Cruise ships utilize salt water in several high-volume capacities, but they primarily rely on sophisticated desalination systems for internal consumption. The most obvious use of salt water is for the ship's ballast tanks and cooling systems for the massive engines. Additionally, most older cruise ships use treated sea water for their pool systems, though many modern luxury vessels are shifting toward fresh-water pools for guest comfort. For drinking, cooking, and showering, ships use "flash evaporators" or Reverse Osmosis (RO) systems to convert thousands of gallons of sea water into pure, potable fresh water every hour. This processed water is often cleaner than municipal tap water because it undergoes intense filtration and UV sterilization. Interestingly, even the water used to flush toilets on many ships is "technical water"—grey water that has been treated but is not quite at drinking grade—to conserve the fresh water supply. So, while the ship sits in salt water, the lifeblood of the vessel's internal economy is a constant cycle of high-tech desalination and conservation.