What is the inverse osmosis?

Reverse osmosis

What is osmosis?

Osmosis is the process by which a solvent passes through a semi-permeable membrane, from a dilute solution to a concentrated one, until the difference in concentrations on both sides of the membrane is equal. The pressure required for this phenomenon to occur is known as osmotic pressure.

What is the inverse osmosis?

Reverse osmosis (RO) is a process in which the flow rate through a semipermeable membrane is reduced and a pushing force greater than the osmotic pressure is exerted in the opposite direction to the osmosis process (Figure 1). In this way it is possible to separate the substances found in the water on one side of the membrane (concentrate) and on the other side a dilute solution low in dissolved solids (permeate) is obtained.

Osmosis and reverse osmosis process diagram

Figure 1. Osmosis and reverse osmosis process diagram.

Reverse osmosis is used to desalinate seawater and brackish water, soften waters, remove organic matter, and separate specific contaminants from the water.

How does a reverse osmosis plant work?

Reverse osmosis plants require pretreatment systems, feed pumping equipment, pressurized tanks (membrane holders or housings) containing membranes, chemical dosing equipment, etc. so that they work properly.

  • Membrane: This element is manufactured by winding membranes in a spiral shape, they are usually 40 or 60 inches long, and the most common diameters are 4 or 8 inches. During operation, the water enters under pressure on one side of the housing, as it flows tangentially to the membrane, part of it passes through the membrane surface towards the permeate collector, while water with a high concentration of salts come out at the other end of the membrane. In the Figure 2 you can see the elements of a membrane.

Typical elements of a RO membrane (John C. Crittenden et al.)

Figure 2. Typical elements of a RO membrane (John C. Crittenden et al.)

What are the types of reverse osmosis membranes?

Reverse osmosis membranes are the most common types for purification of drinking water, brackish water and sea water.

Brackish water membranes

Brackish water is mixed fresh and salt water. Therefore it is saltier than fresh water, but not as salty as sea water. These waters are very common in the aquiferous beds near the sea, or it can be due to brackish fossils.

Brackish Water Total Dissolved Solids (ppm): Brackish: 1,000 - 2,000 parts per million

Fresh water: <500
Slightly brackish: 500 - 1,000
Moderately saline: 2,000 - 5,000
Salt Water: 5,000 - 10,000
Sea Water: 10,000 - 35,000
Brine:> 35,000

Reverse Osmosis Membranes for Brackish Water

Brackish water membranes
Sea water membranes
High rejection membranes
Low energy membranes
Low fouling membranes
2.5 "
2.5 "
2.5 "
2.5 "
2.5 "
4"
4"
4"
4"
4"
8 "
8 "
8 "
8 "
8 "

* Consult for other needs

These elements can be installed in different arrangements, the concentrate from one membrane can be fed to another to increase water recovery. Another arrangement is to feed one membrane with the permeate of another, with the aim of further reducing the concentration of dissolved solids in the water.

  • Pre-treatment: Pre-treatment to reverse osmosis systems is important to extend the life of the membranes and obtain a better performance in reducing dissolved solids.

One of the purposes of pretreatment is to prevent scaling. This phenomenon generally occurs when salts of low solubility, such as calcium and magnesium, deposit and become embedded in the pores of the membranes. Scale control consists of adjusting the pH (modifying the solubility of these salts) or adding antifouling (preventing the formation of crystals or slowing their growth).

Other contaminants that can affect RO membranes are suspended solids, these can plug the feed or saturate the membrane surface. A pretreatment process for this problem is filtration. It is recommended to use filters that retain all particles greater than 5 microns. Generally, 5 micron or nominal 1 micron absolute cartridge filters are used.

Disinfection is another typical pretreatment step used to prevent biological saturation of the membrane. It is extremely important to verify that the membrane material and the disinfecting agent are compatible, as many of these can permanently damage the osmosis membrane.

Filtering classification by particle size


Bibliography

MWH’s water treatment: principles and design. John C. Crittenden et al.

Last update 03/11/2020.