Reverse Osmosis (RO) Filtration Process
The process of osmosis was first described by a French Scientist in 1748, who noted that water spontaneously diffused through a pig bladder membrane into alcohol.
Diffusion is the movement of molecules from a region of higher concentration to a region of lower concentration. Osmosis is a special case of diffusion in which the molecules are water and the concentration gradient occurs across a semipermeable membrane. The semipermeable membrane allows the passage of water, but not ions (e.g., Na+, Ca2+, Cl–) or larger molecules (e.g., glucose, urea, bacteria). Diffusion and osmosis are thermodynamically favorable and will continue until equilibrium is reached. Osmosis can be slowed, stopped, or even reversed if sufficient pressure is applied to the membrane from the ‘concentrated’ side of the membrane.
The process of reverse osmosis (RO) occurs when the water is moved across the membrane against the concentration gradient, from an area of higher contaminant concentration to an area of lower contaminant concentration. To illustrate, imagine a semipermeable membrane with fresh water on one side and a concentrated aqueous solution on the other side. If normal osmosis takes place, the fresh water will cross the membrane to dilute the concentrated solution. In reverse osmosis, pressure is exerted on the side with the concentrated solution to force the water molecules across the membrane to the fresh water side.
Reverse Osmosis Membranes
RO membranes were developed by the University of California in the 1960’s originally to remove salt from sea water to make it potable. The membrane material developed at that time was Cellulose Triacetate, used mostly in the film industry for glossy photographs.
While the process worked, early reverse osmosis systems were very expensive to manufacture, very high pressures were needed to drive the membranes and the membranes did not last very long.
Modern advances in synthetic materials have generally solved these problems, allowing RO membranes to become highly efficient at rejecting contaminants, and making them tough enough to withstand multiple cleanings and other factors necessary for efficient operation.
The real breakthrough in RO membrane technology was when Thin Film Triacetate was developed, or better known as a TFC membrane.
The TFC membrane consists of a selective polyamide active layer formed by interfacial polymerization on top of a polysulfone support layer fabricated by phase separation onto a thin (40 μm) polyester nonwoven fabric. By careful selection of the polysulfone casting solution (i.e., polymer concentration and solvent composition) and tailoring the casting process, a support layer with a mix of finger-like and sponge-like morphologies was produced that give significantly enhanced membrane performance. This new material did not require the high pressures to operate, bringing down the manufacturing and operating costs significantly.
Reverse osmosis membranes continue to improve in both rejection capabilities and robustness. They are able to withstand harsher chemical cleaning, to extend membrane life before replacement is needed. Typical modern spiral wound RO membrane elements and assemblies are illustrated below.
Over the past 20 years, increased competition and better manufacturing efficiency have significantly reduced the price of RO membranes. Reverse osmosis membrane technology is fast becoming the water treatment method of choice throughout the world. With our ever growing population; demand to safe drinking water and the technologies to produce it will continue to increase proportionally.
Potable Water Applications
Since they were first developed in the 1960’s, reverse osmosis membranes have most commonly been used for purifying water and removing salts and other impurities in order to improve the color, taste or properties of the fluid to meet potable water quality standards. However, RO is finding increasing uses in industrial applications because of its effectiveness and cost-efficiency.
Reverse osmosis systems used in industrial and commercial applications, where large volumes of treated water are required at a high level of purity; typically operate at pressures between 100 and 1,000 psig, depending on the membranes chosen and the quality of the water being treated. Most commercial and industrial systems use multiple membranes in series. The processed water from the first stage of treatment can be passed through additional membrane modules to achieve greater levels of treatment for the finished water. The reject water also can be directed into successive membrane modules for greater efficiency, though flushing will still be required when concentrations reach a level where fouling is likely to occur.
Reverse osmosis systems can be used to treat boiler feed water, industrial wastewater, process water and more. A few of the major industrial uses are:
- Boiler Feed Water Treatment: RO is used to reduce the solids content of waters prior to feeding into boilers for the power generation and other industries.
- Pharmaceutical: Reverse osmosis is an approved treatment process for the production of United States Pharmacopeia (USP) grade water for pharmaceutical applications.
- Food & Beverage: Water used to process food products and to produce beverages is often treated by a reverse osmosis system.
- Semiconductor: Reverse osmosis is an accepted component of a treatment process to produce ultra-pure water in the semiconductor industry.
- Metal Finishing: RO systems have been successfully applied to a variety of metal finishing operations including several types of copper, nickel and zinc electroplating; nickel acetate seal; and black dye.
Cost benefits of RO
Reverse osmosis is increasingly being adopted in a wide array of water purification applications owing to the economic benefits of RO membrane filtration systems provide. For example, the prices of acid, caustic solutions and other chemicals used in conventional systems continue to rise, while the prices of RO units and membrane elements continue to decrease.
In terms of operating efficiency, the primary cost for operating reverse osmosis systems is electricity, and since these systems consume very little energy, operating costs are relatively low. Moreover, relative to other technologies RO systems do not require significant downtime, with the exception of quarterly or semi-annual routine maintenance. RO systems are highly automated, requiring minimal operator intervention and generally incorporate advanced remote monitoring and control features.
Given the many economic advantages of reverse osmosis systems and the ever-increasing global demand for clean water, we expect to see continued growth in the use of reverse osmosis water filter purification technologies worldwide.