Wednesday 25 Apr 2018

:: Reverse Osmosis

Reverse osmosis is relatively a new technology compared to distillation and dialysis. It was commercially introduced in the 1970’s.
The process consists in deionizing  water (separating water from a saline solution) by pushing it under pressure through a reverse osmosis membrane. There is no need for heating or any transformation process. The energy required for desalination is used to pressurize feed water.
Reverse Osmosis works by using a high pressure pump to pump feed water in a closed container, increasing the pressure on the membrane. When a part of this water passes through the membrane, the percentage of salt in the remaining water increases. At the same time, some feed water is rejected without passing through the membrane.
Without this rejection, the recurrent increase of salinity in the feed water will cause a number of problems, such as an increase in salinity and in deposits as well as an increase of osmosis pressure on the membranes. Using this method, the amount of rejected water ranges between 20 to 70% of feed water based on the quantity of salt in the feed water.
The reverse osmosis process consists of the following main components:
• Pre-Treatment
• High pressure pumps
• Membrane assembly
• Final treatment
Pre-treatment is important because feed water has to pass through narrow ducts during the operation. Therefore, we need to remove any obstacles and prevent growth and deposits of contaminants, such as organics, microbes and viruses, on the membranes.
Initial chemical treatment includes filtering and adding an acid or other chemicals to prevent deposits
The high pressure pump provides the necessary pressure to allow water to pass through the membranes and to retain salt. This pressure ranges between 17 to 27 bar ( 391.5-246.5 psi) for water in wells, and 54 to 80 bar (783-1160 psi) for sea water.
The membrane assembly consists of a pressure vessel with a membrane that allows feed water to be pressed against it. This membrane can withstand the difference in pressure applied against it.
Reverse osmosis membranes are made in a variety of configurations, with the two most common configurations being spiral-wound and hollow-fiber.
These two types are used in the desalination of seawater and water wells in spite of the difference in the constituents of the membranes and the pressure container, depending on the manufacturer and on the level of salinity in the water to be desalinated.
As for the post-treatment, it is related to conserving water characteristics and preparing water for distribution after filtration. This treatment also helps remove Hydrogen sulfide and adjust the alkaline level
Two developments have contributed to the reduction of the operational cost of reverse osmosis plants during the last two decades: the improvement of the membrane which can be operated effectively at low pressures, and the use of energy recovery devices. Low pressure membranes are used extensively in desalinating water wells.
Energy recovery devices are linked to the concentrate flow when it leaves the pressure container. During the flow, water loses from 1 to 4 bars of the pressure coming from the high pressure pump.
Energy recovery devices are mechanical and generally consist of turbines or pumps that can transform the difference in pressure into energy