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Sludge: From Refuse to Resource
| Article
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16196 |
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Section : |
NATURAL SCIENCE
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| Issue
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6 / 1989 |
3,732 Words |
| Author
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Louise Purrett Carroll
Louise Purrett Carroll is a science writer based in Colorado
and Washington State. |
Sludge just sounds unattractive, and the substance it names is even less attractive. Sludge is slimy, smelly, and unhealthy, packed with diseases and contaminants. Paradoxically, the more we clean up our wastes, the greater our sludge disposal challenge.
Municipal wastes and wastes from many industrial processes are primarily water. As water is removed from the wastes, sludge is what remains. When a city dumps raw sewage into a river, no sludge-type waste is produced. When a paper plant or a coal-fired power plant freely discharges wastes into the air and streams, no sludge-type waste is produced. Only when the waste discharge is cleaned does sludge accumulation begin.
Sludge is the semisolid by-product of wastewater treatment. Sludge is a product of environmental protection, but now the environment needs to be protected from sludge. Massive quantities of sludge are being produced annually, and scientists in the laboratory and in wastewater treatment plants are being challenged to transform sludge from refuse to be disposed of as safely and unobtrusively as possible into a resource to be exploited and turned to beneficial use. The task for researchers today is to rid sludge of its most intractable components and turn it into useful products: fertilizers for fields and forests, burnable char, diesel fuel, even bricks.
This theme even underlies the comprehensive new set of technical regulations governing use and land disposal of municipal waste sludge that the Environmental Protection Agency (EPA) proposed in January. At that time, acting EPA administrator John A. Moore said the new regulations "would encourage local communities throughout the nation to recognize the value of sludge as a salable resource."
Sludge is, by definition, the stuff we don't want in our rivers or our drinking water. Wastewater treatment proceeds through cycles, with each treatment cycle producing its own type of sludge. Primary sludge is what settles out when wastewater is left standing. Secondary sludge is what settles out in the next stage, after microorganisms already in or added to the wastewater decompose and stabilize the organic material in the water. Tertiary sludge is what remains after further purifications to remove pollutants such as phosphorus or nitrogen.
What sludge actually consists of depends on the source, and the process by which it is produced. Although municipal sewage sludge is composed primarily of human wastes, the mixing in of
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