Reverse Osmosis Purification Made Simple

Reverse osmosis is a popular method of purifying drinking water. All of the big guys use it; Coka Cola, Pepsi, Lipton, Evian, etc. Just about any water-based liquid you may consume has its water purified through reverse osmosis first. Reverse osmosis (RO) can also be used to de-salinate sea water.

The reverse osmosis method takes out about everything in water that is one-tenthousandth of a micron (micron = one millionth of a meter) in size or more. That means bacteria, calcium and other minerals, iron, nitrates, chlorine, fluoride and such. Just about anything that is not H2O is removed and all that is left over is pure water. Reverse osmosis is made possible by use of a semi-permeable membrane. Most residential membranes boast a rejection rate of 98% or there abouts. That number is purely arbitrary. Some people read that number to mean 98% of the water that flows through an RO unit is sent down the drain. Others interpret it to mean that 98% of contaminants are removed. Neither is true. The rejection rate is more of an efficiency rating than anything else.

Here Is How A Membrane Works

An RO membrane is a flat sheet of material that is semi-permiable. Obviously the microscopic holes in a membrane sheet cannot be 0.00001 micron in size. I cannot picture a punch press that pokes millions of little holes into a thin sheet of plastic every day. So the membrane is folded or rolled up layer upon layer until the act of forcing water through it produces results down to 0.00001 micron. Most residential RO systems are set up to operate under the typical house water pressure.

As you can well imagine, the process of forcing water through a fractional micron filter at 40psi produces a mere trickle of water. It would take way too long to fill your drinking cup for this method to be of any practical use. So the purified water produced by the membrane is stored in a holding tank. This holding tank has a bladder in it and air pressure on one side of the bladder to force the water out of the top of the tank when you open your RO faucet.

But What Are Those Filters For?

The filters used in an RO system are there to protect the membrane. The membrane can process straight water but its useful life will be drastically shortened if some sort of pre-filter system is not used. Most residential RO systems use a sediment pre filter. This filter traps any particles from your water such as sand and pipe scale before it gets to your membrane. This greatly increases the efficiency and life of your membrane so long as this filter is changed regularly.

Another popular filter is a GAC (Granular Activated Carbon) filter. This is used to remove chemicals in your water before they get to the membrane. This filter also increases the usefull life of your membrane.

Because the membrane is one of the most expensive parts of an RO system, it is a good idea to replace these pre-filter on a regular basis. How often should I replace my RO filters?

The Post Filter

Many RO systems will use a post filter as well. The post filter is usually activated charcoal and is used to add a fresh flavor to the water. The water treated and produced by an RO will often taste bland. That is because virtually everything is removed from it. Many people do not like bland water so a post fitler is added.

RO vs Bottle Water

I recently tested a customer's RO output to help him determine when to replace his pre-filters. I bought a bottle of name-brand water from a gas station on the way there. I tested his RO output with a TDS (Total Dissolved Solids) meter and got a reading of 3. His RO was installed 8 months ago with no filter change as of yet. The nationally available name-brand water tested at 38 with the same TDS meter which is pretty good, but not as good as is available straight out of your tap with the right equipment.

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