During the operation of a reverse osmosis (RO) system, improper operating conditions may cause damage to RO membrane elements. Some types of damage can be restored through chemical cleaning, while others are permanent and cannot be repaired. Once permanent damage occurs, the only solution is to replace the damaged RO membrane elements.
Generally, these types of damage can be classified into two categories: physical damage and chemical damage.
1. What Is Physical Damage?
Physical damage refers to the destruction of the membrane's desalination layer caused by mechanical or physical forces. Once it occurs, it is usually irreversible, and the damaged membrane element must be replaced.
Common types of physical damage include the following:
1. Scratches Caused by Solid Particles
1.1 Particle Damage Caused by Cartridge Filter Failure
When the cartridge filter (security filter) is not properly sealed, or when the filter element operates for a long time under high differential pressure and becomes damaged, solid particles may pass through the filter and enter the RO system.
After being pressurized by the high-pressure pump, these particles may strike the membrane surface at high velocity. This impact can scratch the desalination layer on the surface of the RO membrane element, resulting in a significant decline in salt rejection performance. In severe cases, the membrane element may become completely unusable.
Solution:
Regularly inspect the sealing condition of the cartridge filter elements and avoid operating them for long periods under excessive differential pressure.
1.2 Particle Scratches During Chemical Cleaning
During the chemical cleaning process of an RO system, if the cleaning flow rate is too high, dissolved or detached solid particles and scale deposits may circulate within the system and scratch the membrane surface.
Solution:
At the initial stage of chemical cleaning, the system should be operated at a low circulation flow rate. After the contaminants gradually dissolve, the flow rate can then be increased step by step to improve the cleaning efficiency while minimizing the risk of membrane surface damage.
2. Water Hammer
2.1 What Is Water Hammer?
Water hammer is a phenomenon caused by sudden changes in fluid pressure or pressure fluctuations within a pipeline. When water flows through a long pipeline and a downstream valve is suddenly closed, the flowing water continues moving forward due to inertia. This results in a rapid increase in pressure inside the pipe, creating a shock that impacts pipelines and related equipment.
The intensity of water hammer is related to the flow rate in the pipeline and the head difference (pressure difference between the two ends of the pipeline). The greater the flow rate and pressure difference, the stronger the impact pressure. In severe cases, this may lead to equipment damage. For this reason, systems are usually equipped with pressure relief devices or buffering systems to reduce the effects of water hammer.
Water hammer is not limited to water systems. Similar phenomena may occur in any fluid flow, including liquids, gases, and gas–liquid mixtures, when pressure changes rapidly within a pipeline.
In RO systems, water hammer may also occur if the high-pressure pump starts or stops too quickly. The head of an RO high-pressure pump is typically 1 MPa or higher. If the pump is not equipped with a variable-frequency drive (VFD) or soft-start system, sudden start-ups or shutdowns can cause rapid pressure changes. These pressure shocks may impact the RO membrane elements and sealing components, potentially damaging the membranes and causing a significant decline in salt rejection performance.
Solution:
When opening or closing valves, avoid rapid valve operation. The flow velocity in the pipeline should not change abruptly in order to minimize the risk of water hammer.
3. Membrane Telescoping
3.1 Formation of the Telescoping Effect
Membrane telescoping refers to a structural deformation of a reverse osmosis membrane element caused by excessive pressure difference between the feed side and the concentrate side. When the differential pressure exceeds the design limit of the membrane element, sliding may occur between membrane sheets or between the membrane sheets and the central permeate tube. This leads to axial displacement of the membrane layers inside the element.
When an RO membrane operates for a long period under inter-stage pressure differences exceeding 0.35 MPa, the membrane element experiences strong pressure along the flow direction (from feed side to concentrate side). As a result, one end of the membrane element may compress inward while the other end protrudes outward.
The overall appearance resembles an extended telescope, with one end concave and the other convex, as shown in the figure below.
Under normal conditions, the ends of a standard 8040 RO membrane element remain flat and structurally stable, as shown in the figure below.
The figure below shows a YIME ultra-low pressure smembrane element in the 8040 size. As shown, both ends of the element are flat with no protrusions, which indicates a membrane element in normal condition. This image shows the side view of a properly manufactured membrane product.
3.2 Pressure Difference During System Start-up and Shutdown
During the start-up of an RO system, if the concentrate discharge valve is opened while the high-pressure pump is already running, the pressure on the concentrate side may drop close to zero while the feed side still maintains relatively high pressure. This situation can create a large instantaneous pressure differential across the membrane element.
Similarly, before system shutdown, if the concentrate discharge valve is opened in advance while the high-pressure pump is still operating, a similar pressure shock may occur. Long-term operation under such conditions can easily lead to membrane telescoping.
Solution:
Follow the standard operating procedures when starting or shutting down the RO system, and increase the feed pressure gradually to minimize the impact of sudden pressure differentials on the membrane elements.
4. Back Pressure
Back pressure refers to the reverse pressure generated at the outlet or downstream section of a system. It usually describes a pressure acting opposite to the direction of fluid flow in a closed pipeline due to obstacles or structural changes in the piping system. It can also refer to a pressure condition at the system outlet that is higher than the local atmospheric pressure.
4.1 Back Pressure Caused by Cross-Flow Between Systems
When two or more RO systems share the same permeate header or concentrate header, cross-flow may occur if a system is not equipped with a check valve, or if the check valve does not seal properly.
If cross-flow occurs in the permeate pipeline, the RO unit that is not operating may experience back pressure on the permeate side. In this situation, the pressure on the permeate side may become higher than that on the concentrate side. Long-term operation under such conditions may cause delamination of the membrane's desalination layer.
If cross-flow occurs in the concentrate pipeline, the RO unit that is not operating may remain in a pressurized condition, which may also negatively affect the membrane elements.
Solution:
Install reliable check valves on permeate and concentrate pipelines to prevent reverse flow between systems. Regularly inspect the sealing condition of the check valves to ensure proper operation.
4.2 Forward Osmosis
In systems with high feed water salinity, such as landfill leachate treatment systems, brine reuse systems, or wastewater reclamation systems, if the RO unit is shut down without performing a low-pressure flushing, the high-salinity water on the concentrate side may not be fully displaced.
Under such conditions, not only can organic matter and inorganic salts deposit on the membrane surface, but forward osmosis may also occur.
After shutdown, because the salinity on the permeate side is relatively low, permeate water may move back toward the high-salinity concentrate side due to osmotic pressure. This flow direction is opposite to the normal permeate production direction of an RO system. Long-term forward osmosis may damage the structure of the membrane desalination layer and may even lead to delamination.
Solution:
After shutting down the RO system, perform a low-pressure flushing with clean water or pretreated feed water to replace the high-salinity water on the concentrate side. This helps prevent membrane fouling and reduces the risk of forward osmosis.
5. Membrane Drying and Cracking
5.1 Siphon Effect
If the concentrate pipeline or permeate pipeline is not equipped with anti-siphon protection, a siphon effect may occur during system drainage. This phenomenon can partially or completely drain the water inside the RO membrane system.
When membrane elements remain in a water-depleted condition for an extended period, the membrane surface may dry out and crack, resulting in permanent damage to the desalination layer.
Solution:
Install anti-siphon devices or air-break protection in the permeate and concentrate pipelines to prevent siphoning. In addition, avoid completely draining the membrane elements during routine system shutdown whenever possible.
5.2 Human Error or Control System Failure
Membrane drying may also occur due to operator error or control system malfunction. For example, if the concentrate discharge valve and permeate discharge valve are opened but not closed in time, the membrane elements may remain without water for a prolonged period, which can lead to drying and cracking.
It is worth noting that some RO membrane elements are supplied in a dry condition from the factory, and in this case drying damage will not occur before initial operation. However, after the membrane has been hydrated and operated for the first time, prolonged dehydration can still cause cracking and structural damage.
In practical RO system operation, many membrane failures are not caused by the membrane product itself, but rather by improper system design or incorrect operating procedures.
By properly controlling system pressure differentials, following standard start-up and shutdown procedures, improving pretreatment performance, and regularly inspecting critical equipment, it is possible to significantly reduce physical damage to RO membranes and extend the service life of membrane elements.
Considering these practical operational challenges, YIME integrates intelligent control systems when designing RO system solutions to reduce the risk of operational errors. In addition, when customers purchase YIME RO membrane products, our team also provides professional technical guidance to help ensure proper installation and operation.
