Assessment and control of odour nuisance from waste water treatment: code of practice

ANNEX 2: Guidance on Odour Assessment and Complaint Investigation - Olfactory Screening ('Sniff-testing')

This is a very useful quick test which can provide a subjective "snap-shot" assessment of the presence, strength and character of an odour either within an installation boundary, at the boundary or in the area/community surrounding the site. A record should be kept of the meteorological conditions at the time of testing together with information relating to the operations and activities being undertaken at the time.

Routine assessments can help to build up a picture of the odour impact of the installation on the surrounding environment over a period of time. Assessments which are targeted at adverse weather conditions or particularly odorous cycles of an operation allow "worst case" scenarios to be developed. Ideally the same methodology should be used to follow up complaints.

The general principles are covered below.

General considerations

When undertaking an assessment, the following points need to be considered.

• The person undertaking the assessment should avoid strong food or drinks, refrain from smoking prior to the test, avoid scented toiletries or strong vehicle deodorisers and avoid undertaking an assessment with a cold or sore throat.
• Individuals that have a less than average sense of smell should be excluded. If necessary this can be compared with the general population by means of olfactometry.
• To improve (or to check) data quality the test can be conducted by two persons working independently during the same time period.
• Frequency of assessment should be determined by the potential for odour generation or the number of complaints as required.
• Remember that the nature of an odour can change over distance as the various components are diluted below their individual detection thresholds
• The health and safety of the individual undertaking the assessment should not be compromised. Containers should never be sniffed where there is any possibility of them containing, or having contained, substances which may be harmful.

Testing location

Where possible move from areas of weaker odour strength towards the more intense odour. The exact location will depend on the purpose of the assessment but when investigating off-installation odour, start well down wind and move towards the installation. It should be remembered that an odour may change in character over a distance as a result of dilution and/or conversion. A number of factors may determine the choice of location, including:

• Permit conditions relating to the installation boundary or sensitive receptors
• Complaints received
• Proximity of housing to the installation
• Wind direction at the time of testing

The wind direction should be ascertained and this will assist in selection of the assessment route. An assessment may involve walking along a route selected according to the above factors, or to the conditions found upon arrival. Alternatively points may be fixed in order to evaluate the changing situation over a period of some weeks or months, or may vary from test to test according to local conditions. The latter may be of use in identifying worst case conditions.

Data collection and recording

The intensity, extent (persistence and distance from the plant or installation boundary) of the odour and the sensitivity of the location where the assessment is being made with regard to receptors, should be recorded as well as any external activities such as agricultural practices that could be either the source, a contributor to, or a confounding factor in a particular odour event.

The categories of intensity, extent and sensitivity are:

Intensity

1 No detectable odour

2 Faint odour (barely detectable, need to stand still and inhale facing into the wind

3 Moderate odour (odour easily detected while walking & breathing normally, possibly offensive)

4 Strong odour (bearable, but offensive odour)

5 Very strong odour (very offensive, possibly causing nausea)

Extent (assuming odour detectable, if not then 0)

1 Local & transient (only detected on installation or at installation boundary during brief periods when wind drops or blows)

2 Transient as above, but detected away from installation boundary

3 Persistent, but fairly localised

4 Persistent and pervasive up to 50m from plant or installation boundary

5 Persistent and widespread (odour detected >50 m from installation boundary)

Sensitivity of location where odour detected (assuming detectable, if not then 0)

1 Remote (no housing, commercial/industrial premises or public area within 500m)

2 Low sensitivity (no housing, etc. within 100m of area affected by odour)

3 Moderate sensitivity (housing, etc. within 100m of area affected by odour)

4 High sensitivity (housing, etc. within area affected by odour)

5 Extra sensitive (complaints arising from residents within area affected by odour)

The observation period should be over a standard time, generally 5 minutes at each location. During this time the intensity and extent can be evaluated.

Installation-specific information should be recorded - activities being undertaken, influent flow rate and quality, tank cleaning operations, maintenance etc..

A record should be made of the atmospheric conditions during the assessment. In the absence of an anemometer, the wind speed can be approximated using the Beaufort scale.

Beaufort scale

Force	Description	Observation	km/hr
0	Calm	Smoke rises vertically	0
1	Light air	Direction of wind shown by smoke drift, but not wind vane	1-5
2	Light breeze	Wind felt on face; leaves rustle, ordinary vane moved by wind	6-11
3	Gentle breeze	Leaves and small twigs in constant motion	12-19
4	Moderate breeze	Raises dust and loose paper; small branches are moved	20-29
5	Fresh breeze	Small trees in leaf begin to sway, small branches are moved	30-39
6	Strong breeze	Large branches in motion; umbrellas used with difficulty	40-50
7	Near gale	Whole trees in motion; inconvenience felt when walking against wind	51-61

The key reporting parameters are set out in the following example of a reporting form :