The poop deck: Just what happens when you flush on a cruise?

Creative plumbing:Cruises now need an advanced wastewater treatment system. Photo / Getty Images

If you're one of the 20 million people a year that sit on cruise ship toilet seats, chances are you spent at least a fleeting moment wondering exactly what happens when you flush the loo.

Does raw sewage just go straight into the ocean? Or does it get stored somewhere and released somewhere else? Or does it all stay on the ship until you get back to home port?

Or, as a cruise passenger friend of this journalist once remarked, "Is there a gross reason why the top deck is known as the 'poop' deck?" (No, that's just a nickname via the French word for stern, la poupe.)

If you've ever cruised, chances are you spent at least a moment wondering what happens when you flush. Photo / Getty Images

Cruise ships are literally floating hotels, long criticised as being great harmers of our oceans; statistics from America's Environmental Protection Agency shows "an average cruise ship with 3000 passengers and crew produces about 21,000 gallons of sewage a day.'

It wasn't so long ago that sewage was thrown overboard via "storm valves" attached to the sides of the ship.

But, these days, cruise ships must follow very strict international maritime laws. MARPOL (which stands for marine pollution,) is the International Convention for the Prevention of Pollution from Ships; requiring ships to be three nautical miles (5km) from land before discharging treated sewage.

Natalie Vecchione, environmental manager at Carnival Cruise Line, told news.com.au each Carnival ship has an advanced wastewater treatment system as well as a dedicated environmental officer on-board.

So, the truth about what happens to sewage on the ship isn't terribly different to what happens to sewage at home.

"When you flush the toilet, the wastewater is sent to the wastewater treatment systems on-board. The systems on-board treat the wastewater similarly to how it is treated on land. It goes through a multistage process including biological treatment and disinfection," Ms Vecchione said.

Not long ago sewage was thrown overboard via 'storm valves': P&O's 19th century toilet design. Photo / Getty Images

"Also, the treatment units are designed and approved to stringent International Maritime Organisation standards and they're installed and operated in accordance with the manufacturer's rigorous instructions and procedures."

Put simply, when you flush the loo, the sewage goes straight to the on-board treatment plant which treats it until it's drinkable and then pumped back into the ocean, far away from land.

According to Ms Vecchione, Carnival goes even further than the distance required by international maritime laws and doesn't discharge treated waste water until a ship is greater than 12 nautical miles (22km) from shore and travelling at a speed greater than 6 knots (11km/h).

"Once treated, when the ship is far enough from land, the treated water is discharged. And, once it's discharged, the sea water one metre behind a ship is chemically indistinguishable from the water one metre in front of the ship," Ms Vecchione said.

"Respecting and protecting the waters we sail in and the environment of the destinations we visit goes beyond being an operating necessity, it is also the right thing to do."

So cruise ships have come a long way when it comes to environmental issues and waste disposal.

The self-contained floating hotels are trying to rething their waste management. Photo / Getty Images

And because many people who are curious about cruise ships like to find ways to compare today's cruise ships with the Titanic; if you're keen to read about what happened to sewage on the doomed ship in 1912, you could read this thread devoted to all things Titanic and waste disposal.

A keen Titanic observer has noticed a few things according to the Titanic's deck plan, on G deck there was a "discharge access" on both sides of the ship. But, the plain truth was that in the grand old days of cruising, everything went overboard.