Influence of Biofilms on Porous Media Hydrodynamics - Porous Media: Applications in Biological Systems and Biotechnology - page 190

biofilm plugs. As biofilms grow, they initially decrease the free pore space;
however, the overall porosity might not be significantly affected. If biofilms
form on the inside of large pores or fractures, even a large change in porosity
due to the presence of relatively thick biofilms can have a negligible effect
on overall permeability. In contrast, if biofilms form in regions where their
potential to affect fluid flow is great, such as pore throats, fracture entrances,
and so on, a small change in overall porosity can have a significant effect
on localized or overall permeability. In addition, as discussed by Sharp et al .
(2005), Paulsen et al . (1997) and in Section 5.4.5, areas basically excluded
from flow through thick biofilm growth can become accessible to flowing fluid
owing to sudden detachment events, such as sloughing of large biofilm clus-
ters that previously had blocked certain pores. Hence, methods, which allow
spatially resolved measurements of porosity and differentiation between bulk
and effective porosity are necessary to assess spatial and temporal changes.
Changes in porosity have mostly been assessed through direct microscopic
observation combined with image analysis or are based on tracer breakthrough
curves. Changes in media porosity can vary widely depending on the pore
size distribution of the media as well as the method of measurement. Seifert
and Engesgaard (2007) discuss problems with tracer breakthrough curve-
based porosity estimates of biofilm-affected porous media in detail and discuss
the validity of dual-porosity models for the description of hydrodynamics in
biofilm-affected porous media (Seifert and Engesgaard 2007). Along with many
other authors, Seifert and Engesgaard and Bielefeldt et al . point out that tradi-
tional porosity, permeability relationships, such as the Kozeny-Carman equa-
tion, generally underpredict the change in hydraulic conductivity (Bielefeldt
et al . 2002b; Seifert and Engesgaard 2007).

5.4.3 Permeability
By far the most frequently used parameter to describe the influence of biofilm
formation on porous media properties is permeability ( k ), expressed in dimen-
sions of L 2 (length squared, e.g., m 2 or meter squared). In many published
research papers permeability is expressed in the form of hydraulic conduc-
tivity ( K , L/T, e.g., m/d or meters per day). The conversion is simple and
uses the fluid density ( ρ , M/L 3 , e.g., kg/m 3 ), gravitational constant ( g , L/T 2 ,
e.g., m/sec 2 ), and viscosity ( µ , (M/L)/T, e.g., (kg/m)/sec) according to the
following equation:

µ

k = K

ρ • g

For the purpose of comparing different studies (Table 5.1), 15 ◦ C was uti-
lized as a reference with water as the fluid of interest so that ρ = 999.099
kg/m 3 ; µ = 1.14

10 − 3 (kg/m)/sec; g = 9.807 m/sec 2 . Table 5.1 summarizes

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