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. Because this is not possible, membranes allow some of most dissolved materials to pass through. The treated water therefore is much less contaminated but not completely free of contaminants. The performance of RO depends on the membrane, the size of the unit, pressure, temperature and water quality. The purchaser may need to answer several questions in order to make a wise selection.