Technical conditions concerning biofilters
Aeration in the filter itself
The theory for this type of aeration is in principle not different either from the
aeration in an activated sludge plant, but of course the presence of the filter medium
has a certain effect.
Aeration of the biofilm
In discs the filter medium is covered by a thin water film on the surface of the
biofilm. Hence the conditions for the transfer of oxygen from air to biofilm are very
special. More details on the subject can be found in /16/.
5.10.2 Growth and sloughing off of the biofilm
Biofilters are based on the ability of the bacteria to attach and develop on a solid
medium. Technologically it is desirable that a stable state with equilibrium between
growth and sloughing off is developed. No matter how fundamental these conditions might be it is, however, a fact that too little is known about these conditions,
apart from the very technological empiricism.
Adhesion
In practice, bacteria will adhere to any surface - they have in fact a partiality for
adhesion- irrespective of the type of surface (except where countermeasures have
been taken, for example marine coating). The start of a biofilm is simply that the
medium concerned is in contact with water, in this case with the wastewater. In
practice, the development of a functional film takes approx. 14 days under aerobic
conditions. The building up is commenced faster on surfaces with earlier growth
than on completely clean surfaces. The building up is selective and the bacteria
which are not attached will simply be washed out of the plant. For special substances and processes the starting up may be a big problem.
Types of biofilm
The characteristics of the biofilm are of course a function of the characteristics of the
bacteria which form the biofilm. No applicable examinations are available whatsoever of the relationship between microbiological determination of species and
measured biofilm characteristics. It can only be concluded that two types of biofilm
generally occur: The dense film and the filamentous film. The dense film is a
coherent, immobilized biomass. In its ideal form, it has a plane, smooth surface. For
this type of biofilm there is clear experimental proof of the good, theoretical description of the removal as described earlier in the biofilm kinetics. The filamentous
biofilm is dominated by filamentous bacteria which grow in continuation of each
other and form filaments attached to the filter medium, typically the Chlamydobacteriales Sphaerotilus Natans, (also known from highly polluted streams under the name
of: Sewage fungus). In its extreme form, this type of film forms a mat whose filaments
are moved by the liquid flow. This causes turbulent movements around the filaments
for which reason the transport of substrate into the biofilm is increased compared with
180
Aeration in the filter itself
The theory for this type of aeration is in principle not different either from the
aeration in an activated sludge plant, but of course the presence of the filter medium
has a certain effect.
Aeration of the biofilm
In discs the filter medium is covered by a thin water film on the surface of the
biofilm. Hence the conditions for the transfer of oxygen from air to biofilm are very
special. More details on the subject can be found in /16/.
5.10.2 Growth and sloughing off of the biofilm
Biofilters are based on the ability of the bacteria to attach and develop on a solid
medium. Technologically it is desirable that a stable state with equilibrium between
growth and sloughing off is developed. No matter how fundamental these conditions might be it is, however, a fact that too little is known about these conditions,
apart from the very technological empiricism.
Adhesion
In practice, bacteria will adhere to any surface - they have in fact a partiality for
adhesion- irrespective of the type of surface (except where countermeasures have
been taken, for example marine coating). The start of a biofilm is simply that the
medium concerned is in contact with water, in this case with the wastewater. In
practice, the development of a functional film takes approx. 14 days under aerobic
conditions. The building up is commenced faster on surfaces with earlier growth
than on completely clean surfaces. The building up is selective and the bacteria
which are not attached will simply be washed out of the plant. For special substances and processes the starting up may be a big problem.
Types of biofilm
The characteristics of the biofilm are of course a function of the characteristics of the
bacteria which form the biofilm. No applicable examinations are available whatsoever of the relationship between microbiological determination of species and
measured biofilm characteristics. It can only be concluded that two types of biofilm
generally occur: The dense film and the filamentous film. The dense film is a
coherent, immobilized biomass. In its ideal form, it has a plane, smooth surface. For
this type of biofilm there is clear experimental proof of the good, theoretical description of the removal as described earlier in the biofilm kinetics. The filamentous
biofilm is dominated by filamentous bacteria which grow in continuation of each
other and form filaments attached to the filter medium, typically the Chlamydobacteriales Sphaerotilus Natans, (also known from highly polluted streams under the name
of: Sewage fungus). In its extreme form, this type of film forms a mat whose filaments
are moved by the liquid flow. This causes turbulent movements around the filaments
for which reason the transport of substrate into the biofilm is increased compared with
180