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Air Emissions from Animal Feeding Operations: Current Knowledge, Future Needs
farm in North Carolina is an example of the ambient temperature version of this technology (there are also mesophilic and thermophilic designs). The concentration of ammonia remaining in effluent from that digester may be higher than that which can be volatilized from lagoon effluent once exposed to air. The primary effect of anaerobic digestion is to decrease the amount of volatile solids (corresponding to COD or BOD). Pathogens may also be decreased in the process. Complete anaerobic digestion substantially decreases odor. Emissions from combustion of digester gas should be measured. The State of California recently awarded a $5 million grant to Inland Empire Utilities Agency to develop a centralized waste processing facility in Chino, California, and also provided $10 million as cost sharing for dairy farmers to build anaerobic digesters.
Aeration of liquid or solid waste streams is accomplished by mechanically forcing air through the waste. The objective of aeration is to maintain some concentration of (dissolved) oxygen in the waste stream to support aerobic microbes that digest the organic material in the manure. Aerobic digestion generally produces carbon dioxide rather than methane, and decreases the amount of ammonia produced, producing nitrate and organic forms of nitrogen instead. Aerobic treatment is generally more expensive than anaerobic treatment because of the equipment, electricity, repairs, and management required. Westerman and Zhang (1995) found that the typical electricity cost to completely treat finishing hogs’ manure using aeration was $14 per pig space per year at $0.09 per kilowatt-hour (kWh). This amounts to $5.38 per hog finished at 2.6 groups per year. These authors found that $2.34 in electricity costs per hog finished would be required to attain partial odor control with aeration. Aerobic treatment produces several times the volume of sludge produced by anaerobic digestion. Costs, benefits, and emissions arising from sludge management must be considered.
Solid-liquid separation is used on some farms now and is being considered as part of several alternative manure management systems. Zhang and Westerman (1995) reviewed engineering studies of solids separation. They reported that from none to roughly half of total Kjeldahl nitrogen (TKN), phosphorus, and COD can be removed. Costs of separation increase as the fraction separated increases and as the use of polymers increases. The costs, benefits, and emissions from solids storage and land application after separation are important considerations. Solids separation may decrease the volatile solids load on a subsequent treatment process and may increase the land required to receive swine manure nutrients. Further treatment (composting or dewatering) may add to cost but allow less expensive transport off the site. Effects of solid separation on odor concentrations at the property line remain to be determined.
Composting is a method of stabilizing organic solids and decreasing pathogens by allowing aerobic or anaerobic microbes to digest the material. Composting requires space, labor, and management and can affect emissions positively or negatively. Its primary benefit is to decrease volume and produce a more acceptable soil amendment.