Odor-From Research to Practical Solutions
David Schmidt, MS, PE
Odors from livestock facilities are causing problems throughout the world. Although odors have always been associated with livestock production, no single factor is responsible for the drastic increase in odor related conflicts. Whatever the reason, it is clear that odors from livestock production must be significantly reduced or eliminated in the near future. It is also clear that the need to control odors does not exclude any species or size of operation. Researchers, farmers, and industries throughout the world are attempting to reduce odor production through various technologies. Although many technologies will fall short of their goals, others are sure to significantly reduce odors. This paper will attempt to review odor generation, emission, dispersion, quantification of odors, and some promising odor reduction technologies.
Odor Generation, Emission, and Dispersion
Gases are generated during the decomposition of organic material by microorganisms. Several different microorganisms are responsible for this decomposition. Each set of microorganisms thrive or die depending on their surrounding environmental conditions. Environmental factors typically associated with microbial activity are feedstock, oxygen supply, temperature, and pH. Most odorous gases are created in anaerobic environments. Although the production of nearly two-hundred odorous gases have been documented as a result of manure decomposition, the most notable are volatile organic compounds such as para-cresol, phenol, indole and skatol, and reduced sulfur gases such as hydrogen sulfide. The volatile organic compounds are produced during the first stage of the anaerobic decomposition of complex organic material. Hydrogen sulfide is produced during the final stages of anaerobic decomposition. This final stage of decomposition is also responsible for the generation of methane and ammonia.
The gases created by microbial activity are released into the atmosphere through standard physical and chemical processes. Gases diffuse through the liquid or are carried upward as gas bubbles. These gases must then move through the liquid-air boundary layer and into the atmosphere. The rate at which these processes take place is dependent on the concentration of the particular gas in the air and liquid, the temperatures of the liquid and air, the surface wind speed, relative humidity, and solar energy.
Once these gases are emitted into the atmosphere they are dispersed and transported away from the site. Wind speed, relative humidity and atmospheric stability are the primary factors responsible for the movement of these gases. These factors effect the concentration, frequency, and duration of these gases at the receptor. Concentration, frequency, and duration are all important factors that determine the impact of the odor plume. Depending on the conditions discussed thus far, odors can be detected several miles downwind from an odor source.
Quantification of odors is essential for any research involving the study of odor control technologies. Thus far the only reliable instrument for quantifying odor is the human nose. The nose can detect certain odorous compounds at concentrations far below that achievable with standard chemical analysis. Also, combinations of these gases impact the nasal receptors differently than single gases. These synergistic effects cannot be detected using standard measurement techniques.
The current method for using human noses in quantifying odors is through the use of an olfactometer. An olfactometer is an instrument which dilutes the odorous gas to preset dilution levels and presents this gas to a human panelist through a nozzle or port. Along with the sample of diluted odorous air, the olfactometer presents the panelist with two other choices or clean air. The panelist is then asked to select which of the three samples is different from the other two. If the panelist does not correctly detect the odorous sample the dilution level is decreased (more odor, less clean air) and the panelist is asked once again to select the sample with the odorous air. The dilution level at which the panelist correctly identifies the odorous air is called the dilution to threshold, or odor unit (ou). An odor unit is defined as the volume of clean air required to dilute one volume of odorous air to the point that it can be detected by the human panelist. To be more reliable, this procedure is repeated with six to twelve panelists and the results of all panelists are averaged. Other odor measurements conducted using odor panels include odor intensityhow strong is the odor at full strength; odor characterwhat the odor smells like; persistencehow quickly an odor dilutes to below detection levels; and hedonic tonehow offensive is the odor. These measurements can be added to the standard dilution threshold measurement to give additional information about the odor.
Olfactometry requires that an odor sample be collected in the field and brought back to lab. This method is most accurate when the odor samples are fairly strong. Because of this, samples gathered for olfactometry work are gathered directly from the odor source. A method for measuring odor levels out in the field uses an intensity scale from zero to five and requires the expertise of trained field monitors. These field monitors are trained to match odor intensity with the intensity of a known concentration of a specific gas, commonly n-butynol. The zero to five intensity scale indicates odors ranging from zerono odor detected, to fivea very strong odor. This direct scale method of odor measurement is currently being used at the University of Minnesota to monitor and compare the downwind dispersion of odors from various odor sources.
Odor research projects at the University of Minnesota
Odors are generated and emitted from livestock buildings, manure storages, and manure land application. Promising odor reducing technologies rely on reducing the generation of odorous gases, capturing and treating the gases that are emitted, or maximizing odor dispersion. For example, bacterial, enzyme, nutrient, or chemical additives to the manure storage are an attempt to reduce the amount of odorous gases that are generated. Injection or incorporation of land applied manure rely on "capture and treat" technologies. Odors that are released from the manure are captured and treated by the soil covering the applied manure. Windbreaks around livestock facilities are often cited as methods to increase odor dispersion. Researchers at the Department of Biosystems and Agricultural Engineering at the University of Minnesota currently have projects that address all principles of odor reduction. Below are short descriptions of several odor research projects.
Electromagnetic Pulse This project attempts to destroy the microorganisms in a manure storage by creating a pulsed electromagnetic field. This electromagnetic field destroys the integrity of the microorganisms in the manure.
TOAST Aerobic System This research focuses on a new technology for providing an aerobic environment for the microbes. This aerobic environment will produce carbon dioxide and water as end products instead of odorous gases.
Sulfur Balance Hydrogen sulfide is one of many odorous gases generated in manure storage structures. The objective of this research project is to determine how much impact dietary sulfur sulfates in drinking water affect hydrogen sulfide emissions.
Diet Manipulation This research is attempting to develop a swine nursery diet that will reduce hydrogen sulfide emissions by minimizing the sulfur intake and improving the sulfur digestibility and retention while maintaining pig performance.
Non-Thermal Plasma A non-thermal plasma reactor has been tested in the laboratory and in the field. This reactor produces highly reactive ions and ozone which react with hydrogen sulfide and volatile organic compounds thus significantly reducing the odors released from the system
Biofilters Several projects are currently reviewing the efficiency and effectiveness of biofilters. Biofilters are a method of treating odorous air through the use of aerobic microorganisms growing on an organic media such as compost, peat, or straw.
Manure Storage Covers A lab study and a field study are currently being conducted that attempt to determine the odor reduction from various manure storage cover material. Covers currently being tested include, straw, soybean oil, straw coated with soybean oil, Macrolite‘, Permalon‘, and a geomembrane.
Dust Control Odor is contained on aerosol particles that originate from the animal, the feed, and dried manure. This project attempts to reduce dust emissions by spraying oil on barn surfaces.
Dispersion Modeling This project is evaluating the use of computer models for predicting odor plume dispersion. Field odor measurements made by the direct scaling technique are compared to predictions made by the computer model. This dispersion modeling is part of a larger project attempting to predict odor emissions and dispersion from an odor source based on species, building type, and manure storage type.
Farmstead Odor Database This project attempts to quantify the range of odors coming from buildings and manure storages by sampling odor from several farms throughout the state of Minnesota.
Practical Odor Control
Reducing the impact of odors on the community is an essential part of livestock farming. Unfortunately the generation, emissions, and movement of odors is a very complicated process. And although several technologies currently exist that would virtually eliminate odors from livestock production, most of these technologies are not economically feasible. However, there are emerging technologies that look to be very promising. Although very few of these odor control techniques have been measured with olfactometry, anecdotal evidence from farmers throughout Minnesota suggest that some technologies or management techniques do reduce odors. This anecdotal evidence and on farm research cannot be overlooked. The following section describes some promising options for controlling odor from buildings, storages, and land application.
Livestock buildings are a source of odors that are often overlooked. The most significant problem with reducing odors from buildings is the ability to control gas generation or capture the gases before they are emitted into the atmosphere. Odorous gases are generated from the manure soiled flooring, the animals, and from any manure stored below the flooring. Each of these odor sources requires different control methods. Odor from flooring will be reduced if the floors are kept clean and dry. Anecdotal evidence suggests that some organic bedding, mainly compost or newspaper, may reduce odor emissions.
Control of odors from under floor manure storage depends on the type of manure storage. Manure stored longer than five days will generate more offensive gases. Therefore, to reduce odors from shallow pull plugs, the manure should be removed twice per week. Often this frequent cleaning is not a standard practice but may become a standard practice if odor control is a main objective. One method of shallow gutter management for odor control that is still under debate is the practice of using recharge water. Some facilities use clean recharge water, some recycle recharge water, and others do not recharge their shallow gutters. Anecdotal evidence suggests that using recycled recharge water may reduce odorous emissions from these systems.
There are very few options for reducing odor generation from manure stored below the barn in deep pits. One option that needs more research is reducing pit ventilation. Reducing or eliminating manure pit ventilation will reduce odors but may also create an unhealthy environment for the livestock and the workers. This risk is a function of the building ventilation system.
Building odors can also be captured and treated, provided that the building is mechanically ventilated. Building air that passes through a biofilter is relatively free of odorous gases and dust. Bioscrubbers have also been used to clean this air, however, bioscrubbers are expensive to build and operate. Other industrial air scrubbing technologies such as activated carbon may also be available in the near future to treat exhaust air.
Manure storage facilities can be the most significant source of on-farm odors. However, several technologies can significantly reduce odors from manure storage. One of the ways to reduce these odors is with a cover. A cover on a manure storage acts in one of three ways. A gas impermeable cover will capture the gases as they are released. These gases are then treated and released through the use of a biofilter, a flare, or some other technique. A gas permeable membrane serves to increase the boundary layer between the liquid and air thus decreasing the release of the gases. An organic crust is a combination of a gas permeable cover and a treatment system. The organic cover increases the boundary layer between the liquid and air and forces the gases that are released through an aerobic environment. This aerobic environment is filled with microorganisms that oxidize these offensive gases. This organic crust can develop naturally or can be created using straw or some other organic material.
Anaerobic digestion also offers the possibility of reducing odor from manure storages. An anaerobic digester controls the microbial degradation process and results in the generation of biogas which can be used to generate heat or produce electricity and a stable effluent. It is estimated that anaerobic digestion will reduce odors by 90%.
Aeration is another very effective means of controlling odors from manure storage. Proper aeration of manure will result in the production of odorless gases. Although aeration is very effective, the cost to aerate can be substantial.
Land application of manure typically brings about the most complaints. Fortunately, odors from land application can virtually be eliminated by injection or immediate incorporation. These techniques also increase the amount of nitrogen available for crop uptake because these methods also reduce ammonia volatilization. Unfortunately, injection and incorporation are techniques most easily adapted to liquid manure application. Incorporation of solid manure requires another pass with some tillage implement. This is both time consuming and costly but is necessary to achieve odor control.
One other aspect of manure application that generates odors is the agitation of liquid manure storage facilities. Agitation of these facilities is necessary to reduce the solid buildup in the bottom of the storage, break up any surface crust, and evenly distribute the nutrients throughout the manure. One method to reduce the need for agitation is the use of manure additives. Anecdotal evidence from many livestock producers suggest that some manure additives significantly reduce solids buildup. Although there has been little university research to support this claim, this technique should be viewed as a possible odor control method. Chemical additives also have the potential to reduce specific gas formation such as hydrogen sulfide during agitation. Research has shown significant reductions of hydrogen sulfide emissions during agitation with the addition of ferrous chloride.
The debate will continue on how much odor control is enough. However, one fact remains, odors from livestock production must be reduced significantly in order for livestock production to remain in harmony with the environment. Several technologies are currently available to reduce odors, unfortunately, simple economics often prohibit such technologies from being implemented. Currently there is a substantial effort by industry researchers, university researchers, and producer groups to find and implement odor control technologies. These efforts can be enhanced through the efforts of everyone involved in the livestock industry. Producers, engineers, technicians, consultants, veterinarians, nutritionists, and others must combine their expertise with the basic principles of odor generation, emission, and dispersion to develop and implement practical odor control solutions.