How do cruise ships handle waste water?
To answer the last question first: NO, cruise ships do not recycle the toilet water! Black water, as it is called, is strictly monitored and kept separate from the drinking water on the ship. There is absolutely no crossover with the pipes.
Cruise ships use a multi-step process to filter black water, using mechanical, biological, and chemical treatments. First, filters mechanically remove coarse impurities, or particles, as small as one millimeter. Then, microorganisms speed up the decomposition of the organic matter left in the water. Next, even finer filters sift out everything down to 0.2 microns – including all microorganisms. In the final step, the water is chemically treated to reduce the nitrogen and phophorus content.
After all this, the black water is actually quite clean and the ship’s waste management engineers like to joke that it is perfectly drinkable. But this is only a joke! The water is NOT put into the ship’s supply of drinking water. Cruise ships discharge this (formerly black) water once they are at least 12 miles from land.
If you were paying attention, you will realize that a cruise ship’s wastewater treatment creates by-products – i.e. the solids that are filtered out. These are dehydrated, dried in a centrifuge, and then burned in an incinerator. The crew disposes of these ashes with the other solid waste when the ship docks.
Cruise Ship Water Desalination
OK, with that disturbing topic out of the way, how do cruise ships get a supply of safe, clean water for their passengers?
The average US household uses 300 gallons of water per day. Cruise ships are trending ever bigger and often carry over 8,000 people. That means a cruise ship needs over 600,000 gallons of water per day – enough to fill an Olympic size swimming pool!
Modern cruise ships use two different types of filtration systems.
Flash Evaporator
Flash evaporators utilize heat to turn seawater into vapor, separating the salt from the water.
First, seawater is pumped through pipes and heated to between 158 to 176 degrees Farenheit. This hot water is then put into a tank under vacuum pressure. The pressure causes the water to begin “flashing off”, or turning into vapor. The water molecules become a fine mist, and the solids like salt and other contaminants are left behind. A de-mister collects the water droplets. Any water that remains in this first tank is called brine. It goes into a second flash tank and the process is repeated. There may be a third and fourth tank, depending on the size of the system. After the final tank, the water is tested with a salinometer to measure the salt content. If the water has more than 10 ppm saline, it is dumped overboard.
Flash evaporators can process around 6,000 gallons of water per hour. They are quite efficient because ship designers position the systems close to the engine compartments. This way, the excess heat from the engines helps to warm the water during the initial stages.
Reverse Osmosis
You’ve probably heard of the other type of water filtration system used by cruise ships: reverse osmosis. This is the same technology that many people have in their own homes, just on a bigger scale.
A reverse osmosis system is smaller than a flash evaporator and doesn’t use heat. Instead, salt water is pushed through a series of increasingly finer filters. The final stage utilizes microscopic membranes to remove particles as small as 0.04 microns (40 nanometers).
A reverse osmosis system can produce about 3,500 gallons of water per hour. This is less than a flash evaporator, but as they take up less room, there may be more reverse osmosis systems on the cruise ship.
Final Water Treatment Steps
Once the seawater has been filtered and desalinated – either through a flash evaporator or through reverse osmosis – there is one final stage before the water is drinkable. It is treated with mineralizers for taste, carbon dioxide to control the pH level, and chlorinated for disinfection. After all of that, the water is tested to be sure it meets USPH standards. Most cruise lines try to maintain water with test results even better than required.
Other Water Sources
Depending on the size of the cruise ship, there may be multiple flash evaporators and reverse osmosis systems on board. But despite a huge capacity for making potable water, sometimes, there just isn’t enough to go around. Why? Well, when the desalination systems is operated too close to shore, because the filters may be blocked by pollution and sediment that can be found near the coast. Usually, ships must be at least 4 miles away or in water 165 feet deep to use their desalination system. Local regulations may require the ships are even farther out to sea.
So cruise ships look for other sources of water. A small but significant supply comes from the air conditioning units that keep the temperature inside the ship comfortable. Condensed water from on board cooling systems is collected and may be used for non-drinking purposes, such as in the laundry rooms.
But of course, the easiest way to get extra water is to obtain it from somewhere else. The process of pumping water onboard from an external source – either a tanker or at a terminal – is called bunkering. This is the same term used when referring to pumping fuel on board. In the case of water, there are strict regulations that must be followed to ensure that the water is up to USPH standards. It is stored in separate tanks and not added to the potable water supply until all the tests are complete, which can take 18 – 24 hours. Often, shore water is not used for human consumption but instead supplements what is needed for other ship functions, like laundry, for high-pressure washing, for various machinery throughout the ship, in the fire-fighting sprinkler systems, and as ballast to keep the ship stable.
Drinking Water on a Cruise Ship
Some first time cruisers wonder, where is the best place to get drinking water on the cruise ship? The truth is, it’s all the same. The water in the galley comes from the same tanks as the water that runs from the faucet at your bathroom sink. And as we just discussed, those tanks contain some of the most purified water you’ll find anywhere on earth. The cruise ship crew tests each point in the chain of filtration regularly and maintain strict records to show that the water meets USPH guidelines.
Furthermore, consider the pipes that this water is running through. Back on land, water pipes in your house may be 30, 40, even 50 years old. Some municipalities still have lead pipes within their water system! But a cruise ship’s water pipes will be as new as the cruise ship – and most cruise ships sailing today are well under 20 years old.
Now, one point I will concede is that the water you get at designated water dispensers tastes slightly better. This is because some cruise ships install an extra carbon filter on the dispensers themselves. The filter removes chlorine and reduces the wear and tear on the machines. This can result in a slightly different, and most people agree, better, taste to the water. So if that makes a difference to you, by all means feel justified to refill your water bottle at a drink station rather than using your bathroom tap.
Watch our video about water filtration on a cruise ship:
