Function

• The goal of reverse osmosis is to filter dissolved substances and ions from water. High pressure forces water across a semi-permeable membrane, which is porous enough to allow water molecules through but blocks the passage of dissolved substances like salt ions. This pressure forces the water through the membrane in the opposite direction from the one it would otherwise take. This leaves dissolved solutes and contaminants behind.
  
  

Effects

• Reverse osmosis is an extremely effective filtration technique. Depending on the type of membrane and the system used, reverse osmosis systems can remove 85 percent to 98 percent of total dissolved solids from water, according to the government of Alberta, Canada. Bacteria and viruses are much larger than ions or molecules of dissolved solids, so reverse osmosis removes these contaminants as well, although some gases, like dissolved oxygen or hydrogen sulfide, will pass through the membrane.
  
  

Uses

• Small reverse osmosis systems are often purchased for home use, to filter well water or help ensure the quality of drinking water from other suspect sources. Small reverse osmosis systems are often used in reef aquariums. The U.S. Army's Reverse Osmosis Water Purification Unit (ROWPU) uses reverse osmosis to extract clean drinking water from almost any source, no matter how contaminated. Sea water desalination systems and some sewage recycling systems use reverse osmosis as well.
  
  

Indirect Potable Reuse

• In recent years, a few local governments in the United States have constructed indirect potable reuse (IPR) systems. These systems filter sewage to drinking-water quality using a three-step process that relies on reverse osmosis. The filtered water is subsequently returned to aquifers or other natural formations, where it eventually returns to the drinking water supply.
  
  While critics deride these projects as "toilet-to-tap" schemes, some local government agencies, such as the Orange County Water District, say they have found the water they derive from this process to be of high quality and cleaner than water from sources like the Colorado River, especially since many cities upstream discharge their treated sewage into the river.
  
  

Considerations

• Reverse osmosis systems tend to be energy-intensive, because high pressure is required to force the water through a membrane. If the solute concentrations exceed certain levels, they become markedly less efficient. For home treatment systems, levels of 1500 to 2000 ppm are typically the maximum. There are different types of membranes available on the market, each of which may offer certain advantages. Shop around before deciding which to purchase.