The percentage of feed water converted into permeate in a reverse osmosis or nanofiltration system, calculated as the permeate flow divided by the feed flow.
Recovery rate is the percentage of feed water converted into permeate in a reverse osmosis or nanofiltration system. The calculation is the permeate flow divided by the feed flow, and typical values vary widely by application. Seawater RO systems operate at 35 to 50 percent recovery because of the high osmotic pressure of the feed water. Conventional brackish water RO runs at 70 to 85 percent. High-recovery industrial configurations such as closed-circuit reverse osmosis reach 95 percent or higher. Of all the operating parameters available to a system designer, recovery has the greatest effect on scaling and fouling risk.
As recovery rises, the rejected ions in the concentrate stream are progressively concentrated relative to the feed water. The concentration factor (CF) describes this relationship: CF = 1 / (1 – R), where R is recovery expressed as a decimal. At 75 percent recovery the concentration factor is 4, meaning the concentration of scaling species in the final pressure vessel reaches approximately four times the feed concentration. At 90 percent recovery it rises to 10.
The practical upper limit on recovery for a given installation is set by the saturation indices for the principal scaling species in the feed water, including calcium carbonate, calcium sulfate, barium sulfate, strontium sulfate, and silica. Osmotic pressure of the resulting concentrate stream and any constraints on concentrate disposal further limit recovery in real installations. Operating at higher recovery than the feed water chemistry safely supports, or without antiscalant dosing calculated accurately for concentrate conditions, is one of the most common causes of accelerated membrane scaling in industrial RO operations.
