Biomedical Engineering Reference
In-Depth Information
a
b
c
d
e
f
g
h
FIGURE 5.6
Microscopy images of biofilm forming in continuous-flow porous media envi-
ronments. Flow from left to right in all images. Images (a), (b), (e), (f) are from
Nambi et al. (2003), biofilms appear to be forming preferentially on the down-
stream edge of the porous media elements, which are approximately 300
µ
m
in diameter; pore throats are approximately 35
m in size. (a) 25 days, (b) 32
days, (e) 39 days, and (f) 44 days after inoculation. Images (c), (d), (g), (h)
are depicting thick biofilm development in a porous media reactor over time.
Porous media elements (black) are 1 mm 3 ; cross-section of flow channels is
1mm 2 . The reactor design is explained in more detail in Section 5.4.5. Images
taken at euent, (c and d) and influent region (g and h) of reactor. (Reprinted
with permission Nambi, I.M., Werth, C.J., Sanford, R.A., and Valocchi, A.J.
Env. Sci. Tech. , 37, 2003. c
µ
2003, American Chemical Society.)
On the pore scale, it appears that biofilms accumulate initially in regions
that are somewhat protected (Figure 5.6), that is, areas low in shear stress,
and biofilm growth subsequently expands on the leeward side of porous media
particles (Nambi et al . 2003; Stoodley et al . 2005; Knutson et al . 2007). The
relatively high-shear forces in the pore throats of porous media are generally
not favorable for initial attachment and thick biofilm development. High-shear
environments generally result in relatively thin and smooth biofilms while
lower shear environments usually produce thicker and rougher biofilms (Nam
et al . 2000).
Oscillations in pressure drop across reactors have been related to changes
in biomass distribution, the development of flow channels within the porous
media filled with biomass, and creation of plugged regions where advective flow
had occurred previously (Stewart and Fogler 2001). This behavior is probably
more pronounced under constant flow conditions, which are employed more
commonly in the laboratory than under constant head conditions, which are
more commonly observed in the subsurface.
Under both types of flow conditions (constant head and constant flow),
the most significant effects are usually observed in the influent regions of
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