5. Biofilters
By Poul Harremoes and Mogens Henze
Biofilters are characterized by bacteria being attached to a solid surface in the form
of a biofilm. Biofilms are a dense layer of bacteria characterized by their ability to
adhere to a solid medium and form a fixed film of polymers in which the bacteria
are protected against sloughing off. Biofilters have a short hydraulic retention time
and hence free bacteria in the water will be washed out.
The disadvantage of biofilters is the low efficiency of the biomass. The reason is that
the substances must be carried through the biofilm to be removed by the bacteria.
This transport takes place by molecular diffusion which is a slow process. In
practice it proves that the general rule is that the removal is limited by diffusion.
This phenomenon must be understood in order to understand the functioning of
biofil ters.
5.1. Biofilm kinetics
Fig 5.1 shows an idealized biofilm which is assumed to be homogenous. In the
water, outside the biofilm, the concentration of a substance is assumed to be S. The
transport into the biofilm takes place by molecular diffusion using the diffusion
coefficient D.
bulk
liquid
s
Fig 5.1
carrier
L
N
Svr
Idealized biological film in which the substance is moved by diffusion. The overall
process may be limited by diffusion.
143
By Poul Harremoes and Mogens Henze
Biofilters are characterized by bacteria being attached to a solid surface in the form
of a biofilm. Biofilms are a dense layer of bacteria characterized by their ability to
adhere to a solid medium and form a fixed film of polymers in which the bacteria
are protected against sloughing off. Biofilters have a short hydraulic retention time
and hence free bacteria in the water will be washed out.
The disadvantage of biofilters is the low efficiency of the biomass. The reason is that
the substances must be carried through the biofilm to be removed by the bacteria.
This transport takes place by molecular diffusion which is a slow process. In
practice it proves that the general rule is that the removal is limited by diffusion.
This phenomenon must be understood in order to understand the functioning of
biofil ters.
5.1. Biofilm kinetics
Fig 5.1 shows an idealized biofilm which is assumed to be homogenous. In the
water, outside the biofilm, the concentration of a substance is assumed to be S. The
transport into the biofilm takes place by molecular diffusion using the diffusion
coefficient D.
bulk
liquid
s
Fig 5.1
carrier
L
N
Svr
Idealized biological film in which the substance is moved by diffusion. The overall
process may be limited by diffusion.
143