Enviromental

An emerging issue in the U.S. is the growing environmental threat caused by animal waste. 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.

EPA’s CAFO Rule

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 for Animal Wastes

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, the 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 above 12 and prevent the 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 Can Help Control Odors

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 buildup 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 destroyed.

Lime Treatment is Cost-Effective

Lime treatment of animal waste is economically attractive. For biosolids, lime treatment is often the least expensive alternative. For example, lime stabilization of biosolids is estimated to be less than half the cost of aerobic and anaerobic digestion. Many innovative technologies 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.

Lime effectively treats sewage biosolids as well as industrial sludges and petroleum wastes.

Sewage Biosolids

Quicklime and calcium hydroxide (hydrated lime) have been used to treat biological organic wastes for more than 100 years. Treatment of human wastewater sludges (i.e., biosolids) with lime is specifically prescribed in EPA’s regulations.

How Lime Treatment Works

Lime treatment controls the environment needed for the growth of pathogens in biosolids and converts sludge into a usable product. Treatment of biological wastes with lime is based on several chemical reactions. Calcium hydroxide is an alkaline compound that can create pH levels as high as 12.4. At pH levels greater than 12 and increased temperatures, cell membranes of harmful pathogens are destroyed. The high pH also provides a vector attraction barrier, preventing flies and other insects from infecting treated biological waste. Because lime has low solubility in water, lime molecules persist in biosolids to prevent regrowth of pathogens. When quicklime (CaO) is used with water, an exothermic reaction occurs. As heat is released, the temperature of the biological waste can increase to 70ºC, which provides effective pasteurization. The high pH also will precipitate most metals present in the waste and reduce their solubility and mobility. Lime will also react with phosphorus compounds to prevent eutrophication. The solubility of calcium hydroxide provides free calcium ions, which react and form complexes with odorous sulfur species such as hydrogen sulfide and organic mercaptans. As a result of this reaction, the biological waste odors are destroyed, not just “covered over.” In general, lime stabilization is a non-proprietary process, although patented processes are available.

Lime Can Help Meet EPA’s Part 503 Requirements

EPA oversees federal requirements for the safe treatment, beneficial use, and disposal of biosolids (40 CFR Part 503). Part 503 establishes 2 classes of biosolids — Class A and Class B — that specify performance goals and the degree of treatment before biosolids can be beneficially used or disposed of. Class A biosolids contain extremely low pathogen concentrations and have few or no use restrictions. To meet Class A requirements for pathogen destruction, one can use lime stabilization or other EPA-approved time/temperature processes. Class A biosolids can be used for home lawns and gardens. Class B biosolids contain higher pathogen concentrations than Class A, but have levels low enough for some beneficial uses, such as land application with restrictions. To meet Class B pathogen destruction requirements, lime stabilization is one of five approved processes to significantly reduce pathogens. Specifically, lime is added to raise the pH of the biosolids to 12 for 2 hours. The pH is subsequently maintained at more than 11.5 for 22 hours. Class B biosolids can be used for agricultural or land reclamation use. As EPA notes, “properly prepared biosolids provide a rich source of the essential fertilizer elements needed by plants to produce food.”  [U.S. EPA, “Biosolids Recycling: Beneficial Technology for a Better Environment,” (June 1994).]  Reuse of lime-stabilized biosolids is not limited to use on farmland. Biosolids are also used as a soil substitute for landfill cover and in the reclamation of mining-disabled land. Exceptional quality biosolids can also be sold for public use as a commercial fertilizer or soil conditioner. Most lime treatment facilities have the flexibility to produce either Class A or Class B biosolids, thus increasing disposal and recycling options. The addition of lime also increases the solids content of the waste, making it easier to handle and store. In addition to regulating pathogen concentrations, the Part 503 regulations include requirements for reducing the tendency of biosolids to attract disease vectors such as rodents and insects (Subpart D). To meet vector attraction reduction requirements, the regulations authorize the use of lime to raise the pH to 12 or higher for 2 hours and to subsequently maintain levels above pH 11.5 for another 22 hours without further alkali addition.

Lime Stabilization Is Cost-Effective

Lime stabilization is generally more cost-effective than alternative treatment methods. A series of studies comparing lime stabilization to composting, thermal drying, and digestion technologies found that lime stabilization has unit costs as much as 60 percent lower than these alternatives. Reduced capital cost requirements of lime stabilization are even more dramatic, which is particularly important for municipalities with limited capital budgets.

Industrial Sludges and Petroleum Wastes

Quicklime and hydrated lime can be used to correct the pH of industrial sludges for further treatment, neutralize acidic wastes, and remove or immobilize contaminants. Specific examples include sulfite/sulfate sludges and petroleum waste.

Calcium Sulfite/Sulfate Waste

Calcium sulfite and sulfate wastes resulting from: (1) dry scrubbing of flue gases; (2) lime neutralization of acid waste effluent; and (3) manufacture of superphosphate fertilizers, when untreated, lack bearing strength and are prone to leach objectionable amounts of the sulfate ion into the groundwater. However, when mixed with 2-3% lime and 15-30% pozzolan — such as fly ash, volcanic ash, pulverized slag, etc. — calcium sulfite/sulfate wastes develop considerable bearing strength, erosion resistance, and are non-leaching. The stabilized material can be used in building embankments and earth dams. In addition, sulfite sludges generated when flue gases are treated with lime in wet scrubbers can be crystallized into synthetic gypsum, which is very white and is a saleable product.

Petroleum Wastes

Restoration of waste oil ponds to environmentally safe land for beneficial uses has been achieved using either commercial lime (mainly quicklime) or lime kiln dust. Either material can dewater the oily wastes into a dried sludge that can be compacted and allow the pond area to be converted to useful land.

Lime is widely used to treat hazardous wastes currently generated as well as those that were previously disposed of or abandoned. Lime stabilizes most metals by converting them to more chemically stable forms that are less likely to leach. In addition, lime can react with soils to solidify materials inhibiting the leaching of hazardous constituents. Lime also neutralizes acidic materials within such constituents.

Under EPA’s land disposal restrictions regulations, currently generated hazardous wastes that are to be land disposed must be pretreated using the best demonstrated available technology. For hazardous wastes containing metals, lime is identified by EPA as a suitable treatment method for metals stabilization or metals precipitation. See 40 C.F.R. § 268.42.

EPA also endorses lime stabilization as a key technology for hazardous waste site cleanups. See, e.g., Handbook on In-Site Treatment of Hazardous Waste-Contaminated Soils (EPA/540/2-90/002, Jan. 1990). In 1997, for example, EPA announced a proposed cleanup plan as part of the Anaconda Regional Water, Waste, and Soils Project for 14,000 acres in Anaconda, Montana. A key element of the plan was to treat arsenic-containing soils with lime and organics.

Copper mining created environmental contamination in the 300-square-mile area, and there was concern about potential human exposure. EPA recommended in-place lime treatment over the option of excavating and treating the tailings and contaminated groundwater. Nearby, the Warm Springs Pond was used to capture and treat water contaminated with metals (copper, zinc, and arsenic) that threatened the Clark Fork River. Those contaminated waters were also treated with a lime solution.

Lime plays a key role in many air pollution control applications. Lime is used to remove acidic gases, particularly sulfur dioxide (SO₂) and hydrogen chloride (HCl), from flue gases. Lime-based technology is also being evaluated for the removal of mercury. Lime is more reactive than limestone, and requires less capital equipment. SO₂ removal efficiencies using lime scrubbers range from 95 to 99 percent at electric generating plants. HCl removal efficiencies using lime range from 95 to 99 percent at municipal waste-to-energy plants. There are two main methods for cleaning flue gases from coal combustion at electric generating stations: dry scrubbing and wet scrubbing. Lime is used in both systems. Dry scrubbing is also used at municipal waste-to-energy plants and other industrial facilities, primarily for HCl control.

Dry Lime Scrubbing

In dry scrubbing, lime is injected directly into flue gas to remove SO₂ and HCl. There are two major dry processes: “dry injection” systems inject dry hydrated lime into the flue gas duct and “spray dryers” inject an atomized lime slurry into a separate vessel. A spray dryer is typically shaped like a silo, with a cylindrical top and a cone bottom. Hot flue gas flows into the top. Lime slurry is sprayed through an atomizer (e.g., nozzles) into the cylinder near the top, where it absorbs SO₂ and HCl. The water in the lime slurry is then evaporated by the hot gas. The scrubbed flue gas flows from the bottom of the cylindrical section through a horizontal duct. A portion of the dried, unreacted lime and its reaction products fall to the bottom of the cone and are removed. The flue gas then flows to a particulate control device (e.g., a baghouse) to remove the remainder of the lime and reaction products. Both dry injection and spray dryers yield a dry final product, collected in particulate control devices. At electric generating plants, dry scrubbing is used primarily for low-sulfur fuels. At municipal waste-to-energy plants, dry scrubbing is used for the removal of SO₂ and HCl. Dry scrubbing is used at other industrial facilities for HCl control. Dry scrubbing methods have improved significantly in recent years, resulting in excellent removal efficiencies.

Wet Lime Scrubbing

In wet lime scrubbing, lime is added to water, and the resulting slurry is sprayed into a flue gas scrubber. In a typical system, the gas to be cleaned enters the bottom of a cylinder-like tower and flows upward through a shower of lime slurry. The sulfur dioxide is absorbed into the spray and then precipitated as wet calcium sulfite. The sulfite can be converted to gypsum, a salable by-product. Wet scrubbing treats high-sulfur fuels and some low-sulfur fuels where high-efficiency sulfur dioxide removal is required. Wet scrubbing primarily uses magnesium-enhanced lime (containing 3-8% magnesium oxide) because it provides high alkalinity to increase SO₂ removal capacity and reduce scaling potential.

Comparing Lime and Limestone SO₂ Wet Scrubbing Processes

More than ninety percent of U.S. flue gas desulfurization (FGD) system capacity uses lime or limestone. This trend will likely continue into the next phase of federally mandated SO₂ reduction from coal-burning power plants. In 2003, the National Lime Association sponsored a study by Sargent and Lundy to compare the costs of leading lime and limestone-based FGD processes utilized by power-generating plants in the United States. The study included developing conceptual designs with capital and O&M cost requirements using up-to-date performance criteria for the processes. The results of the study are summarized in two reports: Wet FGD Technology Evaluation and Dry FGD Technology Evaluation. The reports present the competitive position of wet and dry limestone and lime-based processes relative to reagent cost, auxiliary power cost, coal sulfur content, dispatch, capital cost, and by-product production (gypsum and SO₃ aerosol mitigation chemicals). The reports were updated in a 2007 report comparing wet limestone and dry lime-based systems.

HCl Removal

Because lime also reacts readily with other acid gases such as HCl, lime scrubbing is used to control HCl at municipal and industrial facilities. For example, at municipal waste-to-energy plants, dry lime scrubbing is used to control emissions from about 70 percent of the total U.S. capacity (as of 1998). HCl removal efficiencies using lime range from 95 to 99 percent. At secondary aluminum plants, for example, EPA identifies lime scrubbing as a maximum achievable control technology for HCl. EPA tests demonstrate removal efficiencies greater than 99 percent.

Mercury Removal

Different methods for controlling mercury emissions are being evaluated in the United States. One control technology being evaluated combines hydrated lime with activated carbon. The reagent, a registered product, consists of 95-97 percent lime and 3-5 percent activated carbon. Other calcium-based sorbents are also being evaluated as cost-effective alternatives for combined SO₂ and mercury removal.

Lime is extensively used to treat municipal wastewaters and industrial liquid wastes.

Municipal Wastewater Treatment

In advanced wastewater treatment plants, lime precipitation is used in tertiary processes in which phosphorus is precipitated as complex calcium phosphates along with other suspended and dissolved solids. Due to the high pH of wastewater treated with lime (10.5 to 11.0), the stripping of nitrogen, another nutrient, is facilitated. Removal of phosphorus and nitrogen helps prevent eutrophication (algae build-up) in surface waters. When alum and ferric chloride are used to coagulate suspended matter, lime is added to counteract the low pH induced by these acidic salts and to provide the necessary alkalinity for efficient nitrogen removal. When sewage sludge is removed by vacuum or pressure filtration, lime and ferric chloride are used as filter aids to condition sludge and for final clarification of the effluent.

Industrial Wastewater

Lime has numerous applications in treating industrial wastewaters, especially where neutralization of acidic wastes is required.

• In steel plants, sulfuric acid-based waste pickle liquors are neutralized with lime while iron salts are precipitated. Lime is also a neutralizer and precipitant of chrome, copper, and heavy metals in wastewater discharges from plating plants.
• In rayon plants, lime is used to neutralize sulfuric acid wastes.
• In cotton textile finishing plants (dye works), lime neutralizes and precipitates dissolved solids from wastewater.
• In vegetable and fruit canning operations, wastewater can be clarified with lime alone or with supporting coagulants as an alternative to placing the liquid waste in a lagoon. In citrus canning operations, lime can help clarify wastewaters and aid in the processing of citrus pulp by-products.

See the fact sheet on the use of lime to neutralize acidic wastewaters.

Acid Mining Waste

Highly acidic drainage (known as acid mine drainage) from active or abandoned mines is frequently neutralized with lime. Further clarification of the discharged water is achieved by using lime to precipitate iron contained in this pyritic leachate. Coal-washing plants use lime to neutralize the acidic waste or process water to reduce corrosion on steel equipment and to recover the water for reuse.