It can be said that Reverse Osmosis is the pinnacle of water filtration. The phenomenom of osmosis was first recognized by a French scientist in the mid 1700's when he noticed that water spontaneously passed through a pig bladder into a solution of alcohol. But it wasn't until the 1950's when viable membranes were able to be manufactured in the US, that reverse osmosis technology became recognized as a reliable treatment solution for certain water quality issues.
Osmosis occurs when a liquid (for our purposes - water) low in total dissolved solid(TDS) content passes through a semi-permeable membrane or barrier, and dilutes a water source that has a higher TDS content. The passage of the water through the membrane generates a "head difference" between the two water sources. This head difference is a measure of the concentration difference of the two water supplies and is referred to as: osmotic pressure difference.
When pressure is applied to the water source containing the higher dissolved solid content - that exceeds the osmotic pressure - the flow of water is reversed through the membrane...hence; "Reverse Osmosis".
The water that ends up being the finished product from the reverse osmosis process is known as "permeate". Having been squeezed through a semi - permeable membrane; the permeate is low - if not almost absent of TDS. Pure water doesn't truly exist in nature...permeate from an efficient RO system is about as close as you can get to "pure" in practical terms. To put in perspective the level of processing that occurs; the average human hair is 75 microns in diameter. Reverse osmosis can filter the water down to .0001 microns! This is truly filtration at the molecular level. In fact, the degree of filtration is such, that bacteria and viruses can be removed. However; feed water to a reverse osmosis system should be bacteria free, otherwise, the membrane will get fouled and can become "ground zero" for a contamination issue of a home's plumbing system.
This level of processing is not without drawbacks. Because the water is so free of dissolved solids, some consumers think RO water tastes too "flat" or has no character - there are remedies available for this scenario, and it is subjective to the individual. It also takes time for a reverse osmosis system to "make" or process useable water. Remembering that water is squeezed through a semi - permeable membrane, producing enough water for practical use doesn't happen quickly. Depending on certain conditions, it can take two - three gallons of water running to drain for every one gallon of permeate produced. Depending on job site conditions, waste water reclamation may be an option.
The most common residential application of reverse osmosis technology is in the Point of Use (POU) format. This typically involves a compact unit mounted under a kitchen sink with a faucet installed specifically to dispense the permeate. Water dispensers and ice makers in refrigerators can often be hooked up as well.
Depending on the nature of the contaminant, layout of the home, desired end result of the consumer and budget; whole house reverse osmosis or point of entry (POE) reverse osmosis, is an alternative option.
A consideration of POE reverse osmosis systems is corrosion. Permeate is low if not free in TDS content. Water - nature's solvent - becomes more "aggressive" or corrosive as the TDS content is reduced. Feeding permeate through a copper plumbing system can invite corrosion of pipes and fittings while leaving stains in sinks, toilets, and showers. Appropriate measures should be taken to protect metal plumbing systems.
While the reverse osmosis process has a wide array of contaminants it can remove, water must be pre - treated to remove excessive amounts of iron, manganese, and hardness so the membrane doesn't become clogged and fail prematurely. This is especially important when the feed water is from a private water well. If the feed water is municipally supplied, a chlorine resistant membrane (celluose triacetate) or chlorine removal pre treatment stage must be implemented because chlorine will destroy a thin film composite membrane.
Reverse osmosis is a highly effective treatment process. It should be placed downstream from all other treatment stages and thought of as a "polisher" for the potable water. RO should not be utilized as the primary treatment for aesthetic water issues as these contaminants will quickly foul the system. The two most common applications for private water wells in CT are for lead and sodium removal. As is the case with any water treatment device, proper maintenance schedules should be observed to ensure your reverse osmosis system is delivering the highest quality water possible.