And just like in a normal city, all its residents produce a lot of trash.

But there aren't any garbage trucks here to scoop it up and take it away.

We're at sea, obviously.

And since waste can't and shouldn't just be dumped in the ocean, well, what do cruise ships do with all of it?

This is something the industry's been dealing with for years.

Carnival cruise line is coming clean about polluting oceans.

Princess Cruises was fined $40 million in 2016 for illegal dumping, and Carnival got hit with a $20 million fine in 2019 for disposing of plastic waste in the ocean.

Carnival Corporation's issues really brought the need for better technology so that these ships can operate more efficiently.

Cruise lines have been working on systems to purify water and deal with waste inside ships.

Up until now, these types of options weren't available.

All this new tech was built into Royal Caribbean's largest and newest ship, Symphony of the Seas.

The company says it's a zero-landfill ship, which means it uses everything from recycling to water filtration to deal with its own waste.

And this guy's in charge of making sure no single water bottle is unaccounted for.

Welcome to waste and recycling center.

We're down on deck two, a secret, crew-only area of the ship.

Crew members check all the ship's trash cans for recyclables and bring them down here for Alex's team to handle.

Despite being the only waste facility on this massive ship, it's surprisingly quiet.

Alex said the busiest time is in the morning, when things are unpackaged for the day.

This is the waste streams that we have.

Every waste stream has its own way of handling it.

There are separate teams to deal with each incoming recyclable: glass, cardboard, plastic, and metal.

This is our incinerator room. So, we have two incinerators, one and two.

This area is manned 24 hours a day.

We have ten crew members who are working here, five in the morning and five in the evening.

Crew members separate glass into colors: green, brown, and white.

This is the byproduct of it after we crush it.

They can process upwards of 13,000 pounds of glass for a weeklong cruise.

All the small glass pieces are stored in bins until the ship docks.

Plastic goes through this massive compactor.

Even though the ship's gotten rid of plastic straws, it still relies on bottled water because, for health and safety reasons, no cruise ship is allowed to have water fountains.

So, every week, they crush about 528 gallons of water bottles.

We are compacting the cardboard over there.

Throughout the day, cardboard is stacked up in this machine, called a baler.

Once it's full, it's all compressed into bundles.

And used aluminum cans, well, they're sent through this baler.

The machine squeezes them down into big cubes, which are then stored in a fridge just off the waste room.

This area is actually for the items that can produce smell, the garbage.

And the smell could get pretty bad.

The waste is stored for up to seven days at a time, until the ship docks back in Miami, where all the plastic, aluminum, paper, and glass go to recycling partner facilities.

In 2018, Royal Caribbean recycled 43.7 million pounds of waste.

And any rebates earned from these recycling programs go back to the employee retirement fund.

The cruise line is hoping that it's a nice incentive for employees to bring recycling down from their own crew cabins.

So, what about things that can't get recycled? For example, food.

Every week, the ship loads up 600,000 pounds of provisions.

But for the food that's not eaten, well, the company had to figure out how to get rid of all of that, too.

Each one of the ship's restaurants and 36 kitchens has its own suction drain.

Chefs and waiters keep food scraps in separate buckets.

Then, once they've gotten enough, they place it all in this special drain.

All the food waste ends up in one big pipe that runs through the entire ship.

And that pipe leads to what's known as the hydro-processor.

Those pipes over there, so, this is where the food waste is passing through.

This is being processed through here.

This machine has a bunch of tiny layers of mesh to break down the food.

It's being stored in our tank. We have two tanks of comminuted food waste.

And the final step? Incineration.

Now, let's talk about your toilet waste.

Yep, we're gonna go there.

It's all a part of the water-treatment system on board, controlled from the engineering room.

All the wastewater that we are generating on board a ship is being collected. Nothing goes overboard unless we have run it through a treatment plant.

Water is divided into two categories: gray water, from sinks, laundries, and drains, and black water.

That includes everything from the galleys and your toilets, including your urine.

This is then being mixed together and run through the advanced wastewater-purification plant.

The purification system purifies the water to a point above the US federal standard, which is almost safe to drink.

And then it runs several filtration processes before it's being kept on board or it's being discharged overboard when we are at sea with a certain distance from land in order to meet the different local and international regulations.

Anything that can't be recycled or reused on board goes to what's known as a waste-to-energy facility.

Now, we didn't get to see it for ourselves, but Royal Caribbean said "heat or gas from the waste is collected and converted to energy."

It's definitely within their best interests to be the most environmentally friendly because it significantly can reduce the waste on board, the weight that they have to carry, the fuel usage, and it reduces their operational expenses as well.

And after one week at sea, the recycling gets cleared out, incoming provisions are brought on board, and the crew prepares the ship to start the process all over again.