Biofilters
molecular diffusion. These films will therefore remove material according to a zero
order reaction at lower concentrations than calculated according to the biofilm kinetics
as discussed above.
The bacterial density in the film is decisive for the removal rate. What is interesting
is the mass of the bacteria which carry out the process observed. It is difficult to
determine this quantity by measurement, but it can be calculated as a fraction of the
total mass in the same way as for the bacteria in the sludge in an activated sludge
plant. The total mass may vary within wide ranges: 10-100 kg VSS/m 3 ; but data
primarily fall in the range 40-60 kg VSS/m 3 .
The sloughing off of the biofilm
The removal of organic matter corresponds to a continuous growth of bacteria and
hence a growth of the thickness of the biofilm. If this is not balanced by a corresponding sloughing off, the result must necessarily be clogging. The following
conditions are important for the sloughing off and are used technologically, intentionally or unintentionally; as part of the control of the biofilm:
- Hydraulic erosion acts continually on the surface of the biofilm and leads to
a steady sloughing off on the outer side. In its extreme version it is the total
sloughing off of the biofilm on the sand in the recycle flow of a fluidized
filter in a tank where a stirrer creates a very high turbulence level. The
requirements of the hydraulic loading of trickling filters and of the rotational
speed of rotating discs are empirically dictated from a desire to cause
sloughing off, but the flow of water is not so strong that it can by itself cause
the sloughing off. Other factors release the film so that it can be sloughed off
hydraulically.
- Degradation of starved out bacteria in the bottom of biofilms may cause a
weakening of the adhesion. Biofilms will in practice have a tendency to grow
to a thickness where they are only partially penetrated with substrate. In the
aerobic filter, whose reaction rate is controlled by the oxygen, anaerobic
conditions will occur in the bottom of the film. This will degrade the bacteria
in the bottom and destroy the adhesion. Sufficiently weakened, the film will
be sloughed off completely over a smaller area by hydraulic erosion.
- Super saturation and bubble formation in the bottom of the biofilm may
destroy the adhesion - for example methane production under anaerobic
conditions and the production of pure nitrogen by denitrification.
In the last two cases the film is sloughed off completely; and a naked area is left on
the filter medium where new growth starts. The filter is thus continuously in a state
of sloughing off and regrowth. Therefore the biofilm never has a well-defined
thickness in practice which applies to the whole film.
A special phenomenon is the grazing of higher animals on the bacterial mass in the
filter. Very little is known about this, but it is undoubtedly of importance and is for
example reflected in seasonal variations in the biomass in a trickling filter and in
sudden changes in the biofilm in for example nitrifying plants /17 I. A conventional
181
molecular diffusion. These films will therefore remove material according to a zero
order reaction at lower concentrations than calculated according to the biofilm kinetics
as discussed above.
The bacterial density in the film is decisive for the removal rate. What is interesting
is the mass of the bacteria which carry out the process observed. It is difficult to
determine this quantity by measurement, but it can be calculated as a fraction of the
total mass in the same way as for the bacteria in the sludge in an activated sludge
plant. The total mass may vary within wide ranges: 10-100 kg VSS/m 3 ; but data
primarily fall in the range 40-60 kg VSS/m 3 .
The sloughing off of the biofilm
The removal of organic matter corresponds to a continuous growth of bacteria and
hence a growth of the thickness of the biofilm. If this is not balanced by a corresponding sloughing off, the result must necessarily be clogging. The following
conditions are important for the sloughing off and are used technologically, intentionally or unintentionally; as part of the control of the biofilm:
- Hydraulic erosion acts continually on the surface of the biofilm and leads to
a steady sloughing off on the outer side. In its extreme version it is the total
sloughing off of the biofilm on the sand in the recycle flow of a fluidized
filter in a tank where a stirrer creates a very high turbulence level. The
requirements of the hydraulic loading of trickling filters and of the rotational
speed of rotating discs are empirically dictated from a desire to cause
sloughing off, but the flow of water is not so strong that it can by itself cause
the sloughing off. Other factors release the film so that it can be sloughed off
hydraulically.
- Degradation of starved out bacteria in the bottom of biofilms may cause a
weakening of the adhesion. Biofilms will in practice have a tendency to grow
to a thickness where they are only partially penetrated with substrate. In the
aerobic filter, whose reaction rate is controlled by the oxygen, anaerobic
conditions will occur in the bottom of the film. This will degrade the bacteria
in the bottom and destroy the adhesion. Sufficiently weakened, the film will
be sloughed off completely over a smaller area by hydraulic erosion.
- Super saturation and bubble formation in the bottom of the biofilm may
destroy the adhesion - for example methane production under anaerobic
conditions and the production of pure nitrogen by denitrification.
In the last two cases the film is sloughed off completely; and a naked area is left on
the filter medium where new growth starts. The filter is thus continuously in a state
of sloughing off and regrowth. Therefore the biofilm never has a well-defined
thickness in practice which applies to the whole film.
A special phenomenon is the grazing of higher animals on the bacterial mass in the
filter. Very little is known about this, but it is undoubtedly of importance and is for
example reflected in seasonal variations in the biomass in a trickling filter and in
sudden changes in the biofilm in for example nitrifying plants /17 I. A conventional
181