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How To Choose A Reverse Osmosis System

| Articles | December 7, 2009

Choosing a Reverse Osmosis system for your home (or office) depends on so many factors. You can make your best decisions if you have a general understanding of the basics of the process. The technical terms will fall into places as we walk you through some of the most frequently-asked-questions both from customers.

In a nutshell, reverse osmosis is the process where contaminated water is forced through a semi-permeable membrane via pressure to harvest pure clean water at the other end. The membrane strained out TDS (total dissolved solids) composed mostly of salts, metals, microorganisms, minerals, chemicals, etc.)

Water quality

Is the water supply potable?

An R/O system should be installed only if the water supply is deemed bacteriologically safe for human consumption, is disinfected or sterilized on a regular basis.

What is the daily quantity of pure water required?

It should produce at least gallon of drinking water per person/per day.

Is the water supply adequately pre-treated?

The presence of contaminants such as iron, manganese, or hydrogen sulfide should be removed by pretreatment.

What is the level of TDS?

Drinking water should have a TDS of below 500. Water from wells have a TDS of 1000 to 5000, seawater has a TDS of 40,000.

Is the feed water supply chlorinated or unchlorinated?

If unchlorinated, chose a TFC membrane to withstand bacteria attack. If chlorinated, chose a CTA membrane that is not chlorine-sensitive. The membranes have to be replaced around every 2 years, depending on the quantity and quality of water.

What about water acidity?

The average pH is 6.9 to 7.5. Low pH is very corrosive to metals. A pH of 6.9 is ten times more acidic than at a Ph OF 7.0.

Booster pumps

Is a booster pump required?

A booster pump is needed if the water pressure is less than 50 psi, if the TDS in your water supply is over 1000, or if the water is very cold.

Performance factors

In the end, there are four factors that ultimately affect the performance of an R/O unit 1) incoming water pressure, 2) temperature, 3) the TDS number and, 4) the quality of the filters and membranes used.

How much water is produced is governed by the pressure on the net membrane. If the pressure is increased, production goes up. It is also dependent on the temperature. The cooler the feed water, the lower the rate of production.

The force that binds the water molecules to dissolved ions or solids is called osmotic pressure. Now, the higher the number of the total dissolved solids (TDS) in the water, the higher is the molecular force that binds them.

These water molecules that are bound to the molecules of the dissolved solids need to be separated to pass through the pores of the membrane. To break the bond, there must be pressure applied on the water.

In the R/O process, more than one filter and membrane may be used. The pre-filters are for the sediments (sand silt, dirt and others). Then there are the carbon filters which pick up the chlorine which destroys the osmosis membrane.

Finally, the heart of the reverse osmosis system: the reverse osmosis membrane. The most common is the spiral wound. There are two choices, too the CTA (cellulose tri-acetate) which is chlorine tolerant and the TFC/TFM (thin film composite /thin film material) which is not chlorine tolerant.

If the feed water is not chlorinated, a TFC membrane is used because of its greater resistance to bacterial attack. CTA membranes not sensitive to chlorine may be used. Most systems, however, use TFC membranes, with a carbon filter support.

These are the details to consider in the choice of R/O system for your home or office. Don’t forget to also discuss them with your supplier for more inside information on how to choose a reverse osmosis system.

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