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 depend on emission source characteristics.
Odours emit from various sources, such as stacks in an industrial
process, aeration tanks in wastewater treatment plant, composting
piles in a mushroom composting plant, chicken sheds on a poultry
farm and cattle feedlots. 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 such as a discharge stack from abattoir
or a vent from a pig shed. It is important that the pattern
of flow rate and odour concentration is estimated using an appropriate
procedure. Where key factors are unknown, a study should be
carried out to provide a good understanding of the gas flow
and concentration fluctuation pattern on a daily basis, even
in some cases, on monthly and yearly bases.
Odour samples are taken into Tedlar sampling bags loaded in
a vacuum drum through clean Teflon tubing probes inserted into
the stack at different points. The number of points required
is determined by the dimensions at the point where the sample
is taken. As a rule of thumb, the number of sampling points
needed to average air velocity across a stack cross section
can be used as a guide. The odour sample is collected using
an odour sampling system as shown in Figure 1. A vacuum pump
and a 12-Volt battery are built into the sampling drum as shown.
A new and cleaned Tedlar bag is placed into the sealed sampling
vessel. Air is then pumped out of the sampling drum by a battery
operated pump creating a vacuum inside the drum. Sample air
is drawn into the bag by the pressure difference between the
inside and outside of the bag.
Figure 1 Odour sampling system
It is important that air velocity, dimensions of the vent,
temperature and humidity are measured before a sample is taken.
For those samples with a high temperature and pressure, the
gas flow rate is calculated and adjusted to NTP (Normal Temperature
and Pressure ie 20 0C and 1 atmosphere) or STP (Standard temperature
and Pressure ie 0 0C and 1 atmosphere) conditions.
Area sources: Typically an area source will be a water
or solid surface such as the water surface of a slurry storage
tank or a cattle feedlot. A portable wind tunnel system can
be used to determine specific odour emission rates. The principle
of the wind tunnel system is that controlled air, filtered by
activated carbon through a series of devices, forms a consistent
flow over a defined liquid or solid surface. Convective mass
transfer takes place above the surface as odour emission happens
in the natural atmosphere. The odour emissions are then mixed
with clean air and vented out of the hood. A proportion of the
mixture is sucked into a Tedlar bag via Teflon tubing using
the sampling vessel. The air velocity used inside the wind tunnel
is 0.3 m/s. An isometric sketch of a portable wind tunnel system
is shown at Figure 2.
Figure 2. Isometric sketch of portable wind
tunnel system
The Specific Odour Emission Rate (SOER) may be defined as the
quantity (mass) of odour emitted per unit time from a unit surface
area. The quantity of odour emitted 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.
The volume per unit time is calculated from the measured velocity
through the wind tunnel that is then multiplied by the known
cross sectional area of the wind tunnel.
The wind tunnel system is designed to replace the previously
commonly used isolation chamber in the determination of odour
emission from area sources.
Building sources: Typically building sources, such as
chicken and pig sheds, have a number of openings. Prior to about
ten years ago, little research was undertaken on the determination
of odour emissions from buildings. For building sources, measurements
of both odour concentration and air ventilation rate are required.
The air ventilation rate from animal housing is dependent on
operational conditions (e.g. opening or closure of side flaps
or shutters), and ambient wind speed and direction.
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. Additional samples can be taken at different times of
the day or week or to understand the fluctuation of the odour
concentration levels within a day or a week. Similarly sampling
may be carried out for different weeks during a growout cycle
or for different seasons during a year or longer.
Fugitive sources: Typically fugitive sources including
odour emissions from soil bed or biofilter surface. The emission
normally has an outgoing or upward gas flow. In such cases,
wind has less effect on the mass transfer process. It is important
that the outlet concentration of a biofilter bed should not
be diluted in a way that is unrepresentative of the real situation.
