During the operation of a reverse osmosis (RO) system, pressure is one of the most critical operating parameters affecting membrane performance.
Reverse osmosis is essentially a "pressure-driven" separation technology. Only under sufficient pressure can water molecules pass through the RO membrane, while salts, organic matter, bacteria, and other impurities are rejected.
Therefore, changes in operating pressure directly affect:
RO membrane permeate flow, salt rejection rate, system operating stability, energy consumption, and the service life of membrane elements.
If pressure is not properly controlled, it can not only reduce system efficiency, but in severe cases may even cause permanent damage to membrane elements.
I. How Does Operating Pressure Affect Reverse Osmosis Membranes?
If the operating pressure of the RO membrane is below the manufacturer's recommended range, it will directly affect the normal operation of the membrane element.
First, insufficient pressure reduces the driving force for water molecules to penetrate the membrane layer, resulting in a significant decrease in system permeate flow.
Second, the salt rejection rate will also be affected to some extent.
As the water flow through the membrane layer slows down, salt ions have more time to diffuse across the membrane, ultimately causing higher permeate TDS and poorer water quality.
For industrial systems, long-term low-pressure operation may prevent the system from achieving the designed water production capacity, leading to unstable water quality, abnormal operation of downstream equipment, and a serious decline in production efficiency.
In theory, increasing operating pressure can improve permeate flow and enhance salt rejection performance.
This is because higher pressure provides a stronger driving force for water molecules to pass through the RO membrane.
However, it should be noted that:
Both the permeate flow and salt rejection rate of RO membranes have design limits.
Once the system is already operating near the membrane element's optimal condition, continuously increasing pressure will no longer significantly improve water production. Instead, it may introduce a series of risks.
The membrane may fail under excessive pressure, causing structural damage or membrane bag rupture, which can lead to reduced membrane flux. At the same time, high-pressure operation consumes significantly more electricity than low-pressure operation, greatly increasing overall system energy consumption.
Long-term high-pressure operation can also accelerate wear on high-pressure pumps, pipelines, and pressure vessels, thereby increasing maintenance costs.
II. Effect Of Feed Pressure On RO Membrane Salt Rejection
Feed pressure does not directly change the amount of salt passing through the membrane, but it indirectly affects the desalination performance of the system.
When feed pressure increases, the permeate flow of the RO system also increases, and water velocity becomes faster. As a result, the salt passing through the membrane becomes further diluted. Therefore, from a performance perspective, salt passage decreases and salt rejection improves.
However, this improvement is not unlimited.
When feed pressure exceeds a certain range, system recovery continuously increases, and concentration polarization on the membrane surface becomes more severe.
So-called concentration polarization refers to the continuous accumulation of salts on the membrane surface, forming a high-concentration region. This increases salt passage and aggravates membrane fouling, causing rapid decline in membrane performance.
Over time, this can lead to reduced salt rejection and a slowdown or eventual stagnation in permeate flow growth.