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. Examples of semi-permeable membranes are cell walls of a living organism, the membrane on the inside of a chicken egg, the intestinal lining of mammals, or man-made materials (type of plastic) which demonstrate this characteristic.
Osmotic pressure is a measure of how badly the water wants to go from the “clean” side to the “dirty side” (low mineral to high mineral content side of the membrane) and that is governed by the mineral concentration differential. This pressure can be surprisingly high and accounts for one mechanism used by trees to move water from the deepest root to the tallest limb, frequently a vertical distance of 100 or more feet. 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 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 systems utilize 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 RO process, 98% or more of the dissolved minerals are left behind on the “dirty side”.
With rapid developments in membrane and commercial RO system technology during the last 20 years, reverse osmosis has become one of the most cost efficient technologies to deinize 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 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 bisulfite or use of activated carbon filters to achieve dechlorination (i.e., elimination of free chorine). Ion exchange softening may be required to reduce calcium and magnesium carbonates to prevent scaling, although the rapid development of antiscalant chemicals generally makes them the method of choice (eliminates salt consumption). Multi-media filtration may be required if the water contains significant silt. Feedwater should be evaluated using a silt density test (SDI) prior to specifying pretreatment since premature membrane failure and / or frequent membrane cleanings could result from inadequate pretreatment design. The SDI is a unit-less number calculated from several timed collections of water flowing through a 0.45-micron absolute filter while maintaining a pressure of 30 psi.
The product and wastewater flow through an RO are a function of the hydraulic design of the machine and can only be changed modestly. Since water density varies markedly with temperature, warm water (e.g., 77° Fahrenheit) will flow much faster through a membrane than water of 40° Fahrenheit (the flowrate at various temperatures is widely available from manufacturers and is referred to as “flux rate” of the membrane). Most manufacturers list their design specifications based on 77° F water. This has caused many end-users who do not understand this consideration, to improperly select an RO machine since municipal water in many cities may reach lows of 40° F – 45° F during the winter while well water typically averages 55° F year around in much of the US but can be 75° F or warmer in the SE US. It is important this parameter be considered during the design stage. In addition to the RO, this design consideration must be applied to the pretreatment equipment since it must handle higher flows during warmer months.