What contaminant rejection rates should a well-designed RO system achieve?
A properly operating RO membrane rejects 95-99% of dissolved salts, 99%+ of bacteria and viruses by size exclusion, and 95-98% of most pesticides and pharmaceuticals. Heavy metals like lead and arsenic are rejected at 95-99% depending on their ionic form in the feed water. Rejection rates decline as membranes age or foul, which is why regular normalized permeate flow and salt passage monitoring is part of any serious maintenance program.
What is the difference between recovery rate and rejection rate in an RO system?
Rejection rate measures how much of a specific contaminant the membrane keeps out of the product water, expressed as a percentage. Recovery rate is a system-level measurement describing what fraction of feed water becomes usable product versus concentrate discharged to drain. A system running at 75% recovery produces 75 gallons of purified water and 25 gallons of concentrate for every 100 gallons fed to the system.
What operating pressure does a typical RO system require?
Brackish water RO systems treating feed water up to 5,000 ppm TDS operate at 150-400 psi. Seawater systems with feed water above 30,000 ppm require 800-1,200 psi to overcome osmotic pressure and produce meaningful product flow. Municipal water RO systems treating chlorinated tap water at 200-500 ppm typically run at 80-150 psi. AMPAC USA sizes pump and pressure rating to feed water chemistry, not a generic pressure setting.
What pre-treatment is required before water enters an RO membrane?
At minimum, feed water must pass through a 5-micron cartridge filter to prevent particulate damage to the membrane element. Chlorinated feed water requires carbon filtration or sodium bisulfite injection to protect thin-film composite membranes, which degrade rapidly with chlorine exposure above 0.1 ppm. Hard water above 10 grains per gallon benefits from softening or antiscalant dosing to prevent calcium carbonate fouling on the membrane surface.
How long do RO membranes last, and what causes premature failure?
In clean feedwater applications with proper pre-treatment, RO membranes commonly last 3-5 years before replacement is warranted by declining performance. Premature failure is almost always caused by chlorine oxidation, biological fouling from inadequate feed water disinfection, or scaling from insufficient antiscalant dosing or pH control. Running the system outside the manufacturer's specified pressure and temperature limits also accelerates membrane degradation.