What is the Effect of pressure difference across RO membranes?

Reverse osmosis (RO),a pressure-driven separation process has recently become the dominating technique for new desalination installations, and it is used to treat a wide range of salt water resources.The membrane pore size in the reverse osmosis process is in the region of 0.0005 microns. Reverse osmosis membranes are practically non-porous, allowing particles and even many low-molar-mass species such as salt ions, organics, and other molecules to pass through.

Effect of pressure difference across RO membranes

The water flux and salt rejection of RO membranes are both affected by feedwater pressure.

The movement of water across a membrane from the dilute to the concentrated solution side is known as osmosis. To overcome the natural osmotic pressure, reverse osmosis technology applies pressure to the feedwater stream. The concentrated solution is subjected to a pressure greater than the osmotic pressure, and the flow of water is reversed. A part of the feedwater (concentrated solution) is driven through the membrane, resulting in a purified dilute solution product water.

Increase in feedwater pressure enhances water flux across the membrane in a direct proportion. Increased feedwater pressure causes increased salt rejection, however the relationship is less straight than it is for increased feedwater pressure. There is always some salt flow through RO membranes since they are imperfect barriers to dissolved salts in feedwater. This salt route becomes increasingly obstructed as feedwater pressure rises, as water is pushed through the membrane at a faster pace than salt can be delivered.

However, the quantity of salt that can be excluded by increasing feedwater pressure has a limit. Salt rejection no longer grows above a particular pressure level and some salt flow persists linked with water flowing through the membrane.

Possible reasons for increasing the pressure difference across RO membranes

If other parameters like as feed water TDS, feed water temperature, and percent recovery remain constant, more permeate water is produced, and the percent rejection of the membrane increases. However, it should be used in accordance with the manufacturer's design requirements; otherwise, operating at a high pressure will foul or scale the membrane prematurely, lowering membrane life.Fouling of the feed route and a limitation of flow over the membrane surface are indicators of increasing pressure loss. Monitoring pressure reductions between stages allows you to determine if fouling is limited to a specific stage, which can aid in identifying the probable foulant.

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