Reverse Osmosis Overview
Reverse osmosis (RO), sometimes called ultra-filtration, is state-of-the-art water treatment technology. It was originally developed in the late 1950s under U.S. Government funding as a way to desalinate seawater. Today RO is one of the most thorough methods for filtering drinking water — used by most bottling plants, by industries that require ultra-pure water, and increasingly in homes and offices.
How RO Works
To understand reverse osmosis, start with regular osmosis: the natural movement of a solvent (like water) through a semipermeable membrane from a less concentrated solution to a more concentrated one. A semipermeable membrane lets some molecules pass but not others.
In reverse osmosis, pressure is applied to the more concentrated side (for example, salty or contaminated water) to push water molecules against the natural osmotic flow — through the membrane and into the cleaner side. Dissolved contaminants stay behind. It generally takes significant pressure and is relatively slow, but the result is highly purified water.
Contaminant Removal
RO is extremely effective at reducing a wide range of contaminants, including dissolved salts, heavy metals, fluoride, nitrates, and many organic compounds — making it a great choice for dedicated drinking water at the kitchen sink or refrigerator.
Sizing an RO System
A common mistake is assuming the manufacturer's rated output is what you'll get at home. Three variables strongly affect real-world production:
- Total Dissolved Solids (TDS)
- Water temperature
- Incoming water pressure
Membranes are typically rated at 25°C (77°F) and a specific test pressure. Cold water, high TDS, and low pressure all reduce the actual gallons-per-day a system will produce. For RO to function at all, roughly 1 psi of pressure is needed for every 100 ppm of TDS — known as the osmotic pressure (Π = TDS ÷ 100). The expected production of a system in your home can be substantially below the membrane's nameplate rating once these factors are accounted for.
What Affects Membrane Life
Several conditions can foul an RO membrane and shorten its life:
- Suspended solids
- Iron and manganese
- Silica and aluminum
- Colloidal matter
- Calcium sulfate
- Organic matter
- Microbiological fouling
- Oxidation and hydrolysis
- Concentration polarization
Proper pre-treatment — sediment filtration, carbon for chlorine removal, and softening where appropriate — is the single most important factor in maximizing RO membrane life.
Applications
RO is used in a remarkable range of applications, including drinking water, food preparation, restaurants, ice making, photography, car washes, greenhouses, hatcheries, laboratories, pharmaceuticals, semi-conductor manufacturing, boiler feed water, humidification, maple syrup production, aquariums, baby formula, and kidney dialysis — anywhere ultra-pure water is needed.