For whom the smell tolls

The application of advanced control technologies to mitigate odorous emissions has been limited, largely because odour has not been regarded as a major environmental hazard or health risk. However, as the public easily identifies odours, many perceive a risk to exist. Martin Key and Mohit Uberoi of Megtec, an air pollution specialist exhibiting at this year's ET2001 exhibition, review the nature and causes of odours and offer some control and management options.

Assessing the effect of odour emissions is more complex than for the more traditional air pollutants. Direct measurement is difficult because odours are usually caused by a 'cocktail' of chemical species. It is possible to measure known chemical components by mass spectrometry, for example, but the correlation between individual component concentrations, possible synergistic effects and subjective odour perception is almost impossible to predict.

In addition, it is difficult to synthesise the olfactory response of the nose - the olfactory sense is extremely sensitive and varies with the receptor. It is possible to measure the strength of an odour by dynamic dilution olfactometry, whereby a sample is passed into an olfactometer (an instrument which mixes the odorous gas with clean air to provide a wide range of dilutions of the odour for assessment by a panel of pre-screened persons). The olfactometer determines the odour strength by using the subjective response of the panel to identify how many times the odour must be diluted to reach its threshold of detection. Whilst it measures strength, and not 'offensiveness', it is widely used for comparison of different odour sources and for design and verification of any odour control equipment.

Further, odour perception is dependent upon meteorological conditions, and odour generation within a process varies with raw material and process.

A familiar emission control hierarchy can, however, be applied:

  • Eliminate the odour source - an example of this is a chemical manufacturing process where odour was generated by the air transfer of odorous chemicals from a reactor to a storage tank. This was eliminated by inertion of the reactor and vacuum transfer with re-circulation of the vapour phase in a closed circuit.
  • Substitute the cause of the odour with a less odorous alternative - an example being a paint manufacturer who phased out highly odorous acrylates and replaced them with an alternative solvent with similar process characteristics but with much less pungent odour.
  • Minimise emissions by design, process optimisation and management. Containment of emissions is critical as the fugitive emissions from uncontained sources, such as open tanks, lagoons, leakage from buildings, process equipment and pipework may have a significant impact on the site odour profile.
  • End-of-pipe abatement. It is possible to optimise the solution to minimise investment, for example by separation of high intensity and low intensity odours to allow different forms of waste gas treatment. An example of this approach is an animal waste processing facility where high intensity cooker emissions are contained and treated by thermal oxidation, and raw material storage and general building odour are treated by biofiltration.

    Control techniques

  • Masking agents, or the use of one odour to mask another (e.g. the use of deodorant sprays to mask body odour). On an industrial scale this approach is only relevant for very low intensity odours or very diffuse sources. Masking has been successfully used on open effluent lagoons where the masking agent is sprayed into the air around the lagoon edge.
  • Absorption/scrubbing, based upon the absorption of odour into a liquid. There are a number of different types of scrubber used for odour control, including venturi, moving bed, plate, spray tower and packed bed scrubbers. The system relies on odorous gases being in intimate contact with liquor which either reacts with the odour species or in which the odour is soluble. Scrubbers have a wide usage, offer reasonable efficiencies for low to medium odour intensities and can be designed with confidence for a specified duty (indeed they effectively treat 'pure' odours where there is only one chemical group, such as acid gases, ammonia or hydrogen sulphide). However, scrubbers are inflexible and are difficult to adapt to change in emission profile. Also, they require regular addition of chemicals and produce a liquid effluent, which may require secondary treatment.
  • Adsorption, where the gas molecules are retained on a solid surface, which can be carbon, zeolite or a polymer. There are a number of variations of this equipment. Fixed and fluidised bed systems are effective for VOC emissions which can be recovered and re-used or combusted, but are generally inappropriate for mixed odour streams as the regeneration leads to a secondary emission requiring treatment. Disposable cartridge filters (typically activated carbon) have a limited use because they need regular replacement, there is the risk of media degradation from corrosive chemicals re-entrainment, and once saturated breakthrough of odours can occur - plus final disposal concerns.
  • Biofiltration is based upon the treatment of odours by the activity of microorganisms and fungi supported on a carrier media. The technique offers high destruction efficiencies for certain odours but is ineffective on gases which are not water-soluble.

    Odour audit

  • Thermal oxidation is the most flexible for odour control applications, as it can treat high concentrations of most odorous gases and offers the highest odour destruction efficiency (often over 99%). Although the most effective odour treatment system, the barrier to wider application for odour control has always been high-energy consumption. The development of regenerative thermal oxidisers means that the high odour destruction of oxidation can now be combined with a very energy efficient system (up to 95% lower running costs than a traditional thermal oxidiser).

    The first phase of an investigation into a potential odour problem is to carry out an odour audit of the facility. This will include a detailed process assessment and odour analysis by simple sensory means, and if the odour profile is very complex (e.g. where there are multiple sources) it may be necessary to carry out olfactometric monitoring to obtain relative odour quantification.

    In the final analysis, it is necessary to determine the balance between the costs and the environmental benefits of compliance.

    Equipment Type Sensitivity to various operating conditions Typical odour destruction efficiency
    sulphur temperature high loads changes in odour types
    Thermal oxidation No No No No >98%
    Catalytic oxidation Yes No No No >95%
    Biofiltration No Yes Yes Yes >95%
    Adsorption No Yes Yes Yes >95%
    Absorption No No No Yes >90%
    Masking agents No Yes Yes Yes 0-40%

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