Parts & Components
Membranes

The process of reverse osmosis (RO) represents the finest level of liquid filtration available today. While ordinary liquid filters use a screen to separate particles from water streams, an RO system employs a semi permeable membrane that separates an extremely high percentage of unwanted molecules.

For example, the membrane may be permeable to water molecules, but not to molecules of dissolved salt. If this membrane is placed between two compartments in a container as shown in Figure 1, and a salt solution is placed in one half of the container and pure water in the other, water passes through the membrane while the salt cannot.

The, dissimilar liquid systems will try to reach the same concentration of materials on both sides of the membrane. The only way for this to happen is for pure water to pass through the membrane to the salt water side in an attempt to dilute the salt solution. This attempt to reach equilibrium is called osmosis.

But the goal is to remove the salt from water, it is necessary to reverse the natural osmotic flow by forcing the salt water through the membrane in the reverse direction. This can be accomplished by applying pressure to the salt water as it’s fed into the system, creating a condition known as “reverse osmosis.” See Figure 1.

The RO process cannot go on indefinitely unless steps are taken to ensure that the membrane doesn’t become clogged by precipitated salts and other impurities forced against it by the pressurized stream of feed water. To significantly reduce the rate of membrane fouling, RO systems employ cross-flow filtration (shown in Figure 2), which allows water to pass through the membrane while the separate flow of concentrate sweeps rejected salts away from the membrane surface.

Figure 1 Figure 2

Element Construction

Membranes are thin film composite membranes packed in a spiral wound configuration. Spiral wound designs offer many advantages compared to other module designs, such as tubular, plate and frame and hollow fiber module design for most of the reverse osmosis applications in water treatment.
Typically, a spiral wound configuration offers significantly lower replacement costs, simpler plumbing systems, easier maintenance and greater design freedom than other configurations, making it the industry standard for reverse osmosis membranes in water treatment.

Membrane Performance

Thin film composite membranes give excellent performance for a wide variety of applications, including low-pressure tap water use, seawater desalination, brackish water purification, chemical processing and waste treatment. This membrane exhibits excellent performance in terms of flux, salt and organics rejection, and microbiological resistance.
Elements can operate over a pH range of 2 to 11, are resistant to compaction and are suitable for temperatures up to 45°C. They can be effectively cleaned at pH 1 and pH 12. Their performance remains stable over several years, even under harsh operating conditions.

The membrane shows some resistance to short-term attack by chlorine. The free chlorine tolerance of the membrane is < 0.1 ppm. Continuous exposure, however, may damage the membrane and should be avoided. Under certain conditions, the presence of free chlorine and other oxidizing agents will cause premature membrane failure.