x Abu Dhabi, UAEThursday 18 January 2018

Reverse osmosis gains ground on 'flash' process

There are two primary methods to remove salt from water on a large scale: flash distillation and reverse osmosis.

There are two primary methods to remove salt from water on a large scale: flash distillation and reverse osmosis. The multi-stage flash distillation process, sometimes shortened to "flash", was the first to gain widespread acceptance. It was initially used in the 1950s, but it wasn't until the 1980s that the process became commercialised on the scale needed in the Middle East. Most GCC countries, including the UAE, Saudi Arabia and Kuwait, are highly dependent on flash desalination facilities to supply water to their urban areas.

In the flash process, seawater is heated to between 90°C and 110°C by condensing steam onto a bank of metal tubes carrying the water. The heated brine next flows into a vessel inside which the air pressure has been reduced. The transition to lower pressure immediately causes the water to boil, almost exploding or "flashing" into steam. The steam is converted to fresh water by condensing it on the tubes of heat exchangers running through the vessel, and the water is collected for use.

The remaining brine, once it has stopped boiling, is transferred to another vessel, in which the ambient pressure is even lower, and the process is repeated. There can be many such distillation stages, each yielding more steam to be condensed into fresh water. The heat exchangers at each stage warm the seawater flowing into the plant, reducing the amount of fuel needed to heat the brine fed into the first distillation vessel.

The second process, reverse osmosis, was commercialised in the 1970s, but initially had higher costs than flash distillation. With engineering improvements, it started to become cost competitive in the 1990s. Reverse osmosis uses a membrane to separate water from dissolved salts. No heating is required, but energy is needed to power a pump used to pressurise the seawater fed into the plant. As the salt water squeezes against the membrane, some water molecules are pushed through minute pores, with a diameter roughly 100,000 times smaller than a human hair. This creates a stream of fresh water on the far side of the membrane. The salt content of the residual brine increases, so some of the super-salty brine is continuously removed and replaced with incoming seawater.

The process is simple in principle, but the devil is in the details: no membrane is so perfect that it rejects the passage of all salts. Membranes also tend to wear out quickly. Still, significant improvements have been made to the process in the past 10 years, such as creating more efficient membranes, extending the life of membranes, and increasing the energy efficiency of pumps. As a result, state-of-the-art reverse osmosis is beginning to offer significant savings over flash distillation, increasing commercial interest in the technology for seawater desalination and wastewater treatment.