Beneficial Biofilms

Benefits afforded by biofilms in a continuous reactor situation arise chiefly because the cell population is immobilized, and thus the residence time of the cells in the reactor is independent of the fluid-phase residence time. In continuous suspended culture bioreactors (i.e., chemostats), the mean residence time of the system cannot be less than the generation time of the bacterial species, otherwise cells do not have sufficient time to replicate within the reactor and are eventually diluted from the system.

Immobilized and biofilm-bound cells remain in a continuous reactor system independent of the fluid phase, thus the mass loading of limiting substrate (or influent pollutant in the case of a wastewater treatment reactor) can be increased well beyond the growth rate limit imposed on suspended cultures. Consequently, immobilized-cell or biofilm reactors can provide (1) added volumetric reactivity, (2) more stable operating performance, (3) an inherent ease in biomass-fluid separation, and (4) the prospect of staging different bioconversion processes in sequential reactors. Due to these inherent advantages, the use of biofilm reactors is not confined to just bacterial cells, but also comprises plant and animal cell applications.

Bacterial biofilm reactors are employed either in commodities production or in wastewater treatment applications. Biofilm reactors have been reportedly used to produce acetic acid (27), L-lysine (28), gluconic acid (29), kojic acid (30,31), and ethanol (32,33), and in the epoxidation of propene (34). Such biofilm reactors are operated either as packed- or fluidized-bed reactor systems, with cells either attached to an inert support particle or artificially immobilized within a gel matrix.

One major application that relies on a microbial culture's ability to form biofilms is wastewater treatment. Biofilm reactor configurations, applied in both pilot- and full-scale wastewater treatment, include packed-bed trickling filters, high-rate plastic media filters, rotating biological contactors, fluidized-bed biofilm reactors, and membrane-immobilized cell reactors. Examples of biofilm reactors employed for wastewater treatment reported in the literature include polychlorinated hydrocarbon degradation (35), toluene degradation (36), denitrification (37,38), cadmium removal (39), anaerobic butyrate degradation (40), nitrification (41-43), glyphosphate degradation (44), anaerobic propionate degradation (45), phenolic wastewaters (46), uranium removal (47), and anaerobic carbon removal (48,49).

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