Sludge
Sludge
A suspension of solids in liquid, usually in the form of a liquid or a slurry . It is the residue that results from wastewater treatment operations, and typical concentrations range from 0.25 to 12% solids by weight. An estimated 8.5 million dry tons of municipal sludge is produced in the United States each year.
The volume of sludge produced is small compared with the volume of wastewater treated. The cost of sludge treatment, however, is estimated to be from 25 to 40% of the total cost of operating a wastewater-treatment plant. In the design of sludge treatment and disposal facilities, the term sludge refers to primary, biological, and chemical sludges, and excludes grit and screenings. Primary sludge results from primary sedimentation , while the sources of biological and chemical sludges are secondary biological and chemical settling and the processes used for thickening, digesting, conditioning, and dewatering the sludge from the primary and secondary settling operations.
Primary sludge is usually gray to dark gray in color and has a strong offensive odor. Fresh biological sludge—activated sludge, for example—is brownish and has a musty or earthy odor. Chemical sludge can vary in color depending on composition and may have objectionable odor. There are several sludge treatment options available. These operations and processes are principally intended for size reduction, grit removal, moisture removal, and stabilization of the organic matter in the sludge. Examples of these sludge processing methods are sludge grinding and cyclone degritter for the preliminary operations, chemical conditioning and heat treatment, centrifugation and vacuum filtration for dewatering operations, aerobic or anaerobic digestion , composting , and incineration for sludge stabilization.
Other than its organic content, sewage sludge is known to contain some essential soil nutrients, such as phosphorus and nitrogen , that make it suitable for use as fertilizer or soil conditioner after further processing. Sludge may also be disposed of in landfills. Landfilling and incineration, however, are strictly disposal methods and, unlike land application, do not recycle sludge.
Raw sludge contains potentially harmful pathogens, heavy metals and toxic organics; it is regulated by the Environmental Protection Agency (EPA). The EPA has recently proposed new regulations establishing management practices and other requirements for the disposal of sewage sludge.
See also Aerobic sludge digestion; Contaminated soil; Industrial waste treatment; Sewage treatment; Waste management[James W. Patterson ]
RESOURCES
BOOKS
Sundstrom, D. W., and H. E. Klei. Wastewater Treatment. Englewood Cliffs, NJ: Prentice-Hall, 1979.
Weber Jr., W. Physicochemical Processes for Water Quality Control. New York: Wiley-Interscience, 1972.
PERIODICALS
Hasbach, A. C., ed. "Putting Sludge to Work." Pollution Engineering (December 1991).
OTHER
Process Design Manual for Sludge Treatment and Disposal. Washington, DC: U.S. Environmental Protection Agency, 1979.
sludge
sludge / sləj/ • n. thick, soft, wet mud or a similar viscous mixture of liquid and solid components, esp. the product of an industrial or refining process. ∎ dirty oil, esp. in the sump of an internal combustion engine. ∎ sea ice newly formed in small pieces.DERIVATIVES: sludg·y adj.