THE SIGNIFICANCE OF BIOFILMS IN POROUS-MEDIA

The recent literature contains conflicting claims about the characteri stics of attached bacteria in subsurface porous media and how these ch aracteristics affect permeability reduction. Some claim that the bacte ria form continuous biofilms that restrict the pore size, while others claim that bacteria are attached in patchy aggregates that accumulate in pore throats. This contribution applies a recently developed tool from biofilm kinetics, the normalized surface loading, to interpret a wide range of experimental data from porous media experiments and biol ogical filtration. The normalized surface loading is the actual substr ate flux (i.e., rate of removal per unit surface area) divided by the minimum flux capable of supporting a deep biofilm. The analyses show t hat biofilms are continuous for normalized surface loadings greater th an 1.0, but appear to become discontinuous for values less than about 0.25. For the low-load situation, distinguishing between continuous an d discontinuous biofilms is not important when the modeling goal is pr ediction of substrate removal. However, the distinction is more critic al when the modeling goal is to describe the spatial distribution of a ttached biomass and permeability loss.