McMillan Sand Filtration Site

If you have driven around D.C., you’ve probably seen these strange towers at some point, hidden behind a chain-link fence by Michigan Avenue near the hospital complex.

These silos are part of the McMillan Sand Filtration site, a cutting-edge water filtration system when it was built in 1905. It was great for the city, helping to curb typhoid and other infections, and was in use up until 1985. But times have changed, there are better filtration systems, and the demand is now for more commercial and residential building.

The D.C. government bought the land from the Federal government in 1987, and selected a development team back in 2007. One of the goals was to increase the housing options for nearby hospital workers, along with adding community amenities and a hope to curb the flooding that plagues the area. As you might expect, there was substantial opposition to the plans, both because of the historic nature of the site, and because of it bringing new density and gentrification. So it has been a long process through the courts, and it was only last week that the final appeals appear to have been exhausted and the development will proceed.

We’re taking you back to what you can never see again, the amazing underground cells that filtered the water. These photos are from 2012, the last time there were public tours of the site.

You enter the underground through large doors, and walk down into the darkness.

And then it opens up, and you are in a vast underground, a full acre of dark space filled with columns and sand.

This is just one of the cells. There are twenty cells, each reached through separate entrances, and all identical and all vast. Eighty million gallons of water could be purified per day by slowly filtering through the four-foot depth of sand in these cells.

The cells have manholes that can be opened, designed both for light for the workers and for pouring in more sand. There are a total of 2,100 manholes spread among the twenty cells. When open, they produce eerie shafts of light that highlight the sand and dust.

The tall silos above ground were designed to hold additional sand, to periodically refresh the sand in the underground cells. There are twenty silos, one for each underground cell. The mechanism controlling the release of the sand is reached through arched openings in the base of the silo.

There are also four regulator houses, designed to control the flow of water into the filtration system.

What is going to remain?

After the development is completed, the twenty silos will remain but there will be new buildings right next to them.

One of the twenty-five underground cells is going to be preserved. The plans are to eventually turn it into a museum-like exhibit if structural issues permit. The other cells will be destroyed – the century-old concrete is not reinforced and is no longer stable.

The other historic element is a revival of the “Olmstead Walk.” As part of the original creation of the filtration site, Frederick Law Olmstead Jr. designed a walk and gardens to wrap around it. It was apparently beautiful, but hasn’t been seen for a long time, because the site was fenced off during World War II due to the need to protect the water supply from hostile forces, and the fence has been up since then.

You can see the development plans for the site here.