On the Potomac, Egg-Shaped Towers Will Be a High-Water Mark

By Benjamin Forgey
Washington Post Staff Writer
Sunday, March 26, 2006

Washington is about to get a striking new skyline.

Oh, not to worry. We're not talking about changing the profile of the low city, with its hierarchy of heights ranging down from the Capitol dome. Skyscrapers are not about to invade the city's downtown.

Still, if you are standing over in Alexandria's Founders Park and looking north a few years from now, or crossing the new Wilson Bridge, or happen to take the southern approach on a flight into National Airport, you'll notice a dramatic difference.

Lining the Potomac River's eastern shoreline, at the southernmost point in the District, will be two orderly rows of huge industrial structures. Giant egg-shaped sewage digesters, to be precise.

Architecturally, in a city not known for majestic industrial structures, these striking digesters -- each looking a bit like a cocoon from outer space -- will mark quite a change from the norm. They will stretch 303 feet around at their widest point and measure 108 feet from ground level, with sleek elevator towers rising to 120 feet.

They'd be higher still were not the bottoms to be sunk 20 feet into the ground. As the buried bottoms complete the oval form, in full profile the structures look a lot more like giant eggs than the parts that we will see.

Even partially buried, however, these are impressive forms. And there will be eight of them, in two parallel rows of four, plus four additional silolike storage tanks of similar height, though not quite the girth.

Installing the digesters at a cost of $311 million will have significant benefits for the Blue Plains Advanced Wastewater Treatment Plant, the 150-acre facility at the very tip of Southwest Washington that serves more than 2 million folks in the region, and which often bills itself as the largest of its kind in the world. The key word in this boast is advanced. Other, larger sewage-treatment plants do not uniformly carry the processing quite so far; the water that flows into the Potomac from Blue Plains is good for the river.

(The giant containers will process about 350 tons of pre-treated sewage sludge daily -- a feast for the trillions of sludge-loving microbes that actually do the heavy work. Hence the name, digesters. Jerry N. Johnson, general manager of the D.C. Water and Sewer Authority, points out that the efficient digesters will cut odors and reduce the output of sludge -- or biosolids, in today's lingo -- by more than half. This will mean far fewer truck trips to haul the material to Maryland and Virginia farms. As a byproduct, Johnson says, the process will produce enough methane gas to provide 20 percent of the plant's daily power needs.)

In a way, these immense, functional structures resemble the American grain silos that Le Corbusier, the giant of early- to mid-20th-century modern architecture, was so taken with in the 1920s.

Corbusier especially admired the functionality of these buildings, and their sheer massiveness. In "Towards a New Architecture," his important manifesto of 1923, photographs of silos provided background for his famous dictum that "architecture is the masterly, correct and magnificent play of masses brought together in light."

The egg-shape concept actually goes back to the 1920s, when a German engineer discovered the prime advantage of the form -- because it has no angles, it does not provide any "dead zones" where sludge can get stuck. Consequently, the digesters rarely require cleaning. When the Blue Plains structures are fully operational in about four years, Jefferson says, the plant will save about $16 million in annual maintenance.

Judging by photographs of many digester installations in Europe and the United States, it is doubtful that Corbusier would have found them all that appealing. They've been disguised by all sorts of cheap sheathings, so that the odd, rather heavyweight elegance of the basic form rarely comes through.

In Washington, things happily took a different course. Knowing that it had to face review boards such as the Commission of Fine Arts and the National Capital Planning Commission, WASA wisely brought an engineering-architecture team on board in the early planning stages -- BBS Engineers of Columbus, Ohio, and Sorg Architects of Washington.

Not surprisingly, with so many overseers, the design went through several variations. But as sometimes happens, the last one is in most ways the best.

From the beginning, Suman Sorg, with principal responsibility for exterior design, decided to link the eight digesters with bridges at the top, to make worker access more convenient and to bring aesthetic unity to the ensemble. Ditto for the four storage silos, arranged in a separate grouping close by the eggs.

But disagreements arose over the shape and detailing of the bridges and the "caps" at the tops of both digesters and silos. Some designers wanted bridges supported by steel archways ornamented with a sequence of circular forms in steel. Others wanted a slimmer, trimmer, more "industrial" look. And at one point, the eggs were sheathed in a patterned metal, sort of like water towers needlessly decorated in the 1970s age of Super Graphics.

Fortunately, the simplest view prevailed. With their elemental yet elegant steel trusses, the bridges and caps will look right at home. The dipping profiles of the trusses complement the curved form and the ribbed anodized aluminum cladding of the digesters. At the same time, they add a contrasting note of lightness. This is what Sorg humorously refers to as the "slimming effect."

"I wanted a delicate lacy effect to counteract the heavy industrial character of these structures," Sorg says. "More like the effect a necklace or a garland would have on an otherwise not so slender lady."

From most parts of Washington itself, this major addition to the skyline will be invisible, or very nearly so. But because of those prominent views from the south, the change gradually will work its way into the city's consciousness of itself, and into how the city is perceived by the outside world.

It is a change for the better. For all its beauty, monumental Washington sometimes does not quite seem to be a real contemporary city. Huge sewage digesters that stand tall by the Potomac, on the other hand, highlight the importance of sustainable urban infrastructure in a most fundamental way.

Besides, they'll be beautiful, in their way -- and almost magical, maybe, at night.

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