal structure until
the mid-1990s when it was demonstrated that advective flow can occur within
biofilms through channels formed during biofilm growth (Stoodley et al . 1994;
Okabe et al . 1998). Such channels can be tens of micrometers in size, allowing
for significant advective mass transport.
However, in most cases the influence of biofilm growth on porosity has
been investigated on scales at which the internal porosity of biofilms becomes
insignificant. Hence, in most experimental and modeling work, the overall (or
bulk) porosity and effective (i.e., available for advective fluid flow) porosity
have been assessed. However, one recent study suggests the importance of
including the internal porosity (and thus permeability) of biofilms to more
accurately describe permeability changes in biofilm-affected porous media
(Thullner and Baveye 2008).
Differences in effective and overall porosity can be drastic, and their effect
on localized and bulk permeability depends on the location of the formed