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. 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 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 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 antiscalent 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.

Definitions:

Recovery - the percentage of membrane system feedwater that emerges from the system as product water or “permeate”. Membrane system design is based on expected feedwater quality and recovery is fixed through initial adjustment of valves on the concentrate stream. Recovery is often fixed at the highest level that maximized permeate flow while preventing precipitation of super-saturated salts within the membrane system.

Rejection - the percentage of solids concentration removed from system feedwater by the membrane.

Passage - the opposite of “rejection,” passage is the percentage of dissolved constituents (contaminants) in the feedwater allowed to pass through the membrane.

Permeate - the purified product water produced by a membrane system.

Flow - feed flow is the rate of feedwater introduced to the membrane element, usually measured in gallons per minute (gpm). Concentrate flow is the rate of flow of non-permeated feedwater that exits the membrane element. This concentrate contains most of the dissolved constituents originally carried into the element from the feed source. It is usually measured in gallons per minute (gpm).

Flux - the rate of permeate transported per unit of membrane area, usually measured in gallons per square foot per day (gfd).

Dilute Solution – purified water solution, RO system product water.

Concentrated Solution – brackish water solution such as RO system feedwater

Effect of Pressure

Feedwater pressure affects both the water flux and salt rejection of RO membranes. Osmosis is the flow of water across a membrane from the dilute side toward the concentrated solution side. RO technology involves application of pressure to the feedwater stream to overcome the natural osmotic pressure. Pressure in excess of the osmotic pressure is applied to the concentrated solution and the flow of water is reversed. A portion of the feedwater (concentrated solution) is forced through the membrane to emerge as purified product water of the dilute solution side.

Water flux across the membrane increases in direct relationship to increases in feedwater pressure. Increased feedwater pressure also results in increased salt rejection but the rejection is less direct than for water flux.
Because RO membranes are imperfect barriers to dissolved salts in feedwater, there is always some salt passage through the membrane. As feedwater pressure is increased, this salt passage is increasingly overcome as water is pushed through the membrane at a faster rate than salt can be transported.

However, there is an upper limit to the amount of salt that can be excluded via increasing feedwater pressure. As the plateau in the salt rejection curve exceeds a certain pressure level, salt rejection no longer increases and some salt flow remains coupled with flowing through the membrane.

Effect of Temperature

Membrane productivity is very sensitive to changes in feedwater temperature. As water temperature increases, water flux increases almost linearly, due primarily to the higher diffusion rate of water through the membrane.
Increased feedwater temperature also results in lower salt rejection or higher salt passage. This is due to a higher diffusion rate for salt through the membrane.
The ability of a membrane to tolerate elevated temperatures increases operating latitude and is also important during cleaning operations because it permits use of stronger, faster cleaning process.

Effect of Salt Concentration

Osmotic pressure is a function of the type and concentration of salts or organics contained in feedwater. As salt concentration increases, so does osmotic pressure. The amount of feedwater driving pressure necessary to reverse the natural direction of osmotic flow is, therefore, largely determined by the level of salts in the feedwater.
If feed pressure remains constant, higher salt concentration results in lower membrane water flux. The increasing osmotic pressure offsets the feedwater driving pressure. There is an increase in salt passage through the membrane (decrease in rejection) as the water flux declines.

Effect of Recovery

Reverse osmosis occurs when the natural osmotic flow between a dilute solution and a concentrated solution is reversed through application of feedwater pressure. If percentage recovery is increased (and feedwater pressure remains constant), the salts in the residual feed become more concentrated and the natural osmotic pressure will increase until it is as high as the applied feed pressure. This can negate the driving effect of feed pressure, slowing or halting the reverse osmosis process and causing permeate flux and salt rejection to decrease and even stop.
The maximum percent recovery possible in any RO system usually depends not on a limiting osmotic pressure, but on the concentration of salts present in the feedwater and their tendency to precipitate on the membrane surface as mineral scale. The most common sparingly soluble salts are calcium carbonate (limestone), calcium sulfate (gypsum), and silica. Chemical treatment of feedwater can be used to inhibit mineral scaling.

Effect of pH

The pH tolerance of various types of RO membranes can vary widely. Thin-film composite membranes are typically stable over a broader pH range than cellulose acetate (CA) membranes and, therefore, offer greater operation latitude. Membrane salt rejection performance depends on pH. Water flux may also be affected.