4.0 MEASUREMENT AND MONITORING OF ODOUR

4.1 Terms associated with odour measurement

For better understanding of the methods of measurement of odour, definition of following few terms is required:

• Odour Intensity: Odour intensity is the strength of the perceived odour sensation. It is related to the odorant concentration. The odour intensity is usually stated according to a predetermined rating system. Widely used scale for odour intensity is the following:

(Half score is used when the observer is undecided)

• Odour Detectability or Threshold: Odour detectability or threshold is a sensory property referring to the minimum concentration that produces an olfactory response or sensation. With odour intensity at or just above "threshold" odour become difficult to perceive.

• Odour Character: Odour character or quality is the property to identify an odour and to differentiate it from another odour of equal intensity.

• Hedonic Tone: Hedonic tone is a property of an odour relating to its pleasantness. When an odour is evaluated for its hedonic tone in the neutral context of an olfactometric presentation, the panelist is exposed to a controlled stimulus in terms of intensity and duration. The degree of pleasantness or unpleasantness is determined by each panelist experience and emotional associations.

4.2 Sampling of odours

Odours are measured adopting olfactometric testing methods, which are psycho-physical methods. Olfactometery employs a panel of human noses as sensors. In these methods, the olfactory responses of individuals sniffing diluted odour presented by an olfactometer to determine odour strength or odour concentration.

Odour measurement requires representative samples of the air to be drawn into a sample bag and rapidly transported to an odour laboratory for olfactometric testing. Sampling strategies and techniques depends on emission sources characteristics. Each type of source has special requirements for sampling and sample collection.

Point Sources: Typically, a point source will be a stack with a known flow rate. Odour samples are taken into Tedlar sampling bags loaded in a vacuum drum through Teflon tube inserted into the stack at different points.

Area Sources: Area source will be water or a solid surface. A portable wind tunnel system can be used to determine the specific emission odour rate (SEOR). The specific emission odour rate (SEOR) may be defined as the quantity of odour emitted per unit time from a unit surface area. The quantity of odour is not determined directly by olfactometry but is calculated from the concentration of odour (as measured by olfactometry), which is then multiplied by the volume of air passing through the hood per unit time.

Building Sources: Building sources, such as chicken and pig sheds, have a number of openings. For animal sheds odour samples are normally taken from several points within a shed. Experience indicates that one composite sample is sufficient to represent a single shed at a particular time.

Fugitive Sources : Typically fugitive sources include odour emissions from bed or bio-filter surface. The emission normally has an outgoing or upward gas flow. Odorants in the atmosphere or gas stream can be collected by passing known volume of air or gas through a column of activated carbon or by condensing techniques.

4.3 Odour measurement

The olfactometric methods of odour measurement falls into two categories:

a. Determination of the threshold concentration of odoriferous gases:

For determining threshold concentration by the olfactometery testing procedure, a diluted odorous mixture and an odour free gas (as a reference) are presented separately from two sniffing ports at 20 lpm to a group of eight panelists in succession. In comparing the gases emitted from each port, the panelists are asked to report the presence of odour together with confidence level such as guessing, inkling or certainty. The gas dilution ratio is then decreased by a factor of two. The panelists are asked to repeat their judgment. This continues for six different dilution levels, resulting a total of 8x6x2 = 96 judgments (sniffing) from eight panelists. Using panelist responses over a range of dilution settings, odour concentration expressed as odour unit per cubic meter (ou/m³) can be calculated from individual threshold estimates. European threshold concentration ranges for some unpleasant odours are presented in Table 2.

For odour intensity measurement, generally a panel of 6 to 12 persons of normal health are employed. The panel members sniff the air at a given location at the same time and report individually the nature and intensity of the odour. By averaging the values recorded by members of the panel, a single value can be assigned to the odour intensity at a given location.

Generally odour intensity increases with the odorant concentration. The relationship between intensity and concentration can be expressed as:

P = K log S

where : P = Odour Intensity

K = Constant

S = Odour Concentration

Currently, the preferred and internationally standardized methods of measuring odour are the Dutch Standard Method (NVN 2820) and the more recent European Standard Method (CEN TC 264). A joint Australia New Zealand standard based on the draft CEN standard is in the process of preparation.

4.4 Critical factors in measurement of odour concentration

A dynamic olfactometer is a gas diluting apparatus and an interface between a panel of human observers and an odorous gas sample diluted at various concentrations. Olfactometery requires very high standards of testing conditions. These include following requirements:

• Odour free testing: An odour free testing environment is an important element in the olfactometery testing process.

• Olfactometer calibration: The olfactometer must be calibrated against a tracer gas to check the dilution setting of the olfactometer.

• Panelist management: Panelist should be trained and screened using reference air incorporating certified butanol at a concentration of 60 ppm.

4.5 Limitations in odour measurement

Odour concentration is only one of the four dimensions that are used to express odour sensation as experience by humans. The odour concentration is determined in an odour free environment and do not reflects the actual perception of the odour.

Common standardized instrument calibration and panel selection procedure are a pre-requisite for comparison of odour concentration data reported in the literature. In the absence of standardized procedures, odour concentration levels reported might simply reflect the experience of the operator, the design of the olfactometer, its operational mode, its mixing method, the flow rate presented to panelist and the number of panelist employed.