The successive steps of the development of an aerobic biofilm can be described as follows (Elmaleh and Grasmick 1985):
step 1 - The biofilm is composed of a few aerobic bacteria included in a gelatinous matrix, i.e the density is low.
step 2 - Aerobic micro-organisms grow rapidly, and the density is an increasing function of the thickness.
step 3 - As oxygen depletion begins to occur in the biofilm, an anaerobic zone appears near the solid material.
step 4 - Anaerobic and facultative bacteria grow near the solid material as aerobes decay, causing decreasing density.
step 5 - An equilibrium between the anaerobes and aerobes is reached, which means that the density is stabilized.
step 6 - Now the equilibrium is maintained, until the substrate concentration is exhausted in the deeper zone, the anaerobe bacteria will begin to decay, and parts of the biofilm will finally slough away.
step 7 - The newly developed space will be used by new aerobic bacteria which will start all over again and build new biofilm.
The rate of nutrient removal in attached-growth systems depends on the flow rate of the waste water, the organic loading rate, rates of diffusivity of nutrients into the biofilm, and temperature. The depth of penetration of both oxygen and nutrients is increased at higher loading rates. Oxygen diffusivity is usually the limiting factor. Aerobic zones of the biofilm are usually limited to a depth of 0,1 to 0,2 mm, the remaining thickness of the biofilm being anaerobic (see Fig. 5.4). Depending on hydrodynamic conditions, Atkinson and Fowler (1974), found values between 0,07 and 4 mm in thickness. When the biofilm is mechanically or hydraulically controlled, its thickness does not exceed 0,2 mm, which is the maximum depth for having a full aerobic biofilm (Grasmick 1982).
Arvin and Harremoes (1990) reported that the thickness of the biofilm is controlled by the following factors:
1. Growth of active biomass as a result of influx of the substrate.
2. Decay of active biomass.
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Figure 5.2 The different steps in developing a biofilm, shown as transients of a biofilm.
3. Accumulation of inert organic material from the decay of active bacteria.
4. Accumulation of polymers from the metabolism of the substrate.
5. Deposition/flocculation of suspended particles from the bulk liquid.
6. Erosion of small particles from the surface of the biofilm.
7. Sloughing of large fractions of the biofilm.
At present our ability to predict the thickness of a biofilm is relatively low.
It is also difficult to define and measure the thickness of a biofilm. Experimental measurements can be made directly, or can involve a procedure called congelation of the whole reacting medium. Accuracy in the values of a thickness less than 100 ^m is 20%, but on values of about 2 mm, the discrepancy can reach 300 % (Grasmick 1982).
The number of variables affecting the growth of biomass, and subsequently the rate of substrate utilization, makes it difficult to describe the systems mathematically.
Masuda etal. (1987) reported that oxidizing, nitrifying and denitrifying bacteria can exist almost uniformly in the entire biofilm. Oxidation of organics, nitrification and denitrification occur in the same biofilm. Probably the denitrifying bacteria exist in the most anoxic areas, in the deeper layer of the biofilm.
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