Animals generate a significant amount of wastes (urine, feces, etc.) and the management and disposal of these wastes can be problematic. Currently, most livestock waste is applied to fields as fertilizer. However, animal wastes can potentially be used for bioenergy through the capture of biogas (from anaerobic digestion), syngas (from gasification), or bio-oils (from pyrolysis). Components of manure may also be recovered for use in producing bioproducts.
The swine industry in the U.S. has undergone substantial structural change over the past 20 years. During the 1970s and 1980s, production typically occurred in mostly small, birth to market operations. Today, hog production is specialized, occurring at three different locations. Breeding, gestation, and care until old enough for weaning occurs at one site. After weaning, piglets are moved to a nursery facility where they are fed special diets. Once they reach 40-60 pounds in weight (about 8-10 weeks of age), they are transported to a finishing facility where they are fed until they reach market weight (200 to 250 lbs).
Independent producers represent a diminishing portion of total production. Most production today is under contract, where large hog producers (called integrators or contractors) contract with smaller growers to feed the hogs until market weight. The integrator provides management services, feeder pigs, medicine, and other inputs while the grower provides the labor, land, and facilities. In return, the grower receives a fixed payment, adjusted for production efficiency.
Swine production has expanded from traditional areas in the Midwest (mostly Iowa, Illinois, Indiana, and Minnesota, where corn is an abundant feed) to other parts of the country, predominantly the Southeast (especially North Carolina), the Southwest, and the West (figure 1).
The average number of operations producing swine is decreasing, while the size of hog operations is increasing. In 2006, more than 80% of hog operations had more than 2,000 head; operations with 10,000 head accounted for more than half of the total pork production. In 2006, the total number of market and breeder hogs on inventory was 61.69 million, 65% of which were produced in five states: Iowa (16.6 million), North Carolina (9.6 million), Minnesota (6.8 million), Illinois (4.2 million), and Indiana (3.2 million).
The amount and characteristics of wastes produced per animal depends on a number of factors including age, size, type of feed, and whether or not the animal is pregnant (gestating) or nursing (lactating). Physical properties of waste that are of interest include weight, volume, total solids and moisture content, as these properties describe the amount and consistency of the material that must be dealt with by equipment and in treatment and storage facilities. Chemical constituents (nitrogen [N], phosphorus [P], and potassium [K]) are important in use of livestock wastes as fertilizers, as well as for environmental considerations.
A number of studies estimate the average manure produced per animal and the composition of the manure. Both estimated average quantities per unit and manure characteristics differ among studies, due to different assumptions regarding feed ration composition, the use of feed additives to reduce phosphorus levels, and the efficiency of conversion of feed to meat, among other factors. Table 3 provides swine manure characteristics by animal type, as estimated by the USDA. Wastes characteristics are based on average amounts as excreted from the animal (i.e., wastes do not contain bedding or other materials).
The USDA estimates that in 1997, 9.3 million tons (dry matter) of manure were generated by swine operations, of which 8.6 million tons were on operations where the hogs were confined. The manure that is collected from such operations is mostly disposed of via field application as a fertilizer. Sometimes field application is in excess of the nutrient assimilation capacity of the soil, leading to problems associated with nitrogen and phosphorus runoff. Concentrated animal feeding operations (CAFO, which for swine production means 2,500 hogs or more - based on the average annual number of hogs in inventory or sold) must be permitted under the National Pollutant Discharge Elimination System (NPDES) and must have a nutrient management plan for wastes applied to the field as fertilizer. A nutrient management plan is voluntary for non-CAFO operations.
Swine manure is generally collected and stored as a slurry in pits, or as a liquid in lagoons. Swine production often involves housing hogs in facilities with slotted floors, where the waste drops through the slots directly into storage tanks (pits) or falls into a gutter and is flushed into a storage tank. This form of storage generally provides from 3-12 months of storage and is periodically pumped out or drained. Lagoons are designed to encourage anaerobic digestion of the organic material in the manure. Lagoons are generally larger than pits and provide for storage of larger quantities of manure, for longer periods of time. Pit storage is more common in the Heartland, particularly on smaller operations, while lagoon storage is more common in the Southern Seaboard.
The problem of excessive application of manure for fertilizer is
particularly important for large swine operations due to the large volumes of manure produced, the nature of the manure (mostly liquid), and the frequency of manure collection. The USDA identified 68 counties where manure nitrogen levels exceed the soil nutrient assimilative capacity of all the county’s crop and pasture land (primarily in North Carolina, northern Georgia, Alabama, central Mississippi, western Arkansas, and California) and 152 counties (concentrated in eastern North Carolina, northern Georgia, northern Alabama, western Arkansas, central California, and western Washington) where the manure phosphorus levels exceed the county assimilative capacity. Additionally, 155 and 337 counties were identified where manure nitrogen and phosphorus levels, respectively, exceed half of the county soil nutrient assimilative capacity. These counties are most in need of alternative waste management methods.
The EPA estimates that as of 2005, approximately 4,281 swine farms were good candidates for biogas collection and bioenergy production. Nearly 51% of these farms were located in North Carolina and Iowa; about 85% of the candidate farms were located in the top 10 swine production states.