Deer Island Treatment Plant: 9 fun facts about the Boston area’s sewage hub

Nestled amid 60 acres of open space within the Boston Islands National Recreation Area is the Deer Island Treatment Plant, the engineering marvel that sorts, cleans, and collates waste for the Boston region.

Curbed Boston toured the facilities with David Duest, the plant’s director. You can find some Facebook Live videos of the behind-the-scenes tour here and here.

And below are nine fun facts about the plant, which, with a maximum design capacity for 1.35 billion gallons per day, is the nation’s second-biggest (behind Detroit’s, which is designed for 1.6 billion).

• The plant services 2.3 million people in 43 communities, including Boston proper. That is more than 1 in 3 Massachusetts residents.
• Four underground tunnels draw wastewater to the plant from these surrounding communities, and treatment of the waste begins via pumps that lift it 80 to 150 feet to start the process.
• Forty-eight clarifying machines that are 186 feet long, 41 feet wide, and 12 feet deep then remove about half of the pollutants in the wastewater, including pathogens and suspended solids. Gravity is the main power source here.
• Then it’s on to secondary treatment, where Deer Island really shines. Using both gravity and microorganisms, the treatment removes even more pollutants, discharging water that is now more than 85 percent “clean.”
• Finally, a disinfection phase via two basins each approximately 500 feet long and able to hold 4 million gallons, drives out more of the bad—rendering the wastewater that came in only nine hours ago even more pollutant-free. The weirs of the basins are pictured above. Deer Island averages more than 94 percent removal of solids and organics from the wastewater that comes in, per Duest.
• When the time comes, this clean-ish water, now called final effluent, is released into Massachusetts Bay through a 9.5-mile tunnel that is 22.5 feet in diameter (same as the diameter of the Callahan Tunnel). It is discharged through more than 50 individual diffusion pipes, each with eight small ports. The travel time through the tunnel can take 10 hours.

Henry Zbyszynski/Flickr

• And what of the waste parsed from the original wastewater? That’s where Deer Island’s 12 distinctive egg-shaped anaerobic digesters come in. They serve as faux stomachs to break down the sludge and the scum (the plant’s terms) into methane gas, carbon dioxide, solid organic byproducts, and water.
• The methane gas allows the Deer Island plant to service nearly all of its own energy needs in-house, no small feat given the winters here.
• And! Some of the digested sludge from the “eggs” travels through a tunnel, to a factory in Quincy, where it’s processed into organic fertilizer. Look for “Bay State Fertilizer.” You helped make it, after all.

Bonus fact: The Deer Island Treatment Plant is open for tours on weekdays.