Last month, the city of Cambridge expanded it’s so-called Curbside Composting program to pick up food scraps from 25,000 households in the city.

But it turns out all that food isn’t actually being composted.

Cambridge says what is happening is the best option for removing that waste from garbage that would otherwise be destined for landfills. But not everyone agrees.

Tony Manning and Diana Velez have been huge fans of the Cambridge composting program since receiving a small food waste container for their kitchen counter last month.

“We've got some limes, some tea bags,” Manning said as he peered inside the container. “And actually I really like that you can put napkins in there.”

“I love it,” Velez said. “It’s really environmentally friendly.”

But neither of them were quite sure what happens to their food scraps after they’re picked up.

“I trust the city, and hopefully they recycle it into compostable material and soil and things they can use around the city,” Manning said.

Where it goes is the Greater Lawrence Sanitary District wastewater treatment plant in North Andover. Tanker trucks bring in thousands of gallons of liquified food waste at a time.

“It’s being mixed with the primary solids and waste activated sludges that are typical at a treatment facility,” said Cheri Cousens, plant’s executive director.

In other words, the Cambridge food scraps are mixed with sewage sludge.

Tanker truck
Craig LeMoult Jerry Santana of JP Noonan Transportation, Inc. hooks up a hose to deliver a “slurry” of food waste from the Cambridge composting program to the Greater Lawrence Sanitary District, where it will be mixed with sewage waste and go through the anaerobic digestion process.

Cousens descended a staircase to an underground tunnel, and pointed at a large piece of machinery. “These pumps transfer the mixed sludges directly to the digesters,” she said.

That’s where a process called anaerobic digestion eats away at the material, reducing its mass and producing methane gas, which the plant captures and uses as power. Wastewater is separated out and treated before being discharged into the Merrimack River.

And the solid material is dried out and broken down into pellets, that look a little like dirt. “The fertilizer is used traditionally in Massachusetts for fertilizing fields for hay for horses,” Cousens said. “It’s used in other states. For example, up in Maine, they're using it for wreaths and trees and things like that.”

The solid end product of the anaerobic digestion process is pellets, which are used as fertilizer for hay fields, as well as other uses.
Craig LeMoult WGBH News

So Cambridge’s food waste is being removed from garbage, and the end product is used as fertilizer. But the process in between is not composting. So, does that matter?

“The city of Cambridge is doing the right thing by taking food scraps out of the landfill,” said Laura Orlando, who teaches environmental health at Boston University. “It's doing the wrong thing by adding those food scraps to a material that is known to be toxic.”

It’s not just human waste in that sewage sludge. It’s everything else we’re pouring down our drains, and industrial waste from factories. Orlando says an actual composting program — in which the scraps decompose into a cleaner material that can be used for gardening — is possible. “It's happening in San Francisco. It's happening in Seattle,” she said. “Anaerobic digestion is not composting.”

“I don't want people to feel like we're misleading them,” said Cambridge’s recycling director, Michael Orr. “What we're trying to do is get food out of the trash.”

Orr said any contamination in the fertilizer they’re creating falls well below any limits in safety regulations. And composting requires a lot of space and smells pretty bad, so he said there was nowhere close enough that could handle what Cambridge produces. “If we could find a place nearby we'd love to do it,” he said. “I just I don't have anywhere to go.”

Also, using this method, they can include meat scraps that can’t be composted. Orr said they suffered over what to call the program. “We had to figure out how to make this program as effective as possible," he said. "How do we get people to understand it as quickly and as easily as possible?”

Composting is something people want to do. And in just the first month of the program, the city’s trash was cut by 10 percent.

A composting bin sits alongside recycling and trash bins in Cambridge, Mass.
Meredith Nierman WGBH News

As for the people who want compost back for their gardens, Cambridge is providing that. It’s just coming from an actual composting facility in Saugus.

Back in the kitchen, Tony Manning and Diana Velez were both surprised to hear what’s really happening to their food waste. But Manning said his main concern was keeping it out of landfills. “So the fact that it goes into a different stream that doesn't end up in that place but actually ends up being useful, that's, that's fine by me,” he said.

“I don't know, I was really hoping it was going to be composted and used for feed plants or something,” Velez replied. She’s disappointed, but said she’s still going to use the program.

And they both said they hope Cambridge does figure out a way to really, actually compost.