What happens to all that uneaten food on cruises? These lines are working to reduce waste.

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While I watched a crew member aboard Holland America Line’s Rotterdam ship haul away my half-eaten order of french fries, I felt a pang of yearning.

I spent nearly two weeks sailing with the line in October, and made a point to sample nearly every restaurant on board, including the burger joint Dive-In, but never requested a to-go box at the end of a meal like I might on land. With only a minibar in my stateroom and food always at my beck-and-call, it seemed impractical.

Little did I know that elsewhere on board, a machine would likely soon be chomping on my leftovers.

Holland America Line has installed biodigesters that can break down organic material as part of efforts to shrink its food waste footprint, and parent company Carnival Corporation now has more than 600 of the devices throughout its fleet.

For many travelers, food is a key part of the cruise experience with seemingly unlimited options - think buffets - included in the fare. But for all the cuisine passengers enjoy, there is plenty that doesn’t get eaten, and many lines are working to refine their processes for dealing with that waste.

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What happens to food waste on cruise ships?

Carnival Corp., which operates brands including Holland America, Carnival Cruise Line, Princess Cruises and others, generates 1.3 pounds of food waste per person each day on average, but can generate as little as 0.6 pounds per person per day, depending on the line, a spokesperson for the company said in an email.

All of that waste is either processed via biodigesters or dehydrators, or offloaded on shore.

Some of the company’s ships have long had dehydrators, which squeeze the water from food waste and lighten the load that can be taken to landfills, compost sites or waste-to-energy facilities. “And that was good, but not necessarily good enough,” said Bill Burke, the company’s chief maritime officer.

The company began a “three-pronged approach” to food waste in 2019, he said, from the point when the lines stock food to after guests throw away what they don’t eat.

Carnival Corp. analyzed the waste, and worked to determine what was leftover, what they could reuse in other recipes and where they could cut back. “That’s a significant carbon issue if we’re buying food that we’re not using,” Burke said.

The company has reduced food waste by over 30% per person as compared with its 2019 baseline, according to its 2022 sustainability report, and has set new goals of 40% by 2025 and 50% by 2030.

Burke said Carnival Corp., which operates the largest number of U.S. sailings, has also worked to reduce single-use plastics, swapping individual yogurt cups for bulk containers, for instance. The biodigesters, which he called the “holy grail,” have rounded out that work, digesting much of the organic waste that would have previously been ground up and discharged, turning what’s left into a liquid.

Other companies and cruise lines are working toward similar aims.

Royal Caribbean International uses proprietary technology to track how much food is being wasted - by weighing pans of lasagna before and after they are served for instance - and amend production accordingly. The cruise line has also expanded those efforts, including using point-of-sale data to forecast how much food they will use based on passenger demographics, the itinerary and other information.

“(If) we have 10% more kids, we know we’re going to need significantly more chicken fingers,” said Linken D’Souza, the line’s vice president of food and beverage.

Leveraging that intel will allow them to be proactive, D’Souza said, and eliminate waste before it happens.

Some initiatives have been particularly creative. Norwegian Cruise Line launched zero-waste drinks at a bar on its Prima ship last year, reusing items like banana peels and croissants in cocktails.

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How do biodigesters work?

Carnival Corp. uses biodigesters from several different companies, including Recoup Technologies, formerly BioHiTech America. Their product uses microorganisms and other bacteria to rapidly process food waste much like a human digestive system might, according to director of technology operations Bob Joyce.

“The way we tend to describe it is just basically as a metal stomach,” he said. What goes in as salad or steak comes out of a smoothie-like mixture that the cruise ship can then discharge. “If you can eat it, it can eat it,” he said.

But while the machines, which cost between roughly $25,000 and $50,000 depending on the size, can take in a wide range of produce, proteins and starches, Joyce noted that certain items such as walnut shells and steak bones will not break down inside. Crew members sort out inorganic materials beforehand.

The company discharges the liquid at least 12 miles from shore. Holland America’s Rotterdam has nine of them, along with one dehydrator.

“Biodigesters reduce the amount of methane and carbon dioxide emitted into the atmosphere while also reducing the demand on the ocean for complete food waste decomposition,” Carnival Corp. said in its sustainability report. The biodigesters processed around 80 million pounds of food waste in 2022, which would have taken up about one million cubic feet of space had it been sent to a landfill and released around 30,000 metric tons of greenhouse gas emissions, the spokesperson added.

But that doesn’t mean the waste is good for marine life.

“They’re putting nutrients into the oceans that can disrupt aquatic ecosystems,” said Gregory Keoleian, director of the Center for Sustainable Systems at the University of Michigan.

The kind of human sustenance cruise ships discharge is not typically part of fish and other aquatic creatures’ diets, and introducing it can disturb complex food webs, according to Keoleian. “They didn’t evolve to eat human food waste,” he said.

“Our success – and quite literally, our livelihood – depends on doing our part to protect the vibrant marine ecosystems, beautiful communities, and scenic spaces we operate in,” the Carnival Corp. spokesperson said in an emailed statement. “We’re investing in the industry’s smartest solutions to enable sustainable cruising, such as biodigesters, which offer the best available food waste solution on the market today.”

Carnival Corp. also acknowledged in 2019 that its vessels had committed environmental crimes and knowingly allowed plastic to be discharged with food waste in the Bahamas.

What can passengers do?

Keoleian said preventing food waste generation is as important as how it’s managed, and passengers can play a role in minimizing the environmental repercussions.

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That can mean simply wasting less food when they have a meal on board, and also being judicious about what food items they pick. “If you look at the environmental impacts of food, what foods they choose to eat will impact the environment differently,” he said.

Beef, for example, is more carbon-intensive to produce than other sources of protein like chicken or fish or plant-based proteins, he said.

Burke said there in addition to working toward greater sustainability, there are other positive byproducts, such as appealing to younger travelers who prioritize that in particular. “If we want you to cruise on one of our ships, I think it will matter to you how we take care of the ocean,” he said. “So, it’s not just doing good, it's doing the right thing for business as well.”

Do you find it easier to increase or reduce the amount of waste you produce on cruises? How so?

Nathan Diller is a consumer travel reporter for USA TODAY based in Nashville. You can reach him at ndiller@usatoday.com.

This article originally appeared on USA TODAY: How wasteful are cruise ships? Here's how cruises handle food waste