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Agricultural odors: what is the stink about?

Many people dream of country living with its open spaces, clean fresh air, and smells of grasslands, forests, and freshly mown hay. But occasionally the dream is shattered by unpleasant odors from animal feeding operations, food and ethanol processing plants, or other agricultural activities. To meet community expectations, owners and managers need to manage unpleasant odors.


Odor basics

Most odors are comprised of hundreds of volatile compounds, some of which are detected at concentrations as low as parts per trillion. While some people are hypersensitive to odors, others have a poor sense of smell. The complex mixtures that make an odor and the varied reactions complicate odor management.

Odors can evoke strong emotions and stimulate physiological responses. The perfume, personal products, and cleaning industries sell products with odors that people enjoy, that make them feel good, or that give them a sense of cleanliness and freshness. Aromatherapy products give off aromas that are relaxing or stimulating. Unpleasant odors can provoke strong responses, too.


Odors are described in terms of concentration, intensity, persistence, hedonic tone (i.e. pleasantness or unpleasantness), and character descriptors. When managing agricultural odors, it is common to focus on odor frequency, intensity, duration, and offensiveness (FIDO). Odor frequency describes how often, in a month, offensive odors are detected. Intensity is the odor strength, which is analogous to the loudness of a noise. Duration describes how long an odor event is detected, and offensiveness describes how unpleasant the odor is.

Odors can be managed by keeping the concentrations at non-offensive levels most of the time (90 to 98 per cent of the time). Many agricultural communities and units of government recognize that totally eliminating agricultural odors is not economical, so they strive to develop community expectations and regulations that allow offensive odors for limited amounts of time. Mitigation practices may be required if odor sources are expected to emit odors that will exceed community expectations. The setting of community odor expectations is a political process.

Odor measurement

Odors are difficult to measure and describe in a repeat-able scientific way. Most techniques rely on people trained to assess odor presence and intensity. There are laboratory devices, handheld devices, and people trained to measure odor intensity in the field. Care must be taken to prevent odor fatigue of the human olfactometry system, which is when a person no longer detects an odor after an intense odor experience or a long period of exposure to odorous air.

There are devices to measure chemical concentrations in the air, but there is no established relationship between chemical concentration and agricultural odor. Over 300 gases and vapors have been reported to have come from livestock manures. Research continues to develop a usable relationship between chemical concentration and agricultural odor and devices to measure odor.


Managing agricultural odors

Good neighbor relations and communications can help agricultural business owners be aware of odor issues. Being open to neighbor input about odor problems and letting neighbors know what is being done to mitigate odors helps establish effective communications. It is commonly recommended that people planning to build a new animal feeding operation or expand an existing one visit with neighbors to let them know what is being planned, including why and how odors will be managed.

Agricultural odors are generated, emitted, and transported from a source to a receptor. Receptor sites include property lines, neighbors' houses, and nearby public areas, including public roads, schools, parks, and towns.

Key odor sources on animal feeding operations include barns, open lots, manure treatment and storage units, and fields where stored or fresh manure is land applied. Ventilation exhausts and emissions from food processing and ethanol plants can emit odors, too.

Odors can be managed by reducing source generation or emission, capturing and treating odorous air, or by increasing dispersion. The goal of odor management is to reduce odor concentrations to non-offensive levels most of the time at receptor sites.

Several states use tools for assessing the potential odor impact of animal feeding operations. The tools consider feed-lot size, production practices, and local weather conditions that impact odor dispersion. The tools are typically used to compare alternative feedlot sites and the need for odor mitigation to meet local community odor guidelines.

Odor mitigation

Odor mitigation practices are an active area of research and development. One or more practices can be used. Mitigation practices used need to be effective, economical, and fit into the operation.

Animal diet and feed management are used to provide all of the essential nutrients and energy needed for optimum production while minimizing the nutrient emissions in feces and urine, enteric emissions, and subsequent emissions from stored and land-applied manure.

Chemical and biological additives are used to reduce odor generation or emissions during manure transport, storage, agitation, and land application. Chemicals can be added to oxidize odorous volatile compounds, adjust pH, or react with volatile compounds and form precipitates. Biological additives attempt to change the biochemical pathways that produce odorous gases.

Covers on manure storage reduce odor emissions by creating a barrier between the stored manure and the airflow above it. Covers can be cither permeable or impermeable. Permeable covers allow gas molecules and water to pass through, while impermeable covers trap most gas molecules between the cover and the manure. Impermeable covers can cut emissions by nearly 100 per cent.

Gas-phase biofilters can treat ventilation air or air from manure storage units. Biofilters absorb noxious gases into a biofilm where microorganisms break down the gases and use the energy and nutrients to grow and reproduce. Well designed and managed biofilters can reduce odors and hydrogen sulfide by as much as 95 percent and ammonia by 80 percent.

Enhanced dispersion mixes odorous gases in the air to reduce their concentrations to below detection levels. Dispersion is enhanced using vegetative buffers, wind walls, chimneys, and increased separation distances between odor sources and neighbors, towns, schools, parks, and other public areas.

The bottom line

Advances in agricultural odor awareness, research, and development have expanded our capabilities to assess and manage unpleasant smells, but many questions and challenges remain. In the meantime, people will continue to dream of country living, with its open spaces, verdant landscapes, and occasional odors.

ASABE member Kevin Janni is a professor and extension engineer, Department of Bioproducts and Biosystems Engineering, University of Minnesota, St. Paul, USA;
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Author:Janni, Kevin
Publication:Resource: Engineering & Technology for a Sustainable World
Date:Sep 1, 2010
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