Assessing odorous emissions

How to monitor and assess odorous emissions in your permitted activity.

You will usually be required to monitor odorous emissions to comply with your permit and follow your odour management plan (OMP).

You will need to understand the odour:

  • character
  • chemistry
  • variability
  • volume
  • concentration

Doing this will help you:

  • work out if odour pollution is caused by specific site activities
  • make odour control decisions
  • assess the efficiency and performance of your odour abatement
  • use appropriate measures
  • comply with emission limits
  • focus attention on sources with the greatest potential to cause odour pollution

Monitoring odorous emissions can be expensive and time consuming. As with odour control methods, the monitoring you do to meet regulatory requirements must be necessary and proportionate to the risk of odour pollution.

Your monitoring must always be designed to meet a relevant and specific objective, such as those listed previously.

To show you are complying with emissions limits, you must use companies accredited by the Monitoring Certification Scheme (MCERTS) for sampling and flow measurements from ducts and stacks.

Measurements of channelled emissions are a necessary and proportionate part of showing the effectiveness of abatement systems.

Your assessments must be tailored to the abatement system being monitored and the context in which it is being used. For example, carbon filters may be very effective when first installed but stop working once they become saturated. Performance testing immediately after installation or maintenance may show good results but fail to reveal a significant drop in performance after a short period of time.

Monitoring diffuse or fugitive emissions is difficult and often requires specialised methods which require expert interpretation. For example, handheld meters may be useful for identifying emissions hot spots, but unable to capture representative concentrations or measure emissions volumes.

Olfactory monitoring of emissions at source

‘Olfactory monitoring’ means using human noses to assess odorous mixtures.

In most cases, the appropriate method for olfactory monitoring of channelled odorous emissions at source will be dynamic dilution olfactometry in line with BS EN 13725:2022. This method is suitable for short term sampling of relatively consistent and concentrated point source or open biofilter emissions. But it is not suitable for ambient air monitoring. It may also fail to capture peak emissions from variable activities. You must consider this when planning a sampling exercise or interpreting the results. You should carry out this monitoring under reasonable worst-case circumstances.

The Environment Agency maintains MCERTS for stack sampling which covers odour concentrations and flow rates.

You must only use MCERTS certified stack sampling companies to meet regulatory stack olfactory emissions monitoring requirements. These sampling companies will use accredited laboratories to carry out the laboratory analysis.

You must provide representative and safe monitoring points for stack sampling and safe access to biofilters. Follow the guidance on stack sampling in monitoring stack emissions: guidance for selecting a monitoring approach.

Bare nose assessment of odorous emissions is only appropriate and useful under very specific and controlled conditions. Deliberating breathing in concentrated emissions:

  • is ineffective
  • may subject observers to harmful levels of chemicals, biological agents, or atmospheres without enough oxygen

If you do carry out sniff testing, those doing it must be aware of, and take measures to minimise the effects of olfactory adaptation. Because of olfactory adaptation, incidental observations of odour may be useful, but you cannot used it to conclude that there is minimal or no odour potential from emissions.

Some laboratories offer a service to sample and characterise undiluted odours safely.

Instrument (surrogate) monitoring of emissions

As with odour impact assessments, surrogate monitoring of odorous emissions requires you to establish a ratio between:

  • the odour concentration
  • the parameter being measured

Sometimes, it may be possible for you to measure the concentration of a specific odorous chemical with a known odour potential, such as hydrogen sulphide. While this may be useful if the chemical being monitored is known to be present, it is also important to consider if emissions include other odorous chemicals.

Your instruments do not always need to monitor odorous chemicals directly. For example, you may use methane (which is not odorous) as an indication of odorous landfill gas being released.

Your instrument monitoring can take the form of:

  • gaseous samples which are taken to a lab
  • sorbent materials which concentrate chemicals of interest before laboratory analysis
  • point measurements of chemicals with field portable or fixed location instruments
  • spectroscopy instruments which measure chemical concentrations over long paths or areas

Within each of these general approaches, you can use different instruments and analytical methods, each with its own unique capabilities and limitations.

Chemical speciation

A detailed analysis of both bulk and trace chemicals can help you identify prominent odorous and non-odorous components of emissions. But this is not a practical method for working out odour concentrations.

Unless you are dealing with an abatement method which is proven to work within your sector for a particular process, you need to understand what chemicals and physical conditions are present when selecting and designing an abatement system.

Flow rates

To assess the risk of odorous emissions causing pollution, you need to know both the:

  • odour concentration
  • volume of odorous air being released

You must use MCERTS accredited contractors to measure flow rates of odorous air being emitted from stacks.

Less accurate methods, such as handheld devices, may be appropriate for regular ventilation performance checks or surveys.

Process controls

While not specifically a method for monitoring odorous emissions, important process control monitoring parameters can indicate if a process is likely to be odorous.

For example, adequate oxygen levels and moderate temperatures are good indicators that an aerobic composting process is:

  • under control
  • less likely to generate excessive odours

However, this approach is limited. In this composting example, it does not provide any insight into whether the feedstock materials might be odorous before arrival at the site.

You cannot use process control monitoring to directly assess odorous emissions. It can help you to achieve some of the emissions assessment objectives by focusing attention on sources with the greatest potential to cause odour pollution.

You should understand the implications of all important process control parameters. You will need contingency plans for either:

  • taking immediate corrective action
  • investigating further to work out appropriate corrective measures