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

The main components of an ESP are the filter housing, discharge and collecting
electrodes, power supply, gas guides or baffles and a rapping system for cleaning the
collecting plates.
ESPs can be designed to eliminate extensive quantities of smoke, giving collection
efficiencies of 99% at 1 µm and 90% at 0.1 µm. The effectiveness of an ESP is
limited to removing the grease that adheres to smoke and should not be considered
to be a primary source of odour control. Where installed, pre-filters should be fitted
upstream of the ESP to provide some protection from large contaminants that may
pass through the grease filters. The types of ESP that are used to treat kitchen
ventilation emissions are less effective at temperatures over 60 o C and are not
suitable for relative humidity levels of above 75%.
Where an ESP is used to treat oily fume the collecting plates can become fouled,
rendering them less effective. Weekly servicing should be the minimum requirement
and they should be cleaned immediately as soon as there is any sign of deterioration
in fume control. Some manufacturers have incorporated automatic self-cleaning
mechanism into their equipment to facilitate automatic daily cleaning. Plates 6 and 7
(Annex E) provide examples of an electrostatic precipitator.
4.7.4 Adsorption
Adsorption is a process involving the capture of airborne components on to a fine
particulate active surface. There are a number of possible active materials that can
be used for general application but activated carbon is by far the most widely used.
Other materials used are: Zeolites, silicas and polymeric resins. Activated carbon
systems are available that include impregnated oxidising materials e.g. chlorine
dioxide or permanganate, although such materials tend to be more expensive.
Adsorption is a dynamic process in which gaseous molecules impinge on the surface
of the solid and remain there for a period of time before desorbing again in to the gas
phase. An equilibrium is established between the adsorption and desorption
processes. For the majority of odorous emissions, where the organic mass emission
is low, the equilibrium amount of adsorbed material increases rapidly with small
increases in concentration. Thus, the adsorption process allows a positive driving
force for adsorption up to a point where the equilibrium position changes and the
odour concentration in the outlet exhaust increases. This is the point at which the
carbon has been fully utilised and there are insufficient active sites remaining for
further adsorption. The adsorption bed has thus reached the end of its useful life and
will require replacement or regeneration.
The adsorption process, ensuring odorous components adhere to the matrix, can be
physical e.g. adsorbed molecules are held to the surface by Van der Waals forces, or
chemical e.g. a chemical bond forms between the adsorbed molecules and the
surface. Adsorption, using carbon as a matrix, can be a very effective abatement
option, with odour removal efficiencies in the range 80 to 99% achievable.
Design considerations for adsorption systems include:
• Residence time typically of between 0.1 to 0.2 seconds for gas streams containing
relatively low concentration of odorants (this can rise to 1 to 3 seconds for gas streams
containing relatively high concentration of odorants).
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