Odour Control: Not to be Sniffed At

Composting forms an increasingly important component of the organic waste management process in many countries. However, in spite of its many benefits, the process is also notorious for producing odours. however, by following best practice many of these issues can be avoided.

Image credit: benketar

By Jak Fazakerley and Danen Appasamy

In simple terms the composting process involves the controlled biological decomposition, stabilisation and maturation of organic substrates to produce a high-value product that can be utilised in land, horticulture and turf management.

Over recent years there has been a surge in the number of small-scale (households), medium-scale (community groups and farmers) and large-scale (centralised licensed facilities i.e. open-windrow, in-vessel and aerated static pile) composting operations taking place in the UK. As residential and commercial developments expand and begin to encroach on once remote locations, the ability to find suitable sites become ever more difficult; particularly those that suit the desired topography, location to sensitive receptors, vehicle access and site services.

Such positioning difficulties have resulted in a plethora of environmental complaints, most notably those from the perceived issue of foul odours.

Alleged operator negligence has led to opposition from local residents; resulting in complaints to local authorities and the Environment Agency, and in turn Enforcement Notices being served.

Composting and Odour

One of the most commonly cited reasons for failure to achieve planning permission or an environmental permit to develop and operate new composting facilities, is that of the perceived issues surrounding malodour generation. Ultimately, there will always be the potential for odorous releases at composting sites due to the nature of activities, as well as to external environmental factors. However operators must instil confidence with regulatory authorities and the local community that sites can be managed to a high level of efficiency.

In England and Wales, odours arising from composting sites are regulated under a number of laws, including ‘Section 33(1) (c) of the Environmental Protection Act 1990' and ‘The Environmental Permitting (England and Wales) Regulations 2010 (as Amended)'. H4 Odour Management (March 2011) is part of a suite of guidance notes (subsidiary documents to ‘How to comply with your permit') that were issued by the Environment Agency to help holders and potential holders of environmental permits understand their regulatory requirements with regard to odour.

A condition contained within the guidance document is the requirement for a variety of activities to submit an Odour Management Plan (OMP) as part of the permitting process.

Monitoring should form part of every site's working plan in order to assess the nature and extent of an odour

OMP's should clearly demonstrate an operator's understanding of source-pathway-receptor impacts, and a commitment to controlling and minimising odour pollution through the appropriate use of monitoring and contingency measures. It is important to note that the degree of process control varies depending upon the size and location of the site, the nature of the feedstock and the intended use of the end product. The selection of the most appropriate conceptual solution will require careful thought and planning; no two sites have identical problems relative to their layout and design.

The outputs from odour modelling are typically represented as a visual representation which helps show the fluid dynamics of odours
Odour Recognition

Odour is the perception of volatile substances within the olfactory area of the sinuses. Whether or not an odour can be classified as a nuisance depends on a number of factors. The FIDOR acronym is a useful reminder of the factors that will determine the degree of odour pollution:

Frequency of detection - number of occurrences having a certain characteristic Intensity as perceived – strength, concentration, power or force Duration of exposure – continuance or persistence in time Offensiveness – nuisance, displeasure, anger or resentmen Receptor sensitivity.

Sensitive receptors need to be considered when determining measures, as some receptors are more susceptible to the adverse effects of exposure than others. Domestic residences, hospitals, schools and day-care facilities, for example, are more likely to be sensitive than an industrial complex.

More specifically, some individuals may be extremely tolerant of odours at high intensities while others will be unable to tolerate an odour as soon as they identify it. In addition, the degree of pollution increases with the size of the exposed population. Therefore, the more people that are affected, the greater will be the justifiable expenditure on control measures. However, even if only a very small number of individuals are affected, the seriousness of the exposure may require further control measures.

Ultimately, it is important to engage and communicate with the people who may be affected by the activities. Many operators do this as a matter of course and have well-established procedures for interfacing with the public.

Individuals who work in the immediate proximity to odour sources and who are exposed to relatively high concentrations of odorous chemicals for extended periods of time will typically experience long-term adaptation which is specific to those odours. On the other hand, any offsite receptors will experience a constantly changing concentration because of dispersion patterns in the air. They will not tend to experience adaptation to the same degree because the variable experience of odorant chemical concentrations is more episodic and short-lived.

It is important that site operators appreciate both long and short-term adaptation mechanisms as it helps to explain why offsite receptors perceive odours as a nuisance while operators may not find them particularly offensive. It is also important to keep this type of adaptation in mind when selecting individuals to investigate odours; staff who work on-site may be uniquely unqualified to assess these particular odours as they will normally experience both short and long-term adaptation.


Monitoring is the process of observing changes that may occur over time. It should form part of a site's working plan in order to assess the nature and extent of an odour, and consequently the effectiveness of operational practices/procedures to prevent and contain the odour. In order to monitor the composition and concentration of odours effectively one or more different techniques should be employed.

Each method should be used as appropriate under the particular conditions prevalent, for example if one is looking to assess impact, assess exposure, investigate sources and pathways, measures releases and control processes. The records from emission monitoring should be retained and include enough information for use in analysis (date, time, monitoring technique(s), emission source, weather conditions, dispersion conditions, site activities, external factors etc.).

Grab Samples and Dynamic Dilution Olfactometry - The standard method for measuring odour in Europe (BS EN 13725:2003). Odorous air is collected in sealed bags that are inert and have no intrinsic smell i.e. Nalophane bags, Tedlar bags and PTFE bags. These bags are filled using either a pump or by placing the bag in a chamber under negative pressure. The bags are then taken back to the laboratory for analysis. A minimum of four panellists, selected based upon their ability to smell a reference chemical, sniff the odorous air which is diluted with odour-free air. The result is the number of dilutions used when 50% of the panel can detect the odour (thus a dilution detection level of 1ouE m3 is applicable).

Field Assessment - Suitable for daily monitoring purposes and provides a basic overview of odours at the perimeter of the site. Subject to an appropriate person undertaking the survey (i.e. observers who have been trained and tested to understand their own sensitivities), this can determine the presence/absence, character and intensity of an odour. More objectivity may be brought to sniff testing by the use of aids such as a field dilution olfactometer. However, in the event of a complaint being received, the officer investigating may be unable to validate complaints using this technique due to the inherent high variability in releases and dispersion pattern.

Chemical Monitoring Techniques - Electrochemical Detectors (electronic noses), Gas Chromatograph Mass Spectrometer (GCMS), Gold Foil Instruments and Long Path -Length Monitoring.

Odour Diaries - Diaries may be provided to those members of the community who are frequently reporting odours. This can provide valuable insight into the conditions which result in community exposure; allowing for correlation with site activities and meteorological data. Above all, the sooner links can be made with local residences and a dialogue established, trust and confidence can be inspired between the two parties.

Community Surveys - These must be carefully crafted to avoid guiding or biasing the comments of people being surveyed. However, if undertaken properly can provide a useful context across a wide range of residents.

Good Site Practice

Odour management represents a major commitment for most site operators. Effective odour control is achieved by minimising as many of the links between odour formation and annoyance as possible. By applying a systematic approach, rather than single measures, odour management can be more effective, robust and inexpensive. However, it is important that priority is given to controls that can be used at the earliest possible stage in the process and that the approach taken has clear and realistic expectation of desired outcomes.

There are a number of good practices and BAT/appropriate measures that can be used in order to prevent the generation and subsequent release of odours off site. Many techniques are complementary and should be viewed as a toolkit so to allow for effective anticipation, planning and management of the processes. Such approaches include;

Good Housekeeping: I.e. maintain a tidy, secure and safe working environment; contain materials appropriately; reduce spillages to a minimum; and minimise leachate 'ponding';

Managing Inventory: I.e. provide adequate provision for the acceptance of seasonal peak volumes of delivery (during spring and autumn); understand the types of waste the site will receive; know how long the waste was held before it is delivered; recognise the transport conditions; treat odorous materials promptly in a way which reduces their odour potential; keep odorous materials on site to a minimum, rotating stock where appropriate; aerate some feedstock prior to processing (this may be able to extend holding times); keep stockpiles relatively small (depending on feedstock structure and porosity); and carefully monitor for excessive temperature, moisture and aeration and select building materials which can be easily cleaned.

Containment: Enclosure of the composting process, particularly shredding and storing, within a building means that nuisance factors can be better captured and treated. High-speed roller shutter doors are often used to ensure an immediate airlock solution which prevents emissions leaving the building.

In-vessel technologies, i.e. batch tunnels, vertical composting units, rotating drums and enclosed halls, are available for treating animal by-products and catering waste feedstocks. These have the ability to capture and treat emissions during the sanitisation phase (often the most problematic for the generation of odours due to the higher respiration rate experienced within the feedstock) prior to their release to atmosphere.

To assist in the containment of emissions from open-air windrow facilities, the option of covering windrows using a fabric or membrane cover, such as Gore-Tex, is a possible solution.It enables fugitive emissions to be contained within a textile membrane cover that encapsulates the compost windrow. Air can be positively injected into these membranes or drawn out and taken through a biofilter. This option will lessen the release of emissions to atmosphere at a cost that is significantly less than that of erecting a permanent structure.

Abatement (end-of-pipe): I.e. adsorption using activated carbon, zeolite, alumina; chemical scrubbing (single-pass scrubbing, acidic/alkaline scrubbing, oxidising scrubbing, packed column, plate or spray towers); biological treatment (trickling/soil bed/non-soil biofilters); absorption (scrubbing); and odour treatment chemicals (masking agents – deodorisers). Many abatement techniques have the potential for high removal efficiency (85%-90%) and ability to handle highly variable air streams provided that they are proactively maintained.

Community Involvement

In order to inspire confidence, credibility and trust with the local community, operators need to be wholly transparent in their actions. By engaging with the local community, and being thought of as accessible, accountable and caring, a relationship can be established which will help to reduce the perceived level of threat (or other emotive feelings) from the site. It will help if operators can take a more inclusive approach to the local community and encourage them to come on site at an ‘open day' events.

In this way active engagement will improve the understanding of neighbours and allow them to make informed decisions. Following this, and in the event of an odour incident, communication plans should be in place to effectively communicate to all key stakeholders the reason for the odour, the likely duration, strategies to overcome the current problem, preventative plans to avoid a re-occurrence of the problem, the grievance procedure the party can take and who is the responsible person on site to contact.

Modelling Odour Exposure

Air dispersion modelling is often used as part of the planning and permitting process to evaluate the movement of odour from a source; investigate causes of odour nuisance; arrange emissions points (i.e. stacks) at locations which are further away from nearby sensitive receptors - thus increasing the dispersal distance before they reach the ground; and/or explore the relative benefits of various odour minimisation options.

Models often take into account local topographical and meteorological data, emissions concentrations, site layout, operational parameters, and source dimensions in order to determine the movement of odours from the source. Predictions are normally expressed as 98th percentiles of hourly average concentrations over a year.

A 98th percentile concentrations for three odour units (three dilutions to the average detection threshold) means that in a year (8760 hours), the average concentration for 98% of those hours will be below three odour units. For 2% of the hours (175 hours), the average hourly concentration will be three odour units or above. The outputs from odour modelling are typically represented as a visual representation; thus helping as a tool show the fluid dynamics of odorous compounds.


Offensive odour is one of the most difficult areas to regulate due to its subjective nature: what is offensive to one person may not be offensive to another due to variation in people's sensitivity. However, it is important to undertake good working practises and appropriate monitoring, based on methodical decision making and published guidance, to minimise and mitigate exposure.

It is also vital to work with affected communities, both on a formal and informal level, so to keep them aware of actions taken and timescales to resolve issues. Although this article has focused specifically on the composting sector, many of the issues contained can be implemented cross-sector i.e. wastewater treatment plants, agricultural practises etc.

Jak Fazakerley is an environmental consultant and Danen Appasamy is an environmental scientist at Crestwood Environmental

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