Biofilters
the course of the development it was believed to be necessary to use forced ventilation through such filters, but in practice it has proved that the variation in temperature between filter medium, wastewater and the surrounding air is sufficient to
ensure air renewal and reaeration of the water during the trickling from top to
bottom. The trickling filter is very efficient with respect to adhesion of bacteria,
contact between water and biofilm, and reaeration of the water. The most essential
problem with trickling filters is the control of the growth of the biofilm. This is the
function making the greatest demand on design and operation. In the oldest, very
low-rate plants, the control is biological. The biofilm is allowed to develop uninhibited which leads to local cloggings. The clogging will prevent that oxygen is
supplied to the biomass, and it will putrefy locally and be degraded, until there is
passage again. Higher animals, such as worms and larvae, also contribute to the
degradation of the biomass. Thus the filter can become a hatching plant for insects,
especially filter flies, and they can be such a nuisance that this is, in fact, the reason
why the low-rate trickling filter is not commonly used. To this should be added that
due to the low rate the demand for land space is so big that it can only be satisfied
for very small plants.
It is especially bad if the loading is increased (often simultaneously hydraulically
and with respect to organic matter) on a low-rate plant as the cloggings spread at a
higher rate than the rate at which the cloggings can be degraded. But if the load is
increased beyond a certain point, something new happens: From being exclusively
biologically controlled, the thickness of the biofilm will now mainly be controlled
by hydraulic sloughing off. This is the type of biological trickling filter which is still
found in great numbers. In recent years, the trickling filter has experienced a
renaissance because plastics have made it possible to create a large volumetric
surface in filters of little weight, and these can therefore be built high without
incurring considerable costs. Such filters are typically used for the treatment of
wastewater with high concentrations as a pretreatment prior to any other treatment.
5.8.2 Submerged filters
Submerged filters are filters where the filter medium is located under the surface of
the water. Among many different characteristic designs of filters, the most important characteristic is a stationary or movable filter medium.
Stationary filter medium
Fig 5.15 shows a filter design in principle. What is especially important for the
design is: The control of the thickness of the biofilm and the supply of oxygen to the
water.
In respect of the thickness of the biofilm and problems with clogging, there are only
two solutions to the problem: 1) Spacious filter media with sufficient space and
hydraulic conditions, which prevent excessive thicknesses of the film, can be used.
With such filter media it is not possible to obtain a large specific surface area, and
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the course of the development it was believed to be necessary to use forced ventilation through such filters, but in practice it has proved that the variation in temperature between filter medium, wastewater and the surrounding air is sufficient to
ensure air renewal and reaeration of the water during the trickling from top to
bottom. The trickling filter is very efficient with respect to adhesion of bacteria,
contact between water and biofilm, and reaeration of the water. The most essential
problem with trickling filters is the control of the growth of the biofilm. This is the
function making the greatest demand on design and operation. In the oldest, very
low-rate plants, the control is biological. The biofilm is allowed to develop uninhibited which leads to local cloggings. The clogging will prevent that oxygen is
supplied to the biomass, and it will putrefy locally and be degraded, until there is
passage again. Higher animals, such as worms and larvae, also contribute to the
degradation of the biomass. Thus the filter can become a hatching plant for insects,
especially filter flies, and they can be such a nuisance that this is, in fact, the reason
why the low-rate trickling filter is not commonly used. To this should be added that
due to the low rate the demand for land space is so big that it can only be satisfied
for very small plants.
It is especially bad if the loading is increased (often simultaneously hydraulically
and with respect to organic matter) on a low-rate plant as the cloggings spread at a
higher rate than the rate at which the cloggings can be degraded. But if the load is
increased beyond a certain point, something new happens: From being exclusively
biologically controlled, the thickness of the biofilm will now mainly be controlled
by hydraulic sloughing off. This is the type of biological trickling filter which is still
found in great numbers. In recent years, the trickling filter has experienced a
renaissance because plastics have made it possible to create a large volumetric
surface in filters of little weight, and these can therefore be built high without
incurring considerable costs. Such filters are typically used for the treatment of
wastewater with high concentrations as a pretreatment prior to any other treatment.
5.8.2 Submerged filters
Submerged filters are filters where the filter medium is located under the surface of
the water. Among many different characteristic designs of filters, the most important characteristic is a stationary or movable filter medium.
Stationary filter medium
Fig 5.15 shows a filter design in principle. What is especially important for the
design is: The control of the thickness of the biofilm and the supply of oxygen to the
water.
In respect of the thickness of the biofilm and problems with clogging, there are only
two solutions to the problem: 1) Spacious filter media with sufficient space and
hydraulic conditions, which prevent excessive thicknesses of the film, can be used.
With such filter media it is not possible to obtain a large specific surface area, and
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