Surrounded by water but not a drop to drink. Where does a Royal Australian Navy ship source its drinking water?

HMAS Choules’ water-making capability was recently tested when the ship evacuated 1100 people from Mallacoota during Operation Bushfire Assist.

Choules’ Engineering Officer, Lieutenant Commander Darrel Wolter, said the ship could make a substantial volume of water for its own needs and more.

Choules can make more than enough water each day to meet our requirements, and can store a far greater volume on top of that,” Lieutenant Commander Wolter said.

Choules also has the unique capability to be able to fill tankers and then transfer them ashore.”

Most Navy ships employ a phenomenon called reverse osmosis to turn the saltwater they sail through into fresh drinking water.

Osmosis is the process of water moving from an area where it is relatively pure, across a semi-permeable membrane, to an area where the water contains impurities such as salt. The process is passive and therefore doesn't require the input of energy.

To reverse this process and make fresh water, energy is applied to the saltier water. This energy pushes the water through the membrane, leaving the salt behind and desalinating the water.

Reverse osmosis can remove up to 99 per cent of salts, particles, organics and bacteria from sea water.

Choules has two reverse osmosis plants, which can each produce up to 60,000 litres of drinking water a day.

The daily usage of water on board Choules is about 37,000 litres.

With 1100 evacuees on board, the daily usage peaked at 67,000 litres, but with a total capacity for 1.5 million litres, there was plenty to go around.

Operation Bushfire Assist imagery is available on the Defence Image Gallery:
https://images.defence.gov.au/S20192970.