The Animal Waste Problem
An emerging issue in the U.S. is the growing environmental threat caused by animal wastes. The consolidation of the livestock industry created much larger facilities with more waste-producing activities. Concentrated animal feeding operations (“CAFOs”) for beef cattle, swine, and poultry can create numerous environmental problems, including excess nutrient loading of agricultural land, eutrophication of surface waters, groundwater contamination, pathogen release, and offensive odors.
There have been a number of incidents in which large numbers of people have been sickened by water or food contaminated by animal wastes. These problems will only get worse — the amount of animal manure produced annually is estimated to be 10 times the amount of municipal sewage — and much of that manure currently receives little or no treatment. In addition to solid animal manure, there are large amounts of other animal wastes, such as poultry bedding, urine, and carcasses that are estimated to total up to 100 times the amount of human wastewater biosolids and will pose environmental challenges.
The Environmental Protection Agency developed rules to regulate CAFOs. With more than 15,000 facilities to treat animal wastes, cost-effective treatment methods are vital to this industry.
Lime treatment is a multi-functional and cost-effective method to address many challenges posed by animal wastes. Animal wastes contain phosphorus and nitrogen that can be returned to the soil as fertilizer. However, the large quantities of waste at CAFOs means there are excess nutrients that cannot be absorbed by soil and crops. As a result, runoff of animal waste is causing eutrophication of surrounding surface waters. Adding lime to animal waste can volatilize the waste’s nitrogen and convert it into a usable, concentrated fertilizer, and lime can precipitate the phosphorus to an insoluble form, thereby reducing excess nutrients. Lime can also precipitate most metals present in animal wastes and reduce their mobility.
Lime inhibits pathogens by controlling the environment required for bacterial growth. Calcium hydroxide (hydrated lime) is an alkaline compound that can create pH levels as high as 12.4. At pH levels greater than 12, cell membranes of pathogens are destroyed. The high pH also provides a vector attraction barrier (i.e., prevents flies and other insects from infecting treated biological waste). Because lime has low solubility in water, lime molecules persist in biosolids. This helps to maintain the pH above12 and prevent re-growth of pathogens. In addition, when quicklime (calcium oxide, or CaO) is used, an exothermic reaction with water occurs. The heat released during the reaction can increase the temperature of the waste to 70°C, which provides pasteurization and also helps dry out solid wastes.
Lime treatment also reduces odors, particularly hydrogen sulfide, which is not only a nuisance odor but also can be very dangerous if there is a localized build up of high concentrations. In addition to high pH, lime provides free calcium ions, which react and form complexes with odorous sulfur species such as hydrogen sulfide and organic mercaptans. Thus, the biological waste odors are not “covered over” with lime, but actually destroyed.
Lime treatment of animal wastes is economically attractive. For biosolids, lime treatment is often the least cost alternative. For example, lime stabilization of biosolids is estimated to be less than half the cost of aerobic and anaerobic digestion. There are a number of innovative technologies that use lime or lime-derived materials to treat animal wastes and generate a usable agricultural product. Because of lime’s versatility, it can be used to treat most animal wastes, including hogs, cattle, dairy, and poultry.