Trickling Filters
The trickling or biological filter system involves a bed, which is
formed by a layer of filter medium held within a containing tank or vessel,
often cast from concrete, and equipped with a rotating dosing device, as shown
in a stylised form in Figure 6.3.
The filter is designed to
permit good drainage and ventilation and in addition sedimentation and settling
tanks are generally associated with the system. Efflu-ent, which has been
mechanically cleaned to remove the large particles which might otherwise clog
the interparticulate spaces in the filter bed, flows, or is pumped, into the
rotating spreader, from which it is uniformly distributed across the filter
bed. This dosing process can take place either continuously or intermit-tently,
depending on the operational requirements of the treatment works. The
wastewater percolates down through the filter, picking up oxygen as it travels
over the surface of the filter medium. The aeration can take place naturally by
diffusion, or may sometimes be enhanced by the use of active ventilation fans.
The combination of the
available nutrients in the effluent and its enhanced oxygenation stimulates
microbial growth, and a gelatinous biofilm of micro-organisms forms on the
filter medium. This biological mass feeds on the organic material in the
wastewater converting it to carbon dioxide, water and microbial biomass. Though
the resident organisms are in a state of constant growth, ageing and occasional
oxygen starvation of those nearest the substrate leads to death of some of the
attached growth, which loosens and eventually sloughs, passing out of the
filter bed as a biological sludge in the water flow and thence on to the next
phase of treatment.
The filter medium itself is
of great importance to the success of these systems and in general the
requirements of a good material are that it should be durable and long lasting,
resistant to compaction or crushing in use and resistant to frost damage. A
number of substances have been used for this purpose including clinker,
blast-furnace slag, gravel and crushed rock. A wholly artificial plastic
lattice material has also been developed which has proved successful in some
applications, but a clinker and slag mix is generally said to give some of the
best results. The ideal filter bed must provide adequate depth to guarantee
effluent retention time, since this is critical in allowing it to become
sufficiently aerated and to ensure adequate contact between the microbes and
the wastewater for the desired level of pollutant removal. It should also have
a large surface area for biomass attachment, with generous void spaces between
the particles to allow the required biomass growth to take place without any
risk of this causing clogging. Finally, it should have the type of surface
which encourages splashing on dosing, to entrap air and facilitate oxygenation
of the bed.
The trickling filters in use
at sewage works are squat, typically around 8 – 10 metres across and between 1
– 2 metres deep; though these are the most familiar form, other filters of
comparatively small footprint but 5 to 20 metres in height are used to treat
certain kinds of trade effluents, particularly those of a stronger nature and
with a more heavy organic load than domestic wastewater. They are of particular
relevance in an industrial setting since they can achieve a very high
throughput and residence time, while occupying a relatively small base area of
land.
To maximise the treatment
efficiency, it is clearly essential that the trickling filter is properly sized
and matched to the required processing demands. The most important factors in
arriving at this are the quality of the effluent itself, its input temperature,
the composition of the filter medium, detail of the surface-dosing arrangements
and the aeration. The wastewater quality has an obvious signifi-cance in this
respect, since it is this, combined with the eventual clean-up level required,
which effectively defines the performance parameters of the system. Although in
an ideal world, the filter would be designed around input character, in cases where
industrial effluents are co-treated with domestic wastewater in sewage works,
it is the feed rate which is adjusted to provide a dilute liquor of given
average strength, since the filters themselves are already in existence. Hence,
in practice, the load is often adjusted to the facility, rather than the other
way about.
The input temperature has a
profound influence on the thermal relations within the filter bed, not least
because of the high specific heat capacity of water at 4200 J/kg/ ◦ C. This can
be of particular relevance in industrial reed bed systems, which are discussed
in the following, since a warm liquor can help to overcome the problems of cold
weather in temperate climes. By contrast the external air temperature appears
to have less importance in this respect. The situation within the reaction
space is somewhat complicated by virtue of the nonlinear nature of the effect
of temperature on contaminant removal. Although the speed of chemical reactions
is well known to double for every 10 ◦ C rise, at 20 ◦ C, in-filter
biodegradation only represents an increase of 38% over the rate at 10 ◦C. Below
10 ◦C, the risk of clogging rises significantly, since the activity of certain
key members of the microbial community becomes increas-ingly inhibited.
The general properties of
the filter media were discussed earlier. In respect of sizing the system, the
porosity and intergranular spaces govern the interrelation between relative
ease of oxygen ingress, wastewater percolation and nutrient to biofilm contact.
Clearly, the rougher, pitted or irregular materials tend to offer the greatest
surface area per unit volume for microbial attachment and hence, all other
things being equal, it follows that the use of such media allows the overall
filter dimensions to be smaller. In practice, however, this is seldom a major
deciding factor.
In the main, filter systems
use rotational dosing systems to ensure a uniform dispersal of the effluent,
though nozzles, sprays and mechanised carts are not unknown. The feed must be
matched to the medium if the surface aeration effect is to be optimised, but it
must also take account of the fluidity, concentration and quality of the
wastewater itself and the character of the resident biofilm.
Since the biological
breakdown of effluents within the filter is brought about by aerobic organisms,
the effectiveness of aeration is of considerable importance. Often adequate
oxygenation is brought about naturally by a combination of the surface effects
as the wastewater is delivered to the filter, diffusion from atmosphere through
the filter medium and an in-filter photosynthetic contribution from algae.
Physical air flow due to natural thermal currents may also enhance the
oxygenation as may the use of external fans or pumps which are a feature on
some industrial units.
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