Briny, plump, and pricey, oysters feel like a delicacy. Outside of buck-a-shuck happy hours, many run upward of $2 apiece — a prospect that doesn’t deter people from slurping them down. But what many oyster eaters may not realize is that the empty shells left behind retain their value: They’re good for growing baby oysters (or mussels), improving water quality, preventing erosion, and more.

And yet, most of them wind up in the trash. The number of spent shells generated by a single household might be nominal, but restaurants can go through thousands, adding up to tons.

“One big producer in Delaware was throwing them in their basement for years,” said Sarah Bouboulis, habitat project specialist for the Wilmington-based nonprofit Partnership for the Delaware Estuary (PDE). “They’re so heavy that these restaurants, if they can think of something else, they will, but for the most part, they just throw them out.”

In 2016, PDE launched a shell-recycling program that has saved more than 140 tons — roughly 3.5 million shells — from the trash. They collected them from a number of Delaware restaurants, including George & Sons’ Seafood Market in Hockessin, Trolley Square Oyster House in Wilmington, and the Blue Crab Grill in Newark. Last month, PDE partnered with the Philadelphia Water Department to bring this grant-funded program to Philly, where spent shells are found in even greater abundance.

It’s a win-win, saving restaurants fees on trash disposal and keeping a precious resource out of landfills. The calcium carbonate-rich shells are in such short supply that they are sometimes bought by other watersheds, said PDE senior science director Danielle Kreeger. Before it launched its shell-recycling program, PDE had to scrounge for shell.

Storage space had been a limiting factor, but the water department has changed that: They’ve provided a six-acre space in Southwest Philly for the PDE to collect shells — some of which may eventually shore up the banks of local waterways.

The Inquirer spoke with Bouboulis, Kreeger, and PWD Office of Watershed senior scientist Lance Butler about the mechanics of the new program, its potential benefits, and why you should hold onto your oyster shells.

This interview has been edited and condensed for clarity.

Walk me through how the program works.

Bouboulis: Right now we have four participants in Philly — Sansom Street Oyster House, Kensington Quarters, Barclay Prime, and Fishtown Seafood, which is actually open for public drop-offs — and we hope to get more. We collect shells from the restaurants weekly and bring them back to our shell management area near the airport. The shells are very stinky when we get them, and they have to cure for six months to a year to get all the gunk off of them. The sun, the bugs, and other microorganisms clean them up for us naturally. You want to lay the shell as flat as possible so that it gets the most air and sun. We turn them at least once and then they’re ready to go back to our restoration projects. It would be nice if it were a complete circle and we ate them again, but mostly these will be used for increasing habitat, filtration services, and all that good stuff.

» READ MORE: Half a million mussels to be hatched yearly at Bartram’s Garden

Oh, what kind of oysters might grow on these shells in the future?

Kreeger: The native oyster to the Mid-Atlantic region is the Eastern oyster, and that is the main local oyster that is also harvested for seafood. The history of oystering in Delaware Bay is very, very rich. In Philadelphia in the 1890s to the 1920s, almost every city block had an oyster saloon on the corner or someone sitting on a bucket somewhere selling oysters along the road. That was the McDonald’s of that time period — a cheap, nutritious resource. The towns along Delaware Bay where the oysters were landed were wealthy. The town of Bivalve, New Jersey, or Port Norris at one point reached the status of having more millionaires per capita than any town in the United States. Railroads were built to deliver oysters to Camden and Philly and Wilmington. So it was a major thing and these were all this one species. But like many other parts of the country, oyster populations have plummeted since the 1920s. Overharvesting was the big initial reason, but then some nonnative diseases came in in the 1950s and in the 1970s.

Even though we still have a vibrant wild oyster fishery in Delaware Bay today, you rarely see them getting called out locally because they have no trouble selling at markets from Florida and Oklahoma. They’re very, very good. I’m past president of National Shellfish Association and we used to have these tasting tests nationally, even pulling oysters from Southeast Asia and Europe. And Delaware Bay oysters in blind taste tests would routinely rank in the top 5% in quality of meat, for taste, all these properties that oyster connoisseurs really favor. There’s such a demand for these oysters that [farmers and fishermen] don’t have to spend the extra money to market them.

» READ MORE: Best spots to get oysters in Philly

Why are oysters good for the environment?

Kreeger: There’s a commercial value — there’s a lot of jobs associated with oysters in Delaware Bay — but ecologically, their importance is arguably much higher. Fish populations in a healthy oyster reef are 10 times greater than an adjacent area of mudflat that doesn’t have an oyster reef. As filter feeders, each animal can filter up to 10 gallons of water a day during the warm time of the year, so they’re like living waste-treatment plants. They transform pollutants into nonpolluting forms.

Are the shells themselves good for anything else?

Kreeger: Shell is a nationally limited resource that used to be a waste product used for driveways and parking lots. Now, there’s restoration demand nationwide — oyster reefs need shell for baby oyster settlements, but if you want to build an oyster reef, where do you get the shell? There’s no natural sources anymore. The commercial shucking houses, all that shell they generate, they put back on commercial oyster reefs. It’s not available to buy.

Oyster shell itself has calcium carbonate, which helps buffer pH. In fact, we’re using oyster shell in the Philly area, in the Camden area, where it’s freshwater. The shells form the best nature-based building blocks for projects, including living shorelines, where we try to stem erosion along our shorelines by helping the native plants and animals become more vibrant. We’re doing that in the lower Schuylkill, we’ve got projects planned on the Delaware River around Philly. We like to use oyster shell because it’s actually better than other natural materials like wood, recycled Christmas trees, or what have you; it stays in place and has all these nooks and crannies for fish and other invertebrates that helps in stabilizing erosion.

How many shells do you think you’ll be able to recycle through this partnership?

Lance Butler: In Wilmington, the shell management area recycled around 27 tons annually. In Philadelphia, since it’s a significantly larger city, I have this upward goal of 40 tons. But that’s a goal that needs to be thought out between PDE and PWD because it requires work. It requires trucks and personnel. But to reduce 40 tons of “waste” that would ultimately go into a landfill and instead put it to beneficial use in the Delaware Bay, that’s a good goal.

Kreeger: I think we can have even loftier goals than that, but it is totally dependent on labor. We’re not picking up all the shell we could from even the restaurants we’re picking up from in Philly. With four or five times the restaurants and more frequent pickups, I think we could easily recycle over 100 tons a year from the restaurants themselves. Then you add in public drop-offs and other things, we could increase that. But the problem is you need to spend money to collect that shell. We only have funding right now to do this through the rest of this year. But if we can get past logistics, we’re optimistic that this program can become financially self-sustaining.

Butler: We want to develop some metrics to make this a sustainable program. How many tons are removed from the waste stream, the landfills? How many acres of oyster reefs are created? How many linear feet of living shorelines are supplemented with oyster shell bags and so forth? There’s so much more potential. Sarah [Bouboulis] said to us, “We’re gonna start off with a couple of restaurants and see how it goes.” And within two months, we’re up to four already. If we have five to six by the end of the year, I wouldn’t be surprised. The sky’s the limit. I agree with Danielle, 100 tons is a far-reaching goal, but it is attainable.

Restaurants interested in participating in the shell-recycling program should contact Sarah Bouboulis at sbouboulis@delawareestuary.org.