Water, water, everywhere, Nor any drop to drink.

Water, water, everywhere, Nor any drop to drink.

If you had to guess the consumption of water on board a modern cruise ship, per person per day what would you say? You will probably find it hard to believe how much water we actually use while cruising! It is on average about 225 litres(59 gallons) per person every single day. Ok…that figure does include total water consumed each day divided by souls on-board, not just personally but indirectly in the laundry, galley and swimming pools, although pools aren’t emptied and refilled that often and consumption here is usually quite low.

So where does it all come from?

Modern cruise ships do have huge storage capabilities which can be as much as 3,000,000 litres(792516 gallons)! When alongside in port you might notice hoses entering the ship via a shell door as ships do normally top up during port stops. But this is only a drop in the ocean (pardon the pun)

Most of the water consumed on-board is in fact produced on-board from….you guessed it, Seawater!

This can be achieved by two different methods

Flash Evaporators

Flash evaporators are the most popular, these normally produce around 20,000 litres(5283 gallons) per hour, but this can be higher. The seawater is pumped into the evaporator, but before entering the evaporator it is heated usually by the engine jacket water system, this is essentially recovering waste heat as if the evaporator was not running the heat would be passed to seawater using a plate heat exchanger and pumped overboard.

So with the seawater heated to 80°C(176°F) it then passes into the evaporator which is under a vacuum, the vacuum lowers the boiling point of the water to 40°C(104°F). When the water enters it immediately flashes off into steam leaving the salt behind to form a concentrated brine which will later be pumped overboard.

The steam rises through demisters which act as filters and is condensed forming distillate. This distillate water is very pure containing almost zero salt, this isn’t great for our health or very pleasurable for our taste buds. Although we do utilise methods to combat this.

Reverse Osmosis

Reverse osmosis plants are gaining popularity on cruise ships and range in capacity up to 250,000 litres/hour(66043 gallons/hour). Some readers have probably heard or osmosis or remember it from science class. This is a process common in the natural world. If water and a saline solution (sea water) are placed in a beaker divided by a semi permeable membrane, which is a membrane that allows water to permeate but not salt particles as they don’t fit the tiny holes in the membrane

Over time the water will cross to the saline solution diluting it and increasing its height seemingly overcoming gravity, the head created is termed the osmotic pressure. Reverse osmosis is when we pump the seawater at high pressure around 80 bar (must be higher than osmotic pressure) through special membranes, the pure water can fit through the membrane but salt particles cannot. This results in pure water which can then be treated.


Is the water safe to drink??

As previously mentioned the water produced from distillation and to a lesser extent reverse osmosis is low in salt and minerals and therefore not suitable for drinking, it has also not been heated sufficiently to ensure all bacteria is killed off, therefore treatment is required.

The water on-board all cruise ships is safe for drinking unless otherwise stated. The first stage of treatment is re-mineralisation, these are basically steel vessels filled with different grades of rock, which as the water flows through minerals are absorbed into the water which in turn improves the taste.

The water will then be dosed with chlorine to around 2 parts per million, this ensures any bacteria is killed off. The PH of the water is also measured and can be corrected by dosing with a mild acid. The chlorine and PH of the water are continuously sampled and recorded not only when passing from the water makers to the tanks but again when being pumped from the tank around the ship.

Cruise ships under go public health inspections regularly which includes inspections of the vessels water production and distribution equipment as well as past records of chlorine and PH to ensure they have been operating within safe limits.

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