Information center on Pure water technology by reverse osmosis - Leading manufacturer of reverse osmosis water filters

If a choice had to be made, the Ultra-filtration/Reverse Osmosis process would be elected as state-of-the-art in water treatment technology today. Reverse Osmosis (RO for short) was developed in the late 1950s under U.S. government funding, as an economical method of desalinating sea water - a dream long sought after by mankind.

Ultra-filtration/Reverse Osmosis is revolutionary because it uses a completely new mechanism for processing water - the semi-permeable membrane. Surprisingly enough, it looks a lot like common household sandwich wrap and is composed of very similar polymers (plastics). That's where the similarity ends because this near-miracle material is now considered one of the greatest technological achievements of our century.

It's best to look at the semi-permeable RO membrane as providing two distinct water treatment processes. First, it is the ultimate mechanical filter, or ultra-filter, straining out virtually all particulate matter, turbidity, bacteria, microorganisms, asbestos - even single molecules of the heavier organics. To appreciate the fineness of this ultra-filter, as it is referred to in the industry, its pores are on the order of .0005 microns or .0000002 (two ten-millionths) of an inch! That's smaller than can be seen by the best optical microscopes.

Second, it removes dissolved impurities (e.g. mineral salts, toxic metals) - those even smaller than the water molecules themselves - by a remarkable phenomenon known as Reverse Osmosis. With RO, the membrane is said to reject these impurities by repelling them from its surface. It is however permeable to the water molecules so that they diffuse through in a pure state and collect on the opposite side to make the product water.

RO's Claim to Fame

The real claim to fame for Ultra-filtration/Reverse Osmosis membranes is their ability to remove and reject such a wide spectrum of impurities from water and they do it with very minimal energy usage. In fact, it just requires water pressure. With the exception of distillation, RO is the only known process which can effectively remove the following types of impurities:

  • Particulate matter, turbidity, sediment, etc.
  • Colloidal matter
  • Total Dissolved Solids
  • Toxic Metals
  • Radioactive elements
  • Microorganisms
  • Fluoride/fluorine
  • Asbestos
  • Pesticides and Herbicides
  • Heavier organic molecules (MW>300)

How RO Works: In Practice

Long sheets of the semi-permeable membrane are ingeniously sandwiched together and rolled up around a hollow central tube in a spiral fashion. This rolled-up configuration is commonly referred to as a spiral wound membrane or module. They are available in a large selection of sizes for processing different quantities of water. Typically, a module for home water treatment is as small as 2" in diameter and 10" long, while one for industrial use maybe 8" in diameter and 48" long.

Now that the membrane is in a usable form it must be put in some type of container (called a pressure vessel) so pressure can be maintained on its surface. It is this pressure that supplies the energy to force the water through the membrane, separating it from the impurities. The most amazing aspect of RO is that the contaminants left behind are automatically diverted to a waste drain so they don't build up in the system as with conventional filters and purification devices. This is accomplished by using a part of the unprocessed water (feed water) to carry away the rejected impurities to the drain, thus keeping the membrane clean. The flow of impurities to the drain is often referred to as rejected water. This is the secret to why RO membranes can last so long and perform like new with minimum maintenance even after years of operation. It is also the reason behind the low cost of producing RO water.

Variety of Membranes

Membranes are available in a variety of materials. The most common are the so-called cellulosic type. Within this group are cellulose acetate (CA) and cellulose triacetate (CTA).

While CTA is a marked improvement over the original membrane formulation, CA, all cellulosic membranes share one shortcoming. They are susceptible to eventual deterioration from bacterial growth on the membrane surface. While this is rare in regularly disinfected or chlorinated municipal water supplies it can be a real problem when these membranes are used on non-chlorinated or private well supplies. CTA membranes do offer much better resistance to bacterial and chemical attacks than CA membranes and have excellent performance on most municipal water supplies.

The latest advances in membrane technology are the new polyamide thin film composite (TFC) types. Based on an entirely new formation, the TFC membranes not only are completely impervious to bacterial attack but also have the superior rejection of impurities, higher water production, and increased resistance to adverse water conditions such as pH. They do have one disadvantage at this time and that is being susceptible to chemical deterioration from chlorine and other oxidizers.

Even Ultra-filtration/RO Is Not Without Its Shortcomings

The Ultra-filtration/Reverse Osmosis process alone is not without its shortcomings. It is simply ineffective in removing the lighter, low molecular weight volatile organics such as THM's, TCE, vinyl chloride, carbon tetrachloride, etc. They are too small to be removed by the straining action of the ultra-filtration ability of the membrane and their chemical structure is such that they are not repelled by the membrane surface. Since these are some of the most toxic chemical contaminants found in tap water, it is important they be removed by a carbon filter added to the RO system. In fact, nothing could be better for effective carbon filter absorption than having an RO membrane prior to it to remove practically all of the contaminants which would gradually impair the carbon's performance. To address this situation, nearly all reverse osmosis systems built for the home incorporate a series of filters that include one or more carbon filters.

Reverse Osmosis Membrane Rejection Guide

Giardia cysts

Criptosporidium cysts

DDT

PCB

E.coli bacteria

Fecal bacteria

Chlorinated Pesticides

Radium

Arsenic (+3/+5)

Sodium Chloride NaCl 

Chromium (+3 /+ 6)

Glucose

Sodium

Nickel

Chloride 

Copper 

Potassium

Zinc

Bicarbonate  

Sulfate

Calcium

Silica SiO2

Nitrate 3

Chromate  

Magnesium 

Cadmium 

Fluoride

Barium

Iron 

Silver  

Silicate 

Ammonium 

Aluminum 

Mercury

Phosphate 

Lead

Higher Performance RO membranes Begin With Shorter Leaves

All spiral-wound reverse osmosis membranes are comprised of membrane "leaves" —individual sheets of the membrane through which feed water passes and is purified. Short-leaf design is an advantage because less pressure is required to deliver water to the end of each leaf, and more uniform flux is maintained from one end of the leaf to the other. To give an example, APEC's FILMTEC membranes are fabricated with up to twice as many leaves and each leaf is 1/3 to 2/3 shorter than those in other membranes. The result is a highly efficient membrane —one that provides purer drinking water and longer life. Besides overall system designs and parts used, this is the reason why different ROs have different contaminant rejection capabilities.

Flow Rate of Reverse Osmosis Systems

All reverse osmosis systems are rated (and priced) by their flow rates (Gallons of Pure Water Produces Per Day). The effectiveness and productivity of RO systems increase with greater water pressure and temperature. The actual flow rate (in GPD) of any reverse osmosis system depends on your home water pressure and temperature. This is why all of our systems are named as they are, for example 36/45 GPD means it is rated at 36 GPD at 50 psi of water pressure, 45 GPD at 60 psi. While the industry standard* is set at 60 psi, we know that not everyone's cold water is at 60 psi and 77 degrees F. Instead of misleading customers into thinking that a 45 GPD system will guarantee a flow rate of 45 GPD, we remind you that you might get less than 45 GPD �perhaps 36 GPD or even less if your pressure and temperature are very low! So when the flow rate is critical for you, you need to consider all these factors.

*Please note that the industry standard is not necessarily followed by all vendors. We have seen other RO systems claiming 100 GPD at 100 psi —very unrealistic input water pressure! So do check on these facts before you make a buying decision.

 

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