RO Back Pressure Issues In Pure Water And Purified Water Equipment

In the field of reverse osmosis water treatment, when the pressure on the permeate side of the RO membrane becomes higher than the pressure on the feed side, the RO membrane is subjected to reverse pressure, which is known as back pressure.

An RO membrane is similar to a long envelope-shaped membrane pouch, with the open side bonded to the permeate center tube that contains openings. Multiple membrane pouches are wound around the same permeate center tube, allowing feed water to flow across the outside surface of the membrane. Under feed pressure, purified water passes through the membrane into the membrane pouch and then flows into the permeate center tube.

To facilitate permeate flow inside the membrane pouch, a permeate spacer fabric layer is inserted inside the envelope-shaped membrane pouch. In order to ensure uniform feed water flow across the membrane surface and create turbulence, a feed spacer mesh is placed between membrane pouches in the feed channel.

The three sides of the membrane pouch are bonded together with adhesive. If the pressure on the permeate side becomes greater than the pressure on the feed side, these bonded sealing lines may rupture, causing the membrane element to lose its selective permeability. As a result, the salt rejection rate will significantly decrease, affecting water quality.

Therefore, from a safety perspective, reverse osmosis systems should avoid back pressure conditions.

Since reverse osmosis filtration is pressure-driven, back pressure does not normally occur during standard operation. However, during normal shutdowns or emergency shutdowns, improper valve settings or incorrect valve operation may lead to back pressure.

RO membrane technical manuals generally specify a maximum allowable back pressure limit. For example, if the maximum back pressure an RO membrane can withstand is 0.1 MPa (approximately 1 bar), this is equivalent to the pressure generated by a 10-meter water column.

When the RO system is shut down, the water remaining in the permeate pipeline creates gravitational potential pressure on the permeate side of the RO membrane. The higher the storage tank elevation, the greater the pressure becomes. If the permeate pipeline rises more than 10 meters, the resulting back pressure may be sufficient to damage the membrane.

Therefore, during RO system design, engineers should refer to the back pressure limit specified in the membrane technical manual to determine the maximum allowable elevation of the permeate pipeline. In addition, check valves should be installed on the permeate pipeline to prevent water in the pipeline from generating reverse pressure.

In some cases, the check valve may not seal completely, or the permeate pipeline may be required to rise to a greater height. Under such circumstances, installing a drain or vent pipe downstream of the check valve can help prevent back pressure.