Biomedical Engineering Reference
In-Depth Information
different organisms that can facilitate the exchange of metabolites, and the
increased resistance to environmental stresses, make biofilms very attractive
for technology development.
Owing to their increased tolerance to environmental stress and toxic com-
pounds, biofilm reactors are frequently proposed to be used for the treatment
of recalcitrant compounds. The range of such compounds spans from sur-
factants to herbicides and organic solvents to dyes (Mol et al . 1993; Petrozzi
et al . 1993; Jerabkova et al . 1999; Mondragon-Parada et al . 2008). In addition,
biofilms can influence colloid transport (Kim and Corapcioglu 1997; Ste-
vik et al . 2004; Muris et al . 2005; Morales et al . 2007). The transport of
pathogens (biological colloids) and colloid-mediated contaminant transport
through porous media are both areas of importance for public health.
Biofilm technologies are available, and there are ongoing research and
development efforts in the areas of subsurface biofilm barriers, hazardous waste
treatment, biofilters, enhanced oil recovery, acid mine drainage treatment, fil-
ters, and infiltration systems in water and wastewater treatment, biofilms as
biosorbents, air pollution control, and municipal solid waste leachate control.
Changes in porosity, permeability, dispersion, and diffusion can be desir-
able or detrimental for a given application. For instance, in the case of subsur-
face biofilm barriers for the control of groundwater flow, maximum porosity
and permeability reduction is desired. In contrast, in biotrickling filters for
wastewater treatment an optimal biofilm thickness is desired, which results in
maximum removal of solutes while maintaining fairly high permeability.
To limit the discussion of biofilm technologies in porous media in this chap-
ter, it should be pointed out that moving bed reactors, such as suspended
(fluidized) bed reactors, will be excluded from consideration. The extent of
mixing and abrasion of biofilm by the carrier material is significantly differ-
ent from, for instance, filters, soils, and packed bed reactors, and the topic
would become too complex to be discussed in sucient detail here. Mem-
brane systems such as reverse osmosis filtration cartridges are also excluded
from consideration, although these networks of flow channels could be consid-
ered a special form of porous medium, and biofilm growth (biofouling) in these
systems can significantly affect flow as indicated by increasing backpressure
in (bio-) fouled membrane systems.
A major factor in the performance of biofilm reactors is the limited mass
transport of solutes into the biofilm where the active biomass is located.
Transport limitations of electron acceptors and donors can significantly affect
the performance of porous media biofilm reactors, especially if the reaction
depends on oxygen, which has a very limited solubility in water (Kirchner
et al . 1992; Joannis-Cassan et al . 2007). Hence, it is extremely important to
understand flow dynamics and solute reactive transport in biofilm-affected
porous media (Iliuta and Larachi 2004).
It should also be kept in mind that, in the environment and industrial
systems, biofilms are likely to accumulate solutes and possibly significant
amounts of minerals. Iron oxides, calcium carbonate, and sulfur-containing
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