Download a free copy of the Adobe Acrobat Reader. Openings in the membrane are small enough to hold back most dissolved minerals such as calcium, magnesium, sodium, fluoride and chloride. The process is relatively new for home use and has become widely available since about 1980. Water entering the membrane is called feed water and the water passing through the membrane is called permeate, treated or product water. Rejection refers to the percent of the materials removed from the feed water as it passes through the membrane.
Recovery refers to the percent of feed water that becomes permeate.
The rejection rate of most home RO units is about 80 to 90 percent for the minerals common in hard water, a recovery of about 5 to 35 percent. In the water treatment industry, RO is sometimes referred to as hyperfiltration. The process looks like filtration at the submicron, molecule or ionic size range. Membranes do have small openings but also have an electrical charge which helps reject some ions. If the openings in membranes could be controlled with absolute accuracy, all particles larger than the openings would be rejected.