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

4.7.6 In-line oxidation systems
Oxidation using ozone and/or activated oxygen ions has been used to treat odour
emissions from commercial and industrial kitchen processes. Such systems use
electrostatic field or UV lamps to convert oxygen in the ambient air to oxygen ions
with a small amount of ozone. A mixture of oxygen ions and/or ozone is then injected
into the airflow of the exhaust duct or into a reaction chamber where reaction occurs
with the odour and other pollutants. Odours are oxidised in the reaction chamber and
the treated air is expelled.
Ozone and oxygen ions are inherently unstable and high moisture or temperatures
above 35 o C will promote ozone and oxygen ion degradation.
A typical dose rate for an ozone system treating exhaust gases from a restaurant will
be 1 ppm/v [27] . Average reaction times to completely oxidise material can be as high
as 20 seconds [28] . This reaction time needs to be taken into account when designing
duct work down stream of the injection device.
Due consideration needs to be given to the residual ozone that may arise from these
systems. Complete degradation of ozone is unlikely to take place within the duct work
and therefore extraction system must discharge at high level. There will need to be
restricted application of these systems in areas housing multiple commercial
kitchens. Plate 9 (Annex E) provides an example of an UV/ozone system.
4.7.7 Odour neutralising and counteracting agents
There are a number of products on the market claiming that odour emissions can be
‘neutralised’ by the addition of certain components into the air stream. The exact
definition of the neutralisation is slightly misleading and generally refers to a chemical
reaction in which the odorous molecule is rendered non-odorous by chemical
reaction with the added ingredient.
The neutralising agent is incorporated into an air stream by aspiration. A number of
the products on the market refer to the use of ‘essential oils’ derived from plants,
trees, wood etc. There are a number of mechanisms postulated as to what occurs
during the ‘neutralisation’ process, although the exact mechanism(s) are not fully
understood. Particular mechanisms suggested include Van der Walls forces, where
the molecule weakly bonds on to the odorous component, or chemical reaction.
Because the mechanism of this system is not well understood it is difficult to define
performance characteristics. Manufacturer’s data suggests that odour removal rates
of 90% can be achieved when used in conjunction with appropriate particulate
abatement.
Counteracting agents, as the term implies, are added to the air stream and result in a
reduced response to the odour by the human nose by reducing the perceived
intensity. Thus in effect, the counteracting agent desensitises the human nose and
renders the discharged odour undetectable by the olfactory sensors in the nose. As
this type of system does not remove odour the level of odour removal is likely to be
negligible.
With these systems correct chemical dosing is critical as the chemical agent that is
injected into the air stream can also give rise to odour annoyance. Such systems
should be avoided where the dispersion of the discharging plume is severely
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