Modern Water Recycling Methods
Around the world, in dry regions and aboard ocean-going ships, a number of methods are used to recycle water. In order to make waste water, including sewage water drinkable again, the methods used can include freezing, distilling the water, reverse osmosis, ultraviolet light purification to kill viruses and bacteria, oxidation to disinfect water, microfiltration and ultrafiltration to filter out solid waste and larger bacteria, etc.
Reverse osmosis, microfiltration and ultrafiltration all entail forcing water under pressure through fine filter membranes. They differ only in the size of the pores in the filter membrane, and therefore the size of the molecules they allow through the filter. Reverse osmosis can reputedly remove up to 99% of all contaminants. The remaining solid waste can be composted for use in growing plants and food.
Freezing water to purify it requires that the water be frozen slowly. As ice crystals form, impurities are forced out of the crystal structure. Before all the water is frozen, remove the large block of ice, which should be pure water. This method will not necessarily kill all bacteria, however. This method can be used to create potable water from sea or salt water by effectively forcing the salt ions away from the freezing ice crystals. The chunk of ice in the salt water is pure water, and can be drunk once it is melted. This would be very energy efficient in spaceships where the surrounding space is well below freezing, and no energy would be lost in freezing waste water.
Distilling entails heating and boiling the waste water, and collecting and condensing the steam from the process. The steam cools and solidifies into water again, which can be reused as drinking water.
The final steps of water recycling require sterilizing the water to make sure all bacteria and viruses have been killed. Common methods are exposing the water to ultraviolet light, oxidation of the water, or the use of electrical microcurrents to kill germs and viruses.