Air emissions from livestock farms can cause animal and human health concerns which typically impact the immediate farm surrounds, but do not occur in high enough concentration to be of health concerns off-farm. But when emissions, such as particulate matter and ammonia, settle out of the air, impact to the soil and water occurs. Larger particulate matter can impact water quality, and ammonia can cause soil concerns as it is released from the air and settles on land. Particulate matter, in the form of dust, can cause air quality, visibility and odor concerns for neighbors.
Emission concerns from livestock are becoming more problematic due to the specialization and increase in size of livestock operations. As crop production and livestock rearing have become separated, the natural systems which work together to lessen environmental concerns can no longer be effective. Since 1920, the average commodity product produced per farm has decreased from just under six crops to just over one crop, as per the Natural Resources Conservation Service (NRCS) webinar “Air Quality and Animal Agriculture.” Manure isn’t deposited around fields and broken down. Land isn’t covered with pasture year-round, but crops are harvested for feed and fields are tilled and often left bare. Rotating grazing animals and crop production is no longer a focus on many livestock operations.
Generating air pollutants
Air pollutants — particulate matter, carbon dioxide, methane gas, nitrous oxide, oxides of nitrate, odorous sulfur, volatile organic compounds (VOCs), methane gas and ammonia — can all be generated on livestock farms. Animal activity; mixing feeds; manure handling, storage and spreading; and silage all generate air pollutants. Combustion from equipment also causes air pollution.
These pollutants all fall under one or more category of NRCS concern: particulate matter, ozone, greenhouse gasses, or odors. The primary elements causing air pollution are nitrogen, sulfur, and carbon.
Ammonia is generated via manure and urine. This gas has a strong odor, and can directly contribute to secondary particulate matter formation in the atmosphere. Microbial activity converts the nitrogen as manure is deposited, stored, and applied, releasing ammonia.
VOCs, such as alcohols and volatile fatty acids, are naturally emitted in the digestive process, during decomposition of organic materials and during combustion activities. VOCs are formed during microbial breakdown of larger carbon compounds. Over 100 gaseous VOCS are produced on livestock operations, and are ozone concerns.
Oxides of nitrogen are nitrogen dioxide and nitric oxide. Both are formed during combustion, and nitric oxide is a product of microbial processes which occur in soil and manure. Oxides of nitrogen are precursors to ozone, contributing to its formation. Nitrous oxide is a gas formed during microbial denitrification processes in manure.
Aerobic decomposition of manure releases carbon dioxide, as does breathing, combustion, and the fermentation of silage or other biomass. Anaerobic manure decomposition results in the release of both odorous sulfur and colorless, odorless methane gas. Methane, a carbon-based gas, is also emitted by ruminants as part of the digestive process. Sulfur based gases include hydrogen sulfide — the rotten egg smell — and dimethyl sulfide.
Dust and particulate matter are directly emitted from bedding, feeding, and animal dander. This category includes smoke, dust and aerosol particles. Tillage or wind erosion of soil also emits these pollutants. The composting of manure solids is another means of particulate matter generation. Secondary particulate matter is formed in the atmosphere, while primary particulate matter is directly emitted into the atmosphere. Both are air pollutants.
Generation of air pollutant is dependent upon nitrogen, carbon and sulfur levels, moisture levels, temperatures, and pH levels. Addressing the generation of pollutants is more cost-effective than focusing on controlling their emission. Once generated and emitted into the air, transport of contaminants depends upon their reactivity with other compounds, meteorology, topography and air flow.
NRCS can address air pollution generation issues via feed management plans, dust control, nutrient management, the use of roofs and covers, manure separation, manure composting facility design, anaerobic digesters, and manure lagoons. Practices such as prescribed grazing and planting of rangeland can also be used to reduce pollution generation.
Once emitted into the air, facility layout, topography, windbreaks and shelterbelts are the options for reducing the impact of air pollutants generated on the farm.
Direct particulate matter emissions occur during periods of low moisture, high winds, poor combustion, or crushing activities. On feedlots or in barnyards, dust is generated from manure and dirt, and carries odors. Managing dust through manure management practices and via water sprays to keep dust particles from being generated are the primary control mechanisms. Secondary strategies include adjusting feed schedules to reduce activity levels, using mulches on dirt areas, and adding emulsifiers to water to make spraying for dust control more effective. When environmental conditions are favorable, dust can become a major concern for livestock operators, as occurs in the large feedlots found in Texas and surrounding states.
Gaseous emissions such as VOCs, carbon dioxide, and nitrous oxide are dependent upon concentration levels and are diffused into the air. Increased temperature and moisture levels often favor release. Methane and odorous sulfur gases are primarily released from liquids via bubbles which rise to the surface. Agitation can cause release of gases, as well as of liquid aerosols.
Livestock producers can combat air quality concerns via herd management and manure handling practices which make the environment less conducive to the generation of pollutants. Once generated, the emission of pollutants can be controlled to some degree, but is not as cost-effective as prevention. NRCS agents are able to advise farmers on best practices to reduce air quality pollutants on their farms, and provide solutions to address specific concerns.
For further information, refer to this NRCS Factsheet at http://tinyurl.com/hhwlo26