RO (Reverse Osmosis) & SWRO (Sea Water Reverse Osmosis)
What is Reverse Osmosis (RO)?
RO is a filtration technique which uses the particular properties of RO membranes to prevent the passage of a wide range of impurities to a high degree, greatly purifying the feed-water.
What does Reverse Osmosis remove from water?
RO membranes prevent the passage of contaminants in water that are more than about 0.1nm in effective diameter. Typically they remove well over 90% of ionic contamination, most organic compounds and effectively all particulates. RO removal of non-ionic contaminants with molecular weights (MWs) of <100 Dalton can be low. It increases at higher MWs and, in theory, molecules with MWs >300 Dalton, including particles, colloids, microorganisms and large biologically-active molecules, will be rejected completely. Dissolved gases are not removed.
The great advantage of RO is the very wide range of impurities that it removes. As well as reducing the load on subsequent purification technologies it also protects them from contamination and fouling by large organic molecules, for example, humic and fulvic acids, and completely rejects micro-organisms and viruses present in feed water.
Reverse osmosis is the process of forcing a solvent from a region of high solute concentration through a membrane to a region of low solute concentration by applying a pressure in excess of the osmotic pressure. This is the reverse of the normal osmosis process, which is the natural movement of solvent from an area of low solute concentration, through a membrane, to an area of high solute concentration when no external pressure is applied. The membrane here is semipermeable, meaning it allows the passage of solvent but not of solute.
To illustrate, imagine a semi permeable 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.
The membranes used for reverse osmosis systems have a dense polymer barrier layer in which separation takes place. Since Reverse Osmosis does not occur naturally, it must be created by applying pressure to the high solids water in order to force it through the membrane, with pressures from 8 - 14 bar for fresh and brackish water.
SWRO (Sea Water Reverse Osmosis)
According to the UN, water scarcity is one of the major challenges facing the world today and tomorrow. The need to find sustainable solutions for new freshwater supplies is urgent; solutions that take the straight road from saline to fresh water at low cost and with minimum impact on climate and nature.
Due to its high energy efficiency and reliability, Sea Water Reverse Osmosis (SWRO) is one of the most sustainable and cost effective solutions for converting sea water to potable fresh water. At the same time, SWRO technology undergoes rapid development to improve energy efficiency and support large-scale freshwater production.
Desalination through membrane processes represents one of the unique successes in water treatment technology and is probably one of the distinct examples today of the success of process intensification in the water industry.
Seawater and saline aquifers account for 97.5% of all water on the Earth. Hence capturing even a tiny fraction could have a huge impact on water scarcity. Until recently, seawater desalination was only limited to desert areas like Middle East but now it has expanded its use in coastal areas owing to all technological developments and decrease in cost Two basic types of technologies have been widely used to separate salts from ocean water: thermal evaporation and membrane separation . Over the past ten years, seawater desalination using semi-permeable sea water reverse osmosis (SWRO) membranes has gained momentum and currently dominates desalination markets. Membrane treatment process is more efficient, requires less physical space and is less energy consuming than vaporization or distillation.
Seawater desalination to convert seawater into potable water is being used in many parts of the world. Reverse Osmosis process using thin-film composite membranes has evolved over the last 20 years and has brought down the cost of desalination. Major improvements in the membranes, energy recovery, pumps and pressure vessels have brought down the cost of desalinated water significantly.
The key technology in the desalination process is Reverse Osmosis. In this process sea water is forced against semi-permeable membranes under pressure in a continuous flow condition. The high salt content of sea water requires that the operating pressure for Reverse Osmosis must be between 60-70 bar. As the water permeates through the membrane most of the dissolved impurities removed and 99.5% of the total salt is removed. The impurities are left behind in the flowing water and the concentrated stream from the membranes is discharged to the ocean. The design of the complete system must optimize the flows, the area of the membranes and other conditions to keep the system operation at the highest efficiency possible.