Air Emissions from Animal Feeding Operations Current Knowledge, Future Needs (2003) / Chapter Skim
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5. Approaches for Estimating Emissions
5. Approaches for Estimating Emissions
Pages 98-128
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From page 98... ...
Emission factors, expressed as the mass of each substance emitted per animal or other base unit per unit of time, are then used to estimate air emissions from other AFOs, which are assumed to fall into one of the categories in the defined set of average AFOs used to estimate the emission factors. Weaknesses in this approach noted in the committee's interim report (NRC, 2002a)
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From page 99... ...
is "to develop a method for estimating [air] emissions at the individual farm level that reflects the different animal production methods that are commonly used at commercial scale operations." The approach was intended to provide estimates of total annual air emissions from AFOs for defined geographic areas by kind of animal and manure handling practices for each of eight kinds of emissions.
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From page 100... ...
· defining models in which the dependent variable the amount of each air emission per unit of time is closely related to independent variables that accurately depict real feeding operations and that explain a substantial share of the variation in the dependent variable; providing accurate estimates of the relationship between the dependent and independent variables; and · having estimates of the relationships between dependent and independent variables that clearly distinguish among the kinds of AFOs being modeled. A critical requirement for estimating the appropriate emission factors is a statistically representative survey of emissions from a class of AFOs over several iterations of the time period to be represented.
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From page 101... ...
cannot be supported for estimating air emissions from an individual AFO because it cannot account for a great deal of variability among AFOs. In particular, important factors not included in the EPA model include animal feeding and management practices; animal productivity; housing, including ventilation rate and confinement area; use of abatement strategies such as sprinklers to decrease dust; and physical characteristics of the site such as soil type and whether the facility is roofed.
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From page 102... ...
Principal component analysis might be used to determine which factors contribute the most to emission rates and to evaluate whether certain practices give significantly different results than others. Compared to the existing emission factor approach, this alternative would result in more accurate estimates of emissions from individual farms and the identification of strategies to decrease emissions.
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From page 103... ...
should use process-based mathematical models with mass balance constraints for nitrogen-containing compounds, methane, and hydrogen sulfide to identify, estimate, and guide management changes that decrease emissions for regulatory and management programs. · EPA and USDA should investigate the potential use of a process-based model to estimate mass emissions of odorous compounds and potential management strategies to decrease their impacts.
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From page 104... ...
~ Product Herd/FIock \ l Manure / a: ~1 Export FIGURE 5-1 A schematic representation of a process-based model of emissions from an animal production system. Emissions can be to air or water.
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From page 105... ...
An animal feeding operation may not include all of the components shown in Figure 5-1 on one farm. However, a complete system for animal production must include crop production and manure utilization or disposal even if these activities are carried out on separate farms.
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From page 106... ...
The efficiency of animal production has increased continuously and is improving still, resulting in lower methane emissions from the animals and lower nutrient transfers to manure (Havenstein et al., 1994~. Unless a process-based modeling approach is used, it is unlikely that any group will be able to develop a model to represent all of the techniques used to improve the efficiency of nutrient use by animals and to incorporate all of the possible current technologies, or to keep up with the development of new tech
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From page 107... ...
The approaches to models for each pollutant depend on whether the pollutant is important because of long-term global consequences (e.g., methane) , shortterm local consequences (e.g., hydrogen sulfide)
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From page 108... ...
These estimates are expressed as a fraction of precursors available for emission from the farm and may be affected by animal type, soil type, cropping system, cropping area, weather and climatic conditions, and concentrations and amounts of the elements in manure. Ammonia The committee recommends making estimates and directing control strategies toward decreasing total reactive nitrogen emissions to the environment rather than specifically toward decreasing ammonia volatilization (see section "Reactive Nitrogen Emissions".
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From page 109... ...
Total net nitrogen in animal products sold can be determined by multiplying the quantity of each product sold by its nitrogen content and subtracting the nitrogen in animals purchased. Total urine and fecal nitrogen can be determined as total nitrogen intake minus net nitrogen in animal products.
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From page 110... ...
The ammonical nitrogen in manure storage at each time could be estimated as the amount at a previous time plus additions from animal housing and mineralization of organic nitrogen in storage, minus removal and nitrogen emissions. The amount of organic nitrogen in manure storage must be estimated because it affects the rate of ammonical nitrogen formation.
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From page 111... ...
If the stratification of NH3 in manure is negligible, total ammonical nitrogen can be assumed to be the concentration in the bulk liquid. On any given day, the ammonical nitrogen concentration can be calculated from the concentration at the end of previous day, the change in concentration due to influx from animal housing and the degradation of organic nitrogen in stored manure.
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From page 112... ...
Molecular Nitrogen, Nitrous Oxide, and Nitric Oxide As with ammonia, the committee recommends predicting and directing control strategies toward decreasing total reactive nitrogen emissions to the environment rather than specifically toward nitrous oxide and nitric oxide (see discussion of reactive nitrogen emissions)
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From page 113... ...
In many situations, the most cost-effective plans would be those that result in the greatest volatilization of ammonia to the atmosphere in order to decrease the need to "dispose" of excess nitrogen from the AFO in the form of water-soluble nitrate. A systems approach is also needed to decrease total emissions of reactive nitrogen per unit of animal product, rather than simply decreasing reactive nitrogen emissions from individual AFOs.
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From page 114... ...
The reactive nitrogen emissions can be calculated by subtracting estimates of N2 gas formed for different types of management from the total nitrogen emissions. For most animal production systems, only a small percentage of the nitrogen that is not accounted for is thought to be lost as N2.
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From page 115... ...
Control strategies to decrease reactive nitrogen emissions to the environment are likely to be applied to individual AFOs. However, the objective is to decrease emissions from the entire animal production system, which includes off-farm crop production and manure handling.
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From page 116... ...
These strategies can include both performance standards based on individual farm calculations of nitrogen balance and technology standards to decrease total system emissions of reactive nitrogen compounds by quantifiable amounts.
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From page 117... ...
The EPA (2001a) emission factor approach did not consider enteric methane emissions, although these account for
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From page 118... ...
Methane Emissions from Manure Storage Methane is produced from microbial decomposition of animal manure under anaerobic conditions. The major factors that determine methane production are · types and populations of microorganisms present; · storage or retention time;
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From page 119... ...
The characteristics of animal manure are governed by animal diet and by manure handling, collection, and storage methods. The greater the energy content and biodegradability of the feed, the greater is the methane production potential of the manure.
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From page 120... ...
Methane production potential increases in sequence from solid manure to slurry manure to liquid manure systems. Methanogenic bacteria are obligate anaerobes (i.e., they require an absolutely oxygen-free environment to survive)
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From page 121... ...
5-5) where TMCH4= total annual methane emission from manure storage on the farm in cubic meters of CH4 per year; VSi = volatile solids produced annually by animal i in kilograms; Boi = maximum methane production potential of the manure from animal i in cubic meters of CH4 per kilogram of VS; MCFj = methane conversion factor for manure storage j, which represents the extent to which Be is realized (note: O < MCF < 1~; WSij = fraction of animal i' s waste handled in the manure storage j; and CAF = climate adjustment factor for the farm, which represents the extent to which Be is realized under climatic conditions (e.g., temperature, rainfall)
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From page 122... ...
studied the evolution of sulfur gases, including hydrogen sulfide, dimethyl sulfide (DMS) , methanethiol (MeSH)
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From page 123... ...
Particulate Matter As discussed in Chapter 3, primary particulate matter emissions in animal feeding operations result from mechanical generation and entrainment. In cattle feedlots, the activity level of the cattle, along with the moisture content of the ground, affect the amount of particulate emissions.
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From page 124... ...
Given the paucity of research on particulate emissions from AFOs, this effort will require new data on emission rates from AFO components. Hydrogen Sulfide Hydrogen sulfide (H2S)
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From page 125... ...
In waste management systems where a large quantity of water is added to animal manure, sulfate in the water supply can be an important source of sulfur. In aqueous solution such as liquid manure, hydrogen sulfide maintains equilibrium with bisulfide (HS-)
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From page 126... ...
Production of Hydrogen Sulfide in Animal Manure Because of the potential health effects of hydrogen sulfide, prediction models for H2S should have a time scale of hours or days. An approach similar to that for methane production can be used to predict hydrogen sulfide production, except that total sulfur content is used as the manure characteristic instead of volatile solids, a time factor is added to account for the effect of storage time, and the time scale should be on daily basis.
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From page 127... ...
Emission of Hydrogen Sulfide from Animal Manure Continuous emission of hydrogen sulfide from manure to the atmosphere is controlled by the aqueous chemistry of hydrogen sulfide in the manure and convective mass-transfer mechanisms at the manure surface. The pH, manure temperature, air temperature, wind velocity, and relative humidity are major factors that affect the emission process.
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From page 128... ...
One approach is to develop emission factors for different animal production sectors, and estimate emissions as the product of the specific factor for that sector and the number of animals associated with the enterprise or geographic region. The committee found and reported in its Interim Report (NRC, 2002a)
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