Guidance on the Control of Odour and Noise from ... - Defra

• degree of saturation of the liquid; and
• reactivity of the odorant within the absorbing solvent.
Providing the odorant is reasonably soluble in water then an absorber can be
designed to achieve a desired removal efficiency. The problem with using water is
being able to maintain a low enough concentration within the liquid to maintain the
driving force for dissolution. This often results in excessive volumes of water being
used. Water only systems can be employed where the removal is due to
mechanisms other than absorption. For example, cooling can be used to cool an
odorant to a temperature below its boiling point.
The efficiency of absorption can be increased if the absorbing liquid contains a
reagent, which reacts with the odorant present in the air stream. These reagents are
generally oxidising agents, for example:
• sodium hypochlorite;
• hydrogen peroxide;
• ozone; and
• potassium permanganate.
The use of acids and alkalis as absorbing medium is widespread. An oxidising
medium can impart its own odour to the final discharge, which may then contribute to
annoyance. Systems based on oxidising agents can have high operating costs due
to high oxidising agent use. The gas-liquid tower or absorber is designed to provide
the largest possible liquid surface and good counter current flow of gas and liquid. A
number of absorber designs exist and these include:
Spray system. These systems comprise a liquid spray which contacts with an
upward rising air stream. Such systems tend to have limitations in mass transfer
from gas to liquid but can deal with particulates and condensable materials.
Venturi scrubbers. The exhaust gas stream is forced through the venturi throat
where they are intercepted by an atomised scrubbing liquid stream. These systems
are sometimes employed to control oily fume. Removal efficiency depends on the
pressure drop across the venturi throat and particle size. However, noise may be a
problem since air velocity at the throat is high.
Packed tower scrubbers. These scrubbers are filled with specially designed
packing materials to increase the contact surface area between the scrubbing liquid
and the waste gas stream to enhance the absorption efficiency. They can remove
oily fume by absorption as a stream of scrubbing liquid is sprayed on the oily fume
laden gas stream. Packed tower scrubbers generally occupy more space for
installation and are generally heavier because of the weight of the packing.
Other systems. Novel scrubbing systems are under development, for example an inline
brush scrubber (Plate 8: Annex E) has been pilot tested to remove grease and
odour from the exhaust air of commercial burger kitchens. These systems may be
available on a commercial basis.
58