Environment, Climate Change and Water

Index:

Odour control

Please note that this section of the manual may contain out-of-date information. It has been retained to provide general information until a revised version is available. For further up-to-date information on this topic please contact the EPA.

Aim of this document
Measuring odours
Odours, hazards and the law
Assessing odour impact
Controlling odours from area sources
Controlling odours from process industries
Summary
Further information

AIM OF THIS DOCUMENT

This document supplies technical information on how odours are measured and how odour emissions are governed by legislation. It further explains how different types of odours can be controlled so that their polluting effect on the environment can be minimised.

Authorised officers should consider this document as useful background information only; it is not a prescriptive guide, and in many cases officers will need to call in expert consultants in odour control.

MEASURING ODOURS

An odour is defined as a sensation resulting from the reception of a stimulus by the olfactory sensory system. The way the human response to an odour is evaluated depends on the particular sensory property that is being measured, including the intensity, detectability, character, and hedonic tone of the odour. The combined effect of these properties is related to the annoyance that may be caused by the odour.

Odour intensity

Odour intensity is the strength of the perceived odour sensation. It is related to the odorant concentration, which is an entirely different type of measurement. The following equation defines the relationship between the odour intensity (I) and concentration (C), where k is a constant and n is the exponent.

I (perceived) = k(C)ⁿ

Log I = log K + nlog (C)

This is known as Stevens's law or the power law. For odours, n ranges from about 0.2 to 0.8, depending on the odorant. For an odorant with n equal to 0.2, a 10-fold reduction in concentration decreases the perceived intensity by a factor of only 1.6; whereas for an odorant with n equal to 0.8, a 10-fold reduction in concentration lowers the perceived intensity by a factor of 6.3. This is an important concept that is related to the basic problem of reducing the odour intensity of a substance by air dilution or other means.

Odour detectability

Odour detectability or threshold is a sensory property referring to the minimum concentration that produces an olfactory response or sensation. This threshold is usually determined by an odour panel consisting of a specified number of people. The numerical result is typically expressed as occurring when 50 per cent of the panel correctly detect the odour.

With odour intensity levels at or just above 'threshold', odours become difficult to perceive. As a result, the actual values depend on the type of sensory test, the panellist selection, the detectability criterion, and other factors.

Odour character

Odour character or quality is that property that identifies an odour and differentiates it from another odour of equal intensity. The odour character is described by a method known as multidimensional scaling or profiling. In this method, the odour is characterised by either the degree of its similarity to a set of reference odours or the degree to which it matches a scale of various descriptor terms. The result is an odour profile.

Hedonic tone

Hedonic tone is a property of an odour relating to its pleasantness or unpleasantness. A distinction should be made between the acceptability and the hedonic tone of an odour. When an odour is evaluated in the laboratory for its hedonic tone in the neutral context of an olfactometric presentation, the panellist is exposed to a controlled stimulus in terms of intensity and duration. The degree of pleasantness or unpleasantness is determined by each panellist's experience and emotional associations.

ODOURS, HAZARDS AND THE LAW

Odorous air pollutants are often judged important primarily for their nuisance value and the number of complaints they generate. In only a few cases are there adverse health effects documented in measurable physiological terms. However, odours detected from biological processes may indicate contamination of the air by pathogens. Many compounds below odour detection level are now regarded as dangerous because of the results of risk assessment studies (for example, benzene).

Section 15a of the Clean Air Act 1961 makes it an offence for odours to be detectable by authorised officers outside premises scheduled under the Act. Control measures are imposed, particularly when the odour is assessed to be offensive. Scheduled premises are those listed in the Clean Air Act and include fuel burning equipment areas, grinding and milling works, chemical works and a variety of other industries. New legislation is likely to expand the list on an integrated schedule to a new Act. Even for unscheduled premises, depending upon circumstances, intensity and type, an odour could constitute a public or private nuisance.

ASSESSING ODOUR IMPACT

In recent times attention has been devoted to better and more repeatable measurement techniques for assessing odour levels. This, coupled with more realistic modelling of odour dispersion, can provide a more reliable assessment of odour impact.

Mixtures of compounds require dynamic olfactometry for assessment of odour level. This involves exposing a selected and controlled panel of observers to precise variations in the concentrations in a controlled sequence, to determine the point at which only half the panel can detect the odour. This point is called the odour threshold or one odour unit. The number of odour units is the concentration of a sample divided by the odour threshold.

There are several odour thresholds that can be determined using olfactometry. They are:

the detection threshold
the recognition threshold
the description threshold.

The detection threshold is defined as the lowest concentration that will elicit a response without reference to odour quality. This is reproducible and the most widely reported odour measurement in the literature.

The recognition threshold is defined as the minimum concentration that is recognised as having a characteristic odour quality.

The description threshold is the point at which the panellist is asked to distinguish the odour.

As a result of recent work by Sydney Water and the EPA, a criterion (with dynamic olfactometry and predictive modelling) has been developed for determining the likely acceptability of new developments with Section 15A legislation.

New dynamic olfactometry technology (2 port alternate forced choice) using eight panellists has allowed more reliable repeatable determinations of odour thresholds. New protocols provide quality assurance controls as outlined in the Draft Guidelines developed by the EPA and Sydney Water (1994). These developments allow the correlation between the recognition threshold (OU_50R) and the detection threshold (OU_50D) to be calculated. The criterion of one odour unit using the recognition threshold provides a more realistic assessment of the legislative requirements of Section 15A, when averaged over the nose response time of about one second and not exceeded more than 1 per cent of the time.

Odours that are produced from large areas (such as sewage treatment works, cattle feedlots and the spreading of effluent or manure) are classified as odours from area sources. Odours emitted from a process through a chimney or flue are classified as odours from process industries.

CONTROLLING ODOURS FROM AREA SOURCES

For large area sources like sewage treatment farms, cattle feedlots, composting, household or industrial tips and manure spreading, there are only two proven methods that can be used to reduce odour complaints. These are:

excluding development close to the site
ensuring that the operation is carried out under best management practice.

If development close to the site is to be excluded, a reasonable 'buffer zone' around the area source has to be determined. The actual size of this zone will depend upon a number of factors, including the size of the area from which the odour emanates, the intensity of the odours being emitted, the duration and frequency of the odour emissions, the actual process being undertaken, the topography of the site, the weather conditions that prevail at the site and the neighbours' perception of offensiveness of the odours being produced.

For example, a rural community may not regard low intensity feedlot odours as offensive, but the same odours close to an urban community may generate many complaints.

Best management practices (BMPs) will vary according to the industry producing the odour. However, for all new developments, BMPs will start with the site selection and the building of the facility. For example, composting in Holland is being conducted in specially designed, fully enclosed process chambers ('tunnels') instead of in the open.

In some cases these strategies reduce the production of odours because there is closer process control; if the odours are still produced it is often easier to control them.

There are a large number of chemicals and proprietary products that are claimed to reduce odour when they are applied to area sources. Be careful if you are considering using these products. To reduce odours they would have to be applied over very large area sources; the cost of materials and labour would be very high. The large quantities of these compounds required could themselves cause pollution. However, in certain industries it may be feasible to apply biologically active agents to change the odours being emitted to less odorous compounds.

CONTROLLING ODOURS FROM PROCESS INDUSTRIES

The gas stream emitted from a chimney or flue can be collected and made available for treatment.

The choice of method or combination of methods to be used for controlling odours in gas streams will be influenced by the following factors:

the volume of gas (or vapour) being produced and its flow rate
the chemical composition of the mixture causing the odour
the temperature
the water content of the stream.

While most odours are caused by gases, problems may also result from aerosols in the fumes. Odorous air streams frequently contain high concentrations of moisture. If these vapour discharges can be cooled to less than 40⁰C, a substantial quantity of the water vapour will be condensed and so reduce the volume of gases to be incinerated or otherwise treated. Mist filters can be used, also, to remove solids and liquids from the gas stream; if the odour is caused by these particles the odour will be reduced. The methods available to control odours in a gas stream are:

dispersion
scrubbing
incineration
adsorption on to a solid
biofiltration.

For complete control of odours it may be necessary to use more than one of these techniques (for example, scrubbing may be needed before adsorption or biofiltration). If the temperature of the gas stream is high, it may also be necessary to cool the gases before they are treated. This may be needed before the chemical scrubbing or the activated carbon adsorption method is used.

Dispersion

dorous gases require special consideration and treatment. The human nose is capable of detecting wafts of odorous gases lasting as little as one or two seconds. For such very short periods the concentration of gases in a dispersing plume may be up to ten times higher than the design values averaged over a three minute period. Dispersion of odorous emissions via a chimney is not recommended unless the emission source is properly quantified by an odour panel test.

The results of an odour panel test need to give the number of dilutions (odour units) required to disperse the odorous source to levels below the threshold odour concentration for 50 per cent of the odour panel (abbreviated as TOC_50%). It is generally not practical to disperse more than 200 odour units per cubic metre per second of exhaust flow without correctly designed chimneys. Warren Spring Laboratory in the UK suggests the following technique to estimate the uncorrected chimney height h_u.

h_u = (0.1DQ)^0.5

where D is the number of dilutions or odour units required to disperse the emission source to the TOC_50%, and Q is the volumetric flow rate in m³/s at 00C and 760 mm Hg.

An equivalent expression is:

h_u = (0.1M_o/TOC_50%)^0.5

where M_o is the mass emission rate of the odorous gas in g/s, and the TOC_50% has units of g/m³.

There can be considerable variation in the value quoted for TOC_50% in different references, so take care interpreting TOC values. The TD_50% in this example is the detection threshold.

The EPA uses computer modelling to size stacks for special situations such as odour emissions. More than 30 computer models are available, including those in the US EPA 'UNAMAP' version 5 package and the AUSPLUME model, which has been developed in Australia. All these models require detailed meteorological information and specialist assistance.

Incineration

Incineration is the oxidation of the odour into carbon dioxide and water by the combustion of the odour with a fuel and air. In some cases other compounds may be formed, depending on the mixture of fuel and air used, the flame temperature and the composition of the odour. These compounds could include carbon monoxide, oxides of nitrogen, and sulfur oxides.

It is important to reduce the moisture content of any gas stream requiring incineration in order to reduce fuel consumption. The following table demonstrates the additional cost of incinerating odorous vapours saturated with water vapour at various temperatures above 40⁰C.

Cost ratios of incinerating odorous vapours saturated with water vapour at various temperatures.

Saturation temperature	Cost ratio
40⁰C	1.00
50⁰C	1.05
60⁰C	1.15
70⁰C	1.36
80⁰C	1.81
85⁰C	2.22

An afterburner is basically a refractory-lined furnace fitted with one or more burners. The furnace should be fitted with a temperature indicator-controller and an independent high temperature alarm. Explosion protection systems should be installed if the odorous gas is capable of forming an explosive mixture or if gas is used as the fuel.

The furnace will normally consist of two chambers: a mixing section in which the odorous gases are mixed with auxiliary fuel and ignited, and a combustion section in which combustion is completed. The gas velocity in the mixing section is normally 8 to 15 metres per second to ensure adequate turbulence, but reduces to between 6 and 12 metres per second in the combustion section. The afterburner temperature should be measured where the gases leave the combustion chamber with the sensor shielded from furnace radiation.

The critical part of the afterburner is the contact between the odorous gases and the flame and, ideally, all odorous air should be used as primary or secondary combustion air. The efficiency of combustion will decrease with decreasing contact between the odours and the flame.

Catalysts can be used to allow incineration at lower temperatures with consequent fuel savings. However, these are susceptible to catalyst poisoning and plugging by solid or viscous particles, which makes the system ineffective. Designing the afterburner to allow for alternative thermal incineration in such events removes any capital cost advantages of a catalytic incinerator. Proponents of such systems would need to provide clear evidence of successful extended use in similar applications.

A chimney of adequate height should be installed on the afterburner to ensure proper dispersion of combustion products. A reduction in the efficiency of combustion may arise because of a change in volume or composition of the odorous gas stream. Problems may occur with the burner or fuel supply, and this could reduce the combustion efficiency. A tall chimney will help disperse any residual odours.

A boiler or furnace may be used as an afterburner, provided it is operating at a reasonable load when it is required to act as an afterburner. A liquid or gas-fired boiler, which may be on low fire for considerable periods, is unsatisfactory as an afterburner, unless a separate afterburner is installed. Such a system requires a changeover valve actuated by the boiler to divert the odorous gas to the afterburner when the boiler is on low fire. Non-condensed gases should be admitted to a boiler as primary burner air.

A coal-fired boiler with a chain-grate stoker is suitable as an afterburner because the odorous gases can be admitted below the grate as underfire air. Such coal-fired boilers normally have high residence times. The chimney height should be increased, if practicable, when it is known that a boiler will also be used as an afterburner. The additional stack height will improve dispersion and possibly prevent complaints if the odours are not completely incinerated.

Liquid scrubbing

Liquid scrubbing of gases to remove odours involves either absorption in a suitable solvent or chemical treatment with a suitable reagent.

Scrubbing brings the odorous gas stream into intimate contact with the scrubbing liquid. Unless the odorous substances are readily soluble in the liquid, it is imperative that a large liquid surface is exposed to the gas.

It has been suggested that liquid scrubbing becomes economically attractive compared with incineration and adsorption on activated carbon when the volume of odorous gas to be treated is greater than 5000 cubic metres per hour.

It is important that hot, moist streams are cooled before they contact scrubbing solutions. If this is not done the scrubbing solution will be heated and less efficient, and the scrubbing medium will become diluted from condensation of water vapour.

If a hypochlorite solution is used there is the chance that chlorine will be lost and the cost of replenishment can be high. There is also the chance that odours will be released from a hot scrubbing solution.

Condensation of moisture from the air stream can be achieved with either a direct or indirect condenser. An indirect water-cooled condenser separates the condensed water or condensate for the cooling water. In a direct condenser, cooling water is sprayed into the air stream and the cooling water will be contaminated with odorous condensate. If hot, contaminated cooling water is circulated through a cooling tower it is likely that odours will be released to the atmosphere. With an indirect condenser, the smelly condensate is segregated from the cooling water and may be discharged to sewer or a water treatment plant. The use of indirect condensers is preferred.

The principal types of gas absorption equipment are:

packed towers
plate or tray towers
spray towers
venturi scrubbers.

If the gases contain hydrogen sulfide a solution of sodium hydroxide may be used. When the odour is caused by the presence of unsaturated organic compounds, it may be necessary to use an oxidising agent such as chlorine, sodium hypochlorite, potassium permanganate, ozone or hydrogen peroxide. If chlorine, sodium hydroxide, sodium hypochlorite or ozone is used you may need to monitor exit gas streams.

Satisfactory results have been achieved using a sequence of chlorine gas, diluted sulfuric acid and sodium hydroxide to treat odours. The concentration of reagent in the scrubbing solutions must be maintained either by the use of a metering pump or by regular additions of reagent. (For example, you may specify 4 per cent free caustic soda in a sodium hydroxide reagent, and this should be kept consistent.)

Make provision to sample the exit gases from the scrubber to determine if the scrubbing is effective. Exit gases should be discharged through a stack, which should be higher than nearby buildings. This will avoid problems with building downwash, which may cause odour complaints, particularly if the scrubbing efficiency decreases.

A stack will safeguard against process changes or equipment malfunctions. Some additional instrumentation will be needed on the control equipment in order to monitor scrubber pressure drop, liquid flow, pump pressure, temperature and reagent concentration of the scrubbing solution.

Adsorption on to activated solids

A method that is suitable for controlling odorous substances, even at low concentrations, is adsorption on to activated carbon. To be effective, the contaminated air stream must be free of substances (such as dust) that might clog the carbon particles. The cost of replacing the carbon can be high, as simple systems use the carbon once only. More complex and expensive systems allow regeneration of the carbon for re-use. Regeneration can produce either a wastewater, which will require further treatment before disposal, or a concentrated vapour stream, which can be incinerated more cheaply than the original air stream.

One proprietary system uses activated alumina impregnated with potassium permanganate. The alumina adsorbs the odorous substances so that the permanganate can oxidise them, usually to carbon dioxide, water, nitrogen and sulfur dioxide, depending on their composition. The alumina bed is replaced progressively as the permanganate is exhausted. This has an advantage over carbon because no further treatment is needed; this may offset the cost of the alumina.

Proponents wanting to use an adsorption system should provide evidence of successful long-term application of the particular process.

Biofiltration

This method is becoming an acceptable and successful way of reducing odours from biological processes. For odours from process industries, where the odour is caused by one or a simple mixture of known chemicals, techniques are becoming available to increase the scope of biofiltration.

The procedure is similar to chemical scrubbing, except that in biofiltration the odour is removed by bacterial action. The bacteria grow on inert supports, allowing intimate contact between the odorous gases and the bacteria. The process is self-sustaining. In biological industries (for example, rendering works) it is usual to place the biofilter after the condenser.

Biofilters require careful attention to ensure continued operation. The bed may have to be replaced regularly because of mechanical failure.

Another type of biofilter is the soil-bed filter. Here the odorous gas stream is allowed to flow through a porous soil with a typical depth of 60 cm. The bacteria in the soil are responsible for the destruction of the odorous compounds. Typical reductions of odour by 99.9 per cent are claimed, with no decrease after a year's operation.

SUMMARY

Controlling odours is an important consideration for protecting the environment and our community amenity.

It is possible to detect odours scientifically and measure their impact on the environment. Once this is done there are different methods of control that can be implemented, depending on the source of the odour and various other factors. Officers should call in expert consultants if they are at all unsure when assessing development approvals where odours may be a problem.

FURTHER INFORMATION

For more information about odour control contact your nearest office of the EPA. Branches are listed in the Introduction to this manual.

IMPORTANT NOTE

The EPA has prepared this document for the manual in good faith, exercising all due care and attention. No representation or warranty, expressed or implied, is made as to the accuracy, completeness or fitness for purpose of this document in respect of any user's circumstances. Users of this document should carry out their own investigations and where necessary seek appropriate expert advice in relation to their situations. This document should be read in conjunction with other documents in this manual, and any other legislation and/or policies within which authorised officers operate; for example, Local Government officers should also refer to such legislation as the Environmental Planning & Assessment Act 1979, the Local Government Act 1993 and Occupational Health & Safety legislation, as well as specific Occupational Health & Safety policies developed by Local Government for its employees.

Page last updated: 14 February 2008

Operation type	Emissions	Odour control

Boiler	Sulfur dioxide	A
Acid plant	Acid gases	A, B, E
Fertiliser	Fluorides, fertiliser odour	A, B, F
Rendering works	Decomposing flesh, amines	A, C, D, G
Coffee	Aldehydes, amines	A, C, D
Chicken feathers	Amines	B, D
Fish meal	Amines, aerosols	B, C, D, E
Garbage	Decomposing organics, sulfides	C, G
Ammonia	Ammonia	A, B
Detergent and soap	Soapy	A, B, I
Oil refinery	Hydrocarbons, sulfides (complex)	A,C,G
Chemical plant	various (complex)	A, B, C
Rockwool	Burnt oil, aldehydes	A, B, C
Varnish and paint	Burnt oil, hydrocarbons, aldehydes	A, C
Solvent storage	Various solvents	D, A, G
Animal incineration	Amines, aldehydes	A, C, F, I
Grease trap waste	Amines	B, D, G, I
Fermentation	Yeast, fermenting	A, D, G
Non-ferrous foundry	Burnt core odour, aldehydes	A, C
Ferrous foundry		A
Laminated plastic	Phenols, formaldehyde	A, C
Fibreglass	Styrene, acrylates	A, D

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