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Washingtoniana: The McMillan Reservoir

In this week’s edition of Washingtoniana—where we track down answers to your questions about Washington—we get the facts on the grass-covered structures at the southeast corner of North Capitol street and Michigan Avenue.

Photo by Flickr user IntangibleArts 

“What the heck are those grass-covered rounded structures at the southeast corner of North Capitol street and Michigan Avenue? They look like something out of an old war movie.” —Sarah

Although they may look like a backdrop for Tom Hanks in Saving Private Ryan, the structures are actually abandoned sand washers that are part of the McMillan Reservoir’s sand-filtration system.

The reservoir was part of the McMillan Commission’s architectural plan for the development of Washington around the turn of the century. Both the commission and the reservoir are named for Senator James McMillan from Michigan, who was chairman of the Committee on the District of Columbia.

The commission intended for the reservoir to be both an engineering and aesthetic landmark. It succeeded with the reservoir’s aesthetic value by appointing Frederick Law Olmsted, Jr., son of the famous American landscape architect, to design the reservoir and give it the park vibe it still has today.

As for the engineering, the commission employed Allen Hazen and E.M. Hardy to design a sand-filtration system. The result was an intricate water-purification process: Water travels underground from the reservoir to the filtration system via the city’s aqueducts. Once at the filtration system, the water is purified 20 feet underground in 16-by-16-foot cells filled with sand. The sand catches everything that’s bad in the water before sending it back through the aqueducts to the rest of the city.

So how do those grass-covered structures fit into the equation? Well, those silos are actually where the sand was washed of the impurities when it was in the underground cells.

The sand-filtration system was innovative: It was the city’s first water-treatment facility, and a big win for the people who advocated against chemical treatment. Although Washington was one of the last major cities to use a sand-filtration system at a time when most cities were turning to chemical treatment, it proved to be a better choice because it resulted in a dramatic drop in colon bacilli in the public water supply, as well as the eventual end of typhoid and malaria epidemics in the city.

In 1986, a new and faster filtration system was built across the street. Since then, the old system, including the sand washers, has sat idle. But for eighty years the filtration system was used as it was originally designed. This consistency, along with the pleasing aesthetic of the area, is why the McMillan Reservoir was named a DC Historic Landmark in 1991.

Still, the future of the site is uncertain, according to Rebecca Miller at the DC Preservation League. Developers want to build retail stores and living spaces on some of the land. “There are development teams in place, but there are mixed views on how much development there should be,” Miller says.

Have a question about the Washington area? Send an email along with your name and place of residence to We'll try to answer your question in an upcoming column.

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