Water Softener

    De-Chlorination

    Demineralization

    Electrodeionization

    Multi- Media Filtration

    Microfiltration

    Ultrafiltration

    Nanofiltration

    Reverse Osmosis

    Distillation

    Ion Exchange

    Ultraviolet Disinfection

    Aeration

    Deionization

    Activated Carbon Filters

    Ozonation

Reverse Osmosis


To understand how reverse osmosis works, it helps to understand the process of osmosis which is ubiquitous in nature. When two solutions having different dissolved mineral concentrations are separated by a semi-permeable membrane, water flows from the less concentrated solution to the more concentrated solution.

As water moves through the membrane, most minerals it contains are left behind. The mechanisms which enable the water molecules to pass through the membrane leaving most of the dissolved minerals (ions) behind are not fully understood but it is definitely much more complex than simple filtration. Diffusion and active transport are models which play a role. One definition calls osmosis "the migration of water molecules across a membrane caused by the attraction of the dipole moment of water molecules to ions and polar molecules on the other side of a membrane."

Reverse osmosis utilizes man-induced pressure on the "dirty side" (high mineral content side) to overcome the natural osmotic pressure trying to flow the other way, plus some added pressure to speed the process, in order to force water across the semi-permeable membrane to the "clean side". In the process, 98% or more of the dissolved minerals are left behind on the "dirty side".

With rapid developments in membrane technology during the last 20 years, reverse osmosis has become one of the most cost efficient technologies to deionize water. Systems are in place capable of removing salt from seawater (desalination) at flows of several million gallons per day. Since reverse osmosis does not use expensive and hazardous chemicals, it has replaced ion exchange demineralization in many applications such as boiler feedwater treatment, rinse waters, laboratories, etc.

Reverse osmosis uses membranes wound around a core in order to fit large amounts of membrane surface area into a small volume. Such membranes are referred to as "spiral wound" and have largely displaced the early "hollow fiber" systems. Since the membrane prevents 98% of dissolved ions from passing into the clean water stream, a lot of minerals are left on the "dirty" side of the membrane. To sweep these away and minimize scaling (as the minerals become more concentrated, many may exceed their solubility concentration and begin to precipitate or scale onto the membrane, thus decreasing its filtration efficiency and potentially rendering it useless), typically about 25% of the total feedwater is washed across the dirty side of the membrane to drain. In addition to the concentrated minerals, much of the tiny particles of suspended dirt in the feedwater are also swept to drain. This produces a very clean product water since even very small particles (down to 0.0001 micron), including most total organic carbon or TOC is also removed.

When additional mineral reduction is desired, ion exchange demineralization can be used to polish the product water. Since 98% of dissolved ions are removed in the RO process, the ion exchange resin has considerable capacity between exchanges (see Service Deionization or regeneration).

If the feedwater is properly treated upstream of the reverse osmosis system, maintenance is generally minimal since they only have one significant moving part, a pump. The most prevalent RO membranes in use today, are susceptible to destruction by chlorine so pretreatment generally includes either feeding a reducing agent like sodium bi-sulfite or use of activated carbon filters to achieve dechlorination (i.e., elimination of free chorine).

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