Reverse Osmosis is a separation process which uses water pressure to force water molecules through a membrane that has extremely tiny pores, leaving the larger contaminants behind. Purified water is collected from the “clean” side of the membrane, and water containing the concentrated contaminants is flushed down the drain from the “contaminated” side. This is the reverse of the “normal” osmosis process, where water moves naturally . . . through a semi-permeable membrane . . .
from an area of low solute concentration . . . to an area of high solute concentration (with no external pressure applied).The average RO system is a unit consisting of a sediment/chlorine pre filter, the reverse-osmosis membrane, a water storage tank and an activated-carbon post filter. They cost approx. $150 to over $1,500 for point of use systems.


  • Reverse osmosis significantly reduces salt, most other inorganic material present in the water, and some organic compounds. With a quality carbon filter to remove any organic materials that get through the filter, the purity of the treated water approaches that produced by distillation.
  • Microscopic parasites (including viruses) are usually removed by properly functioning RO units, but any defect in the membrane would allow these organisms to flow undetected into the “filtered” water – so they are not recommended for use on biologically unsafe water.
  • Though slower than a carbon or sediment water filter, RO systems can typically purify more water per day than distillers and are less expensive to operate and maintain.
  • Reverse Osmosis systems also do not use electricity, although because they require relatively high water pressure to operate, they may not work well in some emergency situations.


  • Point of Use RO units make only a few gallons of treated water a day for drinking or cooking.
  • RO systems waste water. Two to four gallons of “waste” water are flushed down the drain for each gallon of filtered water produced.
  • Some pesticides, solvents and other volatile organic chemicals (VOCs) are not completely
    removed by RO. A good activated carbon post filter is recommended to reduce these
  • Many factors impact the RO membrane’s efficiency in reducing the amount of contaminant in the water. These include the water’s pH, temperature and pressure; the contaminant’s concentration and chemical properties as well as the membrane type and condition.
  • Although RO filters do not use electricity, they depend on a relatively high water pressure to force the water molecules through the membrane. In an emergency situation where water pressure has been lost, these systems will not function.
  • RO systems require maintenance. The pre and post filters and the reverse osmosis
    membranes must be changed according to the manufacturer’s recommendation, and the
    storage tank must be cleaned periodically.
  • Damaged membranes are not easily detected, so it is hard to tell if the system is functioning normally and safely.