U.S. Sludge Digesters: From Pancakes to Eggs

by Teresa Austin, Associate Editor; Civil Engineering Magazine, ASCE World Headquarters, 345 East 47th Street, New York City, NY.,

Serial Information: Civil Engineering—ASCE, 1992, Vol. 62, Issue 10, Pg. 36-39

Document Type: Feature article


Egg-shaped anaerobic sludge digesters, used extensively in Germany and Japan, cost more to build, but provide better mixing of sludge, require less maintenance and less energy to operate, and take up less space than the conventional U.S. design. Until recently only cylindrical tank digesters with flat or gently sloping bottoms operated in the U.S. Now, from Boston Harbor to the Los Angeles coastline, egg-shaped digesters are beginning to dot the U.S. landscape. Over the past 25 years, more than 100 egg-shaped digesters have been on-line throughout Germany. The Terminal Island Treatment Plant in Los Angeles has had four concrete egg-shaped digesters operating successfully since 1977. The shape did not become popular in th U.S. for several reasons: the shape is difficult to deal with during construction; there was less data available, at that time, to verify their efficiency; and, though potentially less expensive to operate and maintain, egg-shaped digesters are as much as 30% more expensive to construct than conventional tanks. Yet most engineers and plant operators agree that the unusual shape encourages better mixing by virtually eliminating dead zones, which are prevalent in conventional digesters. Also, the shape lessens grit and scum buildup. Conventionally-shaped U.S. digesters need to be cleaned every few years, an expensive and messy process. The eggs, according to German experience, seem to stay on-line indefinitely.

Subject Headings: Sludge

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