OVERVIEW OF MEMBRANE TECHNOLOGY

We hope that you find the following helpful. We rely on a variety of computer programs to design systems tailored to your specific water supply and application needs.

1. Reverse Osmosis. (RO) membranes are the most common and popular in use. Most are polymer based and will remove 98% of all dissolved solids (salts) present in the feed stream in a single pass. This removal percentage is called REJECTION. By taking the product water from one or a number of membranes called the first pass and putting that water through another membrane or group of membranes we create a two pass system which can remove well over 99% of the dissolved solids resulting in deionizer/distilled quality water. Product water is called PERMEATE. The dissolved solids in water are measured as Total Dissolved Solids (TDS). The TDS meter simply passes a small electrical current between 2 probes
1 cm apart and measures how conductive the water is or conversely the resistance of the water. The reading is expressed as Parts Per Million (PPM) of dissolved solids which is essentially the same as milligrams per liter (mg/l). This translates to how much "stuff" is dissolved in the water -- the higher the number the more dissolved solids. NOTE: at this point you can see that debris in the water -i.e. things you can see -- do NOT contribute to the TDS as they do not conduct electricity. Normal system efficiency is 50% -- 2 gallons in, 1 gallon treated and 1 gallon to drain. This is called RECOVERY. With proper pre-treatment, membrane selection, and flow pattern design, the recovery can be 80% or higher. As recovery increases, however, rejection decreases so there is a sacrifice in water quality. As a general rule it takes 1 PSI across the membrane for each 100 parts per million to get the first drop of water to pass through. To achieve a reasonable flow most systems operate in the 125 to 250 PSI range depending on the raw water quality and membrane selection. Fresh water membranes are used to treat water up to about 10,000 PPM. The rejection process is electro-chemical in nature so the small holes (porosity) of the element are there for water passage not to reject dissolved solids. This porosity is in the range of 0.001 micron making it an excellent ":trap" or filter for bacteria and viruses but makes the elements susceptible to plugging. The most critical aspects of a membrane system design are knowledge of the raw water chemistry, customer water needs, membrane selection, membrane array (flow path), pump curve selection and pre-treatment. Though stated last, pre-treatment is vital. Control of suspended solids, colloids, silt and bacteria must be done because of the 0.001 membrane pore size. Calcium hardness must be addressed to avoid membrane scaling. Seldom if ever are softeners employed for the scale control as they have been generally replaced with physical water treatment to suspend the hardness or chemical feed systems feeding a mild acid and surfactant compound called an anti-scale chemical. Iron and manganese control also is imperative and is often treated with simple filtration or the anti-scale chemical. Most membranes are free chlorine intolerant so it too must be eliminated either by activated carbon or with a chemical feed of a sodium sulfide compound. Lastly, feed water temperature must be viewed. Water like oil becomes more viscous as the temperature decreases so pumps and pressure vessels need to change for low temperatures to maintain production levels. Above 110 Deg. F. membranes deteriorate so intermediate cooling may be necessary.

2. Sea Water Membranes. Virtually everything stated above about standard RO membranes applies to the sea water equipment. Because the TDS level of sea water is 32,000 to 45,000 PPM the driving pressure must increase. Using the general rule of 1 PSI per 100 PPM, the first drop of water is passed at 320 PSI. To achieve meaningful flows, the sea water systems operate at 750 to 1200 PSI. This obviously requires stronger internal support of the membrane itself, higher pressure membrane vessels, more robust high pressure pumps, high pressure piping and materials throughout that can tolerate the corrosive effects of sea water. If there is an abundant source of sea water for supply, systems are designed for recovery in the range of 25% to provide the best quality of treated (permeate) water in the range of 150 PPM. Normally, higher recovery levels of 60% are designed and the systems maintain a permeate level less than 500 PPM which is a World Health Organization (WHO) standard. As systems get larger, it is common to route the drain water to a device attached to the high pressure pump motor to recover energy thus reducing electrical costs. Suffice to say, with all of the complexities and high pressures, this type of equipment does not lend itself to novice manufacturers.

3. NANO Membranes. These are hybrid membranes that are specifically designed to remove certain dissolved solids in the raw water while leaving others. All dissolved solids exist in an ion state. Each ion has a specific number of electrons in its outer shell. The number is referred to as the valence. One being mono valent, two being divalent etc. The higher the number the greater the attraction to the membrane and thus the higher the removal rate --rejection. Where standard and sea water membranes are formulated to low as well as high valence numbers, NANO membranes are formulated to selectively reject divalent and higher with only mild rejection of mono valent ions. The most prevalent divalent ions are Calcium, Magnesium, Carbonate and Sulfate. These are all associated with hardness so NANO membranes are often thought of and used as membrane softeners. The total rejection level of the membrane is a function of the mono valent concentration but experience indicates a level of 80% recovery. The NANO membranes operate at about 80-100 PSI so the systems use less power. Pore size is in the 0.01 micron range so bacteria and viruses are essentially blocked.The same cares and concerns relative to pre-treatment and component selection that is expressed for standard RO systems apply.

4. Ultra Filtration (UF) Membranes. Unlike the above listed membranes, the UF membranes are not designed to reduce dissolved solids. They are strictly a filter as the name implies. They are selectively available with pore sizes from 0.01 to 0.20 micron. Recovery rates are in the 90% range and they operate on pressures ranging from 2 to 50 PSI. In water treatment they are employed for colloid, silt, tannin, and bacteria removal often as pre-treatment for reverse osmosis systems. In the processing and waste industries the applications are limitless – whey recovery for cheese plants, dye removal, dairy solids concentration, yeast separation, tertiary waste water purification, etc. etc. Membranes are available in hollow fiber style (bundles of hair like hollow tubes with inside out or outside in flow). Though on the surface the UF process appears simple, in practice it is actually more complicated than reverse osmosis or NANO because of the chemistry and hydraulic knowledge needed to produce exactly what the customer requires.
We invite any and all questions.

To enhance your understanding of membranes and their applications, pre-treatment and expected results we suggest the book "Reverse Osmosis" by Wes Byrne or the Hydranautics web site www.membranes.com.

For more information on water treatment for your home or business, click on one of the following topics:

Reverse Osmosis Overview, Residential Reverse Osmosis, Commercial Reverse Osmosis, Water Softener Overview, Commercial Water Softeners, Residential Water Softeners, Water Filter Overview, Backwashing Water Filters, Non-Chemical Water Conditioners, Aquafer Water Conditioner, Maxi-Cure Water Filter, Scale Prevention Devices, Ultra Violet Disinfection, High Purity Water, De-ionization, Specialty Water Filtration, Reverse Osmosis Repair, Commercial Water Softener Repair, Residential Water Troubleshooting, Commercial Water Troubleshooting.

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