Lime effectively treats sewage biosolids as well as industrial sludges and petroleum wastes.
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 actually 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. 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 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. 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 pH of industrial sludges for further treatment, neutralize acidic wastes, and to 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.
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.