Biomedical Engineering Reference
In-Depth Information
of time as indicated by elevated absorbance readings at large eluted volumes
after 7 and 9 days. This prolonged retention of dye is likely due to no flow
regions of the biofilm-affected reactor and diffusion-limited transport of dye
back into the main flow path after the initial front of dye has passed through
the reactor. Prolonged retention of dye is even more evident in the biofilm
reactor operated under constant head. While the initial breakthrough of dye
seems to be relatively unaffected under constant head conditions throughout
the experiment, dye retention due to slow (likely diffusion-limited) transport of
dye out of the biofilm toward the end of each tracer study becomes significant
after 3 days already.
It is clear from the research in our laboratories that there are significant
differences between biofilm growth patterns under the different flow regimes
and that there is a need for a more thorough understanding of the reasons for
these differences. Under constant flow conditions a decrease in effective poros-
ity will increase the fluid velocity and thus in most cases the influent pressure
as well as shear stress within the porous medium (data not shown). Biofilms
grown under continuous-flow conditions in a two-dimensional flat-plate reac-
tor appear to reach a pseudosteady state in which the average hydraulic res-
idence time changes only slightly although the location of the primary flow
path seems to be changing with time (Sharp et al . 1999a, 2005; Arnon et al .
2005b). The time until a pseudosteady state is reached and the extent of
porosity reduction depends on the microorganism(s) present, growth condi-
tions (e.g., temperature, electron acceptor availability, pH, etc.), flowrate, and
similar parameters. Equivalent observations for constant flow conditions were
obtained in our laboratories using MRM in pseudo one-dimensional porous
media columns (Seymour et al . 2004b, 2007).
The existence of a “critical shear stress,” that is, a shear stress above which
significant detachment occurs, has been proposed (Kim and Fogler 2000) and
makes intuitive sense under constant flow conditions. Once biofilm growth
begins to restrict pore spaces the localized flowrate and associated shear stress
increase with decreasing effective porosity and can result in increased biofilm
detachment.
In constant head systems, such as many shallow aquifers, a decrease in pore
space results in a decrease in overall porosity and, likely, permeability. Since
the influent head remains constant, such a decrease in overall permeability
will result in a decrease in flowrate according to Darcy's Law.
KA dh
dl
Q =
where Q is the flow rate (L 3 /T, e.g., m 3 /sec), K is the hydraulic conductivity
(L/T, e.g., m/d), A is the cross-sectional area of flow (L 2 , e.g., m 2 ), dh is the
difference in hydraulic head across the reactor (L, e.g., m), and dl the length
of the reactor (L, e.g., m).
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