The Biofilm

Nitrifier, denitrifier, oxidizer or a combination of these types of bacteria can attach themselves to different types of medium and grow into dense films of a viscous, gelatinous matrix called the biofilm. Waste water passes over this film in thin sheets, with dissolved organics, NH4+ or N03" passing into the biofilm due to diffusion gradients within the film. Suspended particles and colloids cannot penetrate the surface of the biofilm, but will be decomposed on the surface of the biofilm into soluble products. Oxygen from the waste water and from air in the void spaces of the medium, provides oxygen for the aerobic reactions at the surface of the biofilm. Figure 5.3 show a diagrammatic representation of a biofilm with involved processes.

Waste products from the metabolic processes diffuse outward and are carried away by the water or air. Growth of the biofilm is restricted to the outward direction from the solid surface. As the film grows thicker (see Fig. 5.2), concentration gradients of both oxygen and nutrient develop. Eventually, when the biofilm is of an appropriate size, both anaerobic and endogenous metabolism occur in the interface of the biofilm.

In a well developed biofilm, the attachment mechanism to the solid medium is weakened, and the shearing action of the waste water flowing across the film pulls it from its attachment and washes it away. This process is called sloughing, and is a function of both the hydraulic and the organic loading rates. But the biofilm is quickly re-established, and, therefore, sloughing is a beneficial mechanism for development of new biofilm.

Trickling Filter

Rotating Disk Unit

Trickling Filter

Submerged Filters


Fluidized Filter id hi down - flow

Figure 5.1 Biofilm reactors used in waste water treatment, a) Trickling filter, b) Rotating Biological Conductor (RBC), c) Submerged filters with down-flow or up-flow application, and d) Fluidized filters.

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