"enjoy ultra-pure water in your home with reverse osmosis"
HOW DOES REVERSE OSMOSIS WORKReverse osmosis is considered to be the most effective and efficient way to purify water for commercial use as well as
drinking and cooking, but how does reverse osmosis work? This article explains the reverse osmosis process and
answers the question, “how does reverse osmosis work in theory and practice?”
What is Reverse Osmosis?Reverse osmosis (RO) is a method of purifying water by squeezing the water through a semi-permeable reverse osmosis
membrane to separate impurities from the water. Unlike osmosis that occurs naturally, RO is a process developed by man.
Semipermeable MembraneSemipermeable means that a membrane permits certain materials to travel through it while selectively blocking other materials.
In practice, a great variety of molecules can travel through the semipermeable membrane, but they travel at different rates.
This causes a separation of impurities and water to occur. The faster-moving water molecules travel through the membrane
much faster than the contaminants, leaving the contaminants behind.
Why the term “Reverse Osmosis”?In nature when water that contains little or no salt is separated by a semipermeable membrane from water containing greater
amounts of salt, the water with less salt will naturally flow through the membrane to dilute the saltier water.
This is osmosis.
Osmosis takes place because the chemical potential of the saltier water is lower than the water containing less salt.
Osmotic pressure causes the less salty water to pass through the semipermeable membrane until the chemical
potential of the two solutions equilibrates.
Osmosis is evidenced by the roots of plants taking water from the surrounding soil, and also the wrinkling of our skin
as the water passes through our skin (a membrane) when we bath for long period of time.
[image showing osm &”RO]
Reverse osmosis occurs when pressure is applied to water forcing it through a man-made semi-permeable membrane
in a direction that is the reverse of that which would occur in nature.
Reverse Osmosis TheoryThe process of separating impurities from water using a reverse osmosis semipermeable membrane is still not fully understood.
Scientists hypothesize two models – porosity and diffusion.
The porosity model theorizes that the water passes through minute physical pores in the membrane. A man-made semipermeable
membrane contains pores of approximately 0.00001 microns, or 0.000000004 inch – about the size of water molecules.
By comparison, pores in other filter media are generally 0.5 – 1.0 micron. As a point of reference, bacteria ranges
from 0.2 – 3 microns, a human red blood cell is 5 microns across, and a human hair is 75 microns.
Under the porosity theory, only molecules as small as water molecules can pass through the membrane, preventing impurities
from passing through.
The diffusion model suggests that the chemical makeup of the membrane allows it to absorb and allow water to pass freely from
one bonding site in the membrane to another due to the weak chemical bonding of the water to the membrane. By comparison,
the stronger chemical bonding of the impurities causes them to remain attached to the membrane.
By either theory, a gradient forms across the solid/liquid interface. The chemical and structural construction of the membrane is
responsible for the membrane’s ability to separate impurities from the water.
Practical ApplicationIn practice, water is forced through the semipermeable membrane by hydraulic pressure. This is usually the accomplished by the
household water pressure in home systems or pumps in commercial RO systems. To withstand the hydraulic pressure the
semi-permeable membrane must be supported inside of a container. The best reverse osmosis systems for home use,
such as those produced by APEC Water, are engineered to require minimal water pressure resulting in a system that,
unlike inferior systems, does not require electricity to pump water through the system.
A common type of semipermeable membrane construction is the spiral wound membrane. Long sheets of membrane are layered
and rolled into a hollow tube in a spiral. For home use, these spiral wound membranes are approximately 2 “diameter and 10” long.
Larger industrial membranes can be up to 4” diameter and 40” long. The most effective spiral wound membranes for home use are
Filmtec membranes made by Dow. These membranes and RO systems that use them are available from APEC Water.
As a matter of full disclosure the writer of this article may receive monetary compensation
if you use any links on this site to make a purchase.
As contaminants build up on the entry side of the semi-permeable membrane, some of the water to be purified (feed water) is flooded
across the membrane surface to wash the contaminants to the drain, constantly renewing the surface of the membrane and extending
its useful life.
The Reverse Osmosis SystemReverse osmosis must be used in a system along with other water purification methods to be fully effective.
Unfortunately, the reverse osmosis process by itself does not remove all contaminants, odor or taste. Reverse osmosis will
not remove solvents and volatile organic compounds (VOC) such as formaldehyde, isopropyl alcohol, benzene, toluene and
other aromatic chemicals. Nor, will the RO process remove pesticides such as 1,2,4 trichlorobenzene or atrazine. Some
chemicals can damage the semi-permeable membrane. Chlorine, present in most city and municipal water systems, will
destroy most reverse osmosis semi-permeable membranes! Since the membrane can easily be damaged or destroyed
by chlorine and the reverse osmosis process BY ITSELF is incapable of removing all contaminants, reverse osmosis is
used in a system that also includes other filters and media. These filters and media work in conjunction with the RO
process to protect the reverse osmosis semi-permeable membrane and remove virtually all undesirable
contaminants from water.
Another reason for using reverse osmosis in a system is that RO removes all minerals from tap water, leaving it with a flat taste.
Superior reverse osmosis systems such as those offered by APEC Water include methods to replace those minerals needed
for great-tasting water!
Insurance Against Unhealthy Drinking WaterWe continue to hear about tap water contamination that occurs without warning like the lead situation in Flint, Michigan.
A good reverse osmosis system connected to your tap water will ensure that YOU are never caught off guard
and your cooking and drinking water will always be absolutely safe to drink.
How does reverse osmosis work, and why is it important? An RO system is the most dependable and economical insurance
against consuming contaminated tap water available. This article has provided information that answers the question,
“how does reverse osmosis work to provide the purest water possible for both commercial and human consumption?”
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