Biofilters
5.8. Types of plants
Biofilters must be able to carry out four functions to be able to treat the water
satisfactorily to remove organic matter. The tank should be designed so that:
- the bacteria necessary for the required process can be attached to a carrier
(filter medium),
- the water gets into efficient contact with the sludge attached to the carrier
(the biofilm/slime),
- the growth of the biofilm can be controlled so that no clogging will occur,
- oxygen is supplied to the water for the degradation of the organic matter.
Bacteria have a remarkable ability to adhere to solid surfaces, whether it concerns
stone, wood or plastics which are the three types of media used in practice. The
engineering control of the adhesion of such bacteria which carry out the required
process is very practical in water and wastewaster treatment - as opposed to
industrial production by means of biological reactors which are often based on
carefully selected types of bacteria. In water and wastewater treatment such conditions are established which from experience are known to favour the bacteria which
carry out the required process. This also applies to biofilters: the bacteria which
cannot adhere to a surface will be washed out of the system. The bacteria which
cannot degrade the material concerned cannot compete, see Chapter 3, Section 3.1.2.
This may occasionally cause difficulties when starting up biofilter plants.
To achieve an efficient removal of the materials in the water, the water must pass the
surface of the biofilm in order that a continuous renewal of water at the biofilm
surface takes place. For this purpose the medium is either solid and the water passes
the medium as is the case in the conventional biofilters (trickling filters), or the
medium is continually carried through the water as the case is in rotating discs. The
efficiency in the plant type depends to a large extent on how this contact between
biofilm and water is arranged.
Control of the growth of the biofilm and the conditions concerning the oxygen
supply are further discussed in Sections 5.10.2 and 5.10.1.
5.8.1 Trickling filters
The trickling filter is the conventional biofilm reactor and many different types of
trickling filters have been used during the last century. Fig 5.14 shows the characteristic type which is a typical three-phase system. The filter medium is stationary,
typically consisting of 5-20 em large stones. Any other conceivable materials have
been tried out- only plastics are, however, widely used as an alternative to stone.
The wastewater is distributed over the filter and trickles down over the stones to be
collected under the stones and carried out. The ventilation through the bottom of
the filter is made efficient so as to ensure a steady air current through the filter. In
167
5.8. Types of plants
Biofilters must be able to carry out four functions to be able to treat the water
satisfactorily to remove organic matter. The tank should be designed so that:
- the bacteria necessary for the required process can be attached to a carrier
(filter medium),
- the water gets into efficient contact with the sludge attached to the carrier
(the biofilm/slime),
- the growth of the biofilm can be controlled so that no clogging will occur,
- oxygen is supplied to the water for the degradation of the organic matter.
Bacteria have a remarkable ability to adhere to solid surfaces, whether it concerns
stone, wood or plastics which are the three types of media used in practice. The
engineering control of the adhesion of such bacteria which carry out the required
process is very practical in water and wastewaster treatment - as opposed to
industrial production by means of biological reactors which are often based on
carefully selected types of bacteria. In water and wastewater treatment such conditions are established which from experience are known to favour the bacteria which
carry out the required process. This also applies to biofilters: the bacteria which
cannot adhere to a surface will be washed out of the system. The bacteria which
cannot degrade the material concerned cannot compete, see Chapter 3, Section 3.1.2.
This may occasionally cause difficulties when starting up biofilter plants.
To achieve an efficient removal of the materials in the water, the water must pass the
surface of the biofilm in order that a continuous renewal of water at the biofilm
surface takes place. For this purpose the medium is either solid and the water passes
the medium as is the case in the conventional biofilters (trickling filters), or the
medium is continually carried through the water as the case is in rotating discs. The
efficiency in the plant type depends to a large extent on how this contact between
biofilm and water is arranged.
Control of the growth of the biofilm and the conditions concerning the oxygen
supply are further discussed in Sections 5.10.2 and 5.10.1.
5.8.1 Trickling filters
The trickling filter is the conventional biofilm reactor and many different types of
trickling filters have been used during the last century. Fig 5.14 shows the characteristic type which is a typical three-phase system. The filter medium is stationary,
typically consisting of 5-20 em large stones. Any other conceivable materials have
been tried out- only plastics are, however, widely used as an alternative to stone.
The wastewater is distributed over the filter and trickles down over the stones to be
collected under the stones and carried out. The ventilation through the bottom of
the filter is made efficient so as to ensure a steady air current through the filter. In
167