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What happens when you flush the toilet on a cruise ship?


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With passengers and crew included, the biggest modern cruise ships can carry almost 9,000 people. That’s a healthy size for a small town, let alone a floating piece of metal. So what happens to all the waste these people produce?

It all goes in the sea... doesn’t it? Well, not quite. Large cruise ships produce around 1,200 tons of waste a day, and they'd be a bit unpopular if they were spewing out trails of un-treated detritus that big in their wake.

Instead, the waste undergoes a multi-step filtration process, ensuring that it is non-toxic and reasonably ‘clean’ before being released back to nature.

First, the less heavy-duty waste (water from kitchens, laundries and bathrooms) is mixed together with the more undesirable lavatory waste.

This mixture is then sent to bio reactors within the ship, where the nasty stuff is filtered out by bacteria. Then the remaining liquid is disinfected – using UV radiation instead of chlorine, as it’s not harmful to marine life.

Finally, the result is discharged into the sea, clean and non-toxic. In fact, it’s so clean that some ports allow it to be released closer than the statutory 12 miles from land.

Just 0.25% of the original 1,200 tons of waste is left at the end of the process. This is incinerated, or stored until it can be disposed of.

The way in which cruise ships dispose of their waste is regulated by strict international laws – governed by the UN International Maritime Agency. Those enforced by the Cruise Lines International Association are even stricter, and forbid the discharge or untreated sewage into the sea at any time.

So now you know.

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