Odour Complaints

• Perception •          • Testing •         • Dispersion •       • Control Measures •         • Discussion •

Introduction

The number of odour complaints received annually by the Ministry of the Environment (MOE) is increasing and this trend will likely continue as housing developments encroach on industrial lands. In the past, open spaces between the smokestacks and the houses provided natural noise and odour buffer zones. Now, the city planners are permitting residential housing developments that abut industry and the residents take exception even though the industrial development pre-dates their existence.

Odour emissions have become a contentious issue between the regulatory bodies, manufacturing facilities and concerned neighbours. Some of the more controversial topics include: How many complaints are required before corrective action must be taken? How much damage can an individual inflict on a company? How do you handle a chronic neighbourhood complainer?

In Ontario, the Environmental Protection Act (EPA) is worded quite clearly and Section 14 (1) states:

Despite any other provision of this Act or the regulations, no person shall discharge a contaminant or cause or permit the discharge of a contaminant into the natural environment that causes or is likely to cause an adverse effect.

An adverse effect is defined as the loss of enjoyment of property, injury or damage to human, plant or animal life, and impairment of the safety or health of any person. This may range from actual physical ailments or symptoms to simple nuisance odour complaints. Therefore, if your plant emissions cause an adverse effect you are in contravention of the Act. This should not to be taken lightly as the EPA has provisions for large personal and corporate penalties.

Unlike most emission contaminants, odour is not well defined. Analytically, it cannot easily be quantified, there is no regulated limit and everyone can detect it. This presents unique challenges for clients and environmental engineers when developing odour abatement programs. The results must be effective at eliminating off-property nuisance odours, a somewhat fuzzy target, while maintaining reasonable capital and operating costs.

Odour Perception

The brain registers an odour response when an odorous compound comes in contact with the olfactory sense organs that are located in the upper respiratory tract. Different compounds can trigger widely varying responses amongst people, making odour assessment very subjective. What one person perceives as being very intense may be barely detectable by another. As well, one person may find an odour pleasant while another unpleasant.

Odour assessment properties include intensity, detectability, character and hedonic tone, which is the perceived pleasantness or unpleasantness. Detection is the most important parameter from an engineering control perspective and the goal is to reduce the off-property odours below the detection levels or to an acceptably low frequency of occurrence.

Some common compounds and their associated odour detection, and recognition levels in parts per million, are as follows:

As can be seen from above, the detection concentration varies significantly between compounds. To put things into perspective, an evaporated droplet, the size of a pinhead, of ethyl mercaptan could be detected throughout a 3,000 ft2 house, whereas it would take an evaporated cup of ammonia to reach detection throughout the same house.

Odour Testing

Odour testing can be accomplished in a number of different ways, however, the MOE approved procedure is to collect samples directly from the exhaust source followed by a multi-person odour analysis.

The odour samples are collected into inert sample bags for transport back to a laboratory for analysis. Typically, the odorous gas is diluted at the source to prevent condensation and maintain sample integrity. Analysis of the samples is usually conducted within 24 hours of collection under carefully controlled laboratory conditions. An odour panel of pre-screened participants sits in an odour-free room where they are presented with two or three sniffing ports. One of the ports, randomly selected, supplies the odorous sample and the others supply pure nitrogen. The odorous gas samples are introduced at increasing concentration to a point where half of the panellists detect the odour. This point statistically represents the odour threshold value (OTV) which is the number of times the pure exhaust sample needed to be diluted to reach 50% detection by the panellists. It is important to note that the pleasantness or unpleasantness of the odour, character, or intensity plays no part in the analysis.

Atmospheric Dispersion

Once exhausted, odorous stack gases mix, or disperse, with ambient air thus reducing the odour concentration. The degree of dispersion is dependent on a number of factors such as the distance between the exhaust point and the receptor, the exhaust gas and ambient air temperatures, the exhaust gas velocity, and the atmospheric conditions. As well, buildings and structures in the vicinity of the exhaust point can influence the air dispersion patterns and careful consideration is required when employing dispersion as an emission control option.

In Ontario, demonstration of regulatory compliance requires application of atmospheric dispersion models that predict the concentration of contaminants and odour at off-property receptors. Currently, the MOE specifies the use of a US EPA dispersion model ("Screen 3") for the prediction of off-property odour concentration. The MOE has selected a 10-minute time averaging period for results from the model. Inputs to the model include variables such as stack and building parameters and property lines, and the odour emission rate as determined from the odour panel assessment.Results are then compared to the maximum allowable MOE guideline concentration of 1 odour unit per cubic metre (ou/m3). One must keep in mind that marginally meeting the MOE guideline does not eliminate the probability of nuisance odour complaints. A concentration of 1 ou/m3 statistically infers that 50% of the population could detect the odour. Therefore, other factors must be considered when developing actual performance targets for an odour abatement program, such as, the economics of setting lower targets, frequency of exceeding target levels, spatial or critical receptor frequency distribution, impact of production variables, etc.

Control Measures

A wide variety of proven odour control technologies are available, however, care should be exercised in the selection process to ensure satisfactory results. Although it seems obvious, it is imperative that a thorough odour assessment be conducted so that the odour removal requirements can be matched to capabilities of the control equipment. As well, if there are multiple odour sources at a facility, the emissions from all of these sources must be considered since all will contribute to the off-property impact. Presented below is a brief overview of some of the more common odour abatement options available. This list is by no means complete and some process exhausts may require application of several techniques.

- Production Shifting, Reformulation and Process Retrofitting

Production shifting, reformulation and process retrofitting are generally classified as "front-end" control measures and are usually considered at the early development stages of an abatement program. These measures typically yield significant reduction in peak odour emissions with favourable economic returns. When back-end treatment is required, the combined implementation of front-end control can substantially reduce capital and operating expenditures while providing more effective control of odour.

- Enhanced Dispersion

Improved dispersion is a cost-effective option under certain circumstances, particularly where the odour is considered a nuisance and has no health related issues. Enhanced dispersion can be accomplished by increasing the exhaust stack height, increasing the exhaust gas velocity, or moving the source away from critical receptors. Although enhanced dispersion is currently acceptable to the regulatory agencies in Ontario, it is not favoured.

- Additives

A wide variety of additive products exist in the marketplace which are designed to mask or alter the characteristics of the process odour. Some of the products are injected into the exhaust gas stream to alter the intensity of the odour or to change the character or hedonic tone of the odour. Care should be exercised when using these products as they may actually increase or change the odour characteristics of your emissions thus causing a heightened nuisance. It is suggested that you contact reputable suppliers with references before considering these measures.

- Condensation

Condensation can be an effective control measure where the odorous constituents of the exhaust stream have a relatively high condensation or dew point temperature. Condensation involves cooling the gas stream down, converting the vapour to liquid, and then removing the liquid phase. Air-to-air or air-to-liquid heat exchangers are commonly used, as are water injection systems (scrubbers). Condensation is typically combined with particulate removal equipment (i.e., mist eliminators, electrostatic precipitators, etc.) to separate the condensed vapour from the gas stream after which the effluent is reprocessed, reused or disposed.

Odour removal efficiency of condensation systems should be cautiously considered since odours can be associated with gaseous phase constituents that are not readily condensed. Depending upon the nature of the exhaust gas stream, fouling of the heat exchangers and maintenance may become an issue. Also, the performance of the system may be seasonally affected by weather conditions such as with ambient air to gas heat exchangers. Finally, since condensate is created, disposal or recycling costs should be considered.

- Absorption

Absorption commonly refers to the intimate mixing of a gaseous exhaust stream with a liquid. The liquid is selected on its ability to dissolve or absorb specific constituents of the stream. Wet scrubbers are examples of absorbers whereby the exhaust stream is routed through a vessel and forced into contact with an injected finely atomized liquid solution.

The generally lower capital and space requirements make this technology attractive for odour control under certain circumstances. However, removal efficiency is generally low (<85%) and, secondary treatment or disposal of the effluent may be necessary depending upon the gas stream constituents. Corrosion and fouling may also impact maintenance costs.

- Adsorption

Adsorption is the process where gases and particles are attracted to and retained on the pore surfaces of an adsorbent. Typically, the adsorption capacity increases with the active surface area of the adsorbent. Common types of amorphous adsorbents are charcoal, silica gel, alumina and synthetic molecular sieves, such as crystalline zeolites.

Most adsorption systems operate similarly in that the contaminant laden gas stream passes through a collection media that traps or adsorbs the contaminants. The bed progressively saturates until contaminant breakthrough occurs and capture efficiency diminishes. Adsorption beds are either disposed of or regenerated.

High contaminant removal efficiencies (>90%) can be achieved by adsorption, and the regeneration and recycling aspects can prove to be advantageous. However, some contaminants will foul the bed making regeneration difficult and in some cases impractical.

- Biofiltration

Biofiltration is a natural process where odour laden gases pass through media containing biological organisms that consume the organic constituents as a food source. Biofilter systems are relatively simple in design and can provide high removal efficiencies (>95%). An advantage of these natural control systems is that the micro-organisms are somewhat self-adjusting to the food supply. However, as with all living organisms, they are slow to respond to rapid changes, thrive in a fairly narrow temperature and humidity range and can be poisoned by a number of chemicals. Biofilters are well suited for high volumetric flow rate, low organic loading exhaust streams and where the characteristics of the exhaust gases remain relatively constant.

- Oxidation

A number of oxidation technologies exist on the market with the common ones being thermal recuperative, thermalregenerative and catalytic oxidation. Oxidation is the process of separating or dissociating harmful organic chains, such as hydrocarbons, into innocuous compounds such as water and carbon dioxide. Oxidation is usually accomplished thermally and sometimes aided by catalysts.

Destruction and removal efficiencies (DRE's) for oxidizers are usually greater than 95% and commonly approach 99% or better. The amortized costs of purchasing and operating a thermal/catalytic oxidizer should be considered over strictly the capital cost.

Discussion

Odour is one of the most difficult contaminants to control since there is no well-defined regulatory or otherwise acceptable limit. The Ministry of the Environment odour concentration guideline of 1 ou/m3 using a 10-minute averaging period, is only useful when applying for a Certificate of Approval (Air) and for demonstrating initial compliance. However, ongoing compliance is established by the absence of odour complaints. If complaints occur, you are in contravention of the Environmental Protection Act. Regardless of the preventative measures already implemented, and more importantly the capital invested, odour complaints may necessitate additional control measures. Engineering design of odour control systems becomes challenging due to the subjective nature of odours. Conservative design inputs can be used, however, a sensitive nose in the neighbourhood, a slight change in the formulation, or peculiar weather conditions, can prompt complaints rendering the control system apparently worthless to the owner.

Effective odour control involves the development of an encompassing abatement strategy where the assessment of several control options with associated annualized costs and risk factors are considered. As well, it is prudent to discuss contingency plans in the event of odour complaints. This is particularly important where the margin of predicted compliance is minimal or where the odour is generally perceived as offensive. People are usually more tolerant when the odours are pleasant, however, there are no guarantees.

Finally, regardless of the abatement strategy selected, get written guarantees on the performance of control equipment. Frequently we are contracted to implement an abatement program only to find that our client had already purchased inappropriate equipment. Thorough engineering assessment of the odour producing processes will ensure that appropriate control technologies are applied. Successful implementation is further guaranteed through effective design, tendering and construction project management.

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