Telescoping or Element Deformation

What it looks like

The membrane leaves inside the element shift relative to the center tube. One end of the element may cave in while the other bulges outward, creating a telescope-like appearance. This is more than a cosmetic defect: it can reduce salt rejection and, in severe cases, permanently damage the element.

What causes it

The primary cause is excessive differential pressure across the membrane element. The resulting axial force pushes the membrane leaves out of alignment.

How to prevent or address it

• Track operating data daily. If differential pressure rises approximately 15% above the clean or initial baseline, investigate fouling and assess the need for chemical cleaning.
• Stay within the membrane manufacturer’s limits. Never exceed the specified maximum differential pressure.
• Investigate the root cause. Minor telescoping may remain serviceable, but severe deformation requires replacement. Check for failed cartridge filters, heavy scaling, or another cause of the pressure-drop increase before installing new elements.

Concentration Polarization

What it is

As water permeates through the membrane, salts and other rejected substances accumulate near the membrane surface. This creates a concentrated boundary layer that resists water transport and increases salt passage, reducing both permeate production and rejection performance.

How to reduce it

• Maintain adequate concentrate-side crossflow. Sufficient surface velocity creates shear that limits accumulation at the membrane surface.
• Keep recovery within the design range. Excessive recovery increases concentrate salinity and accelerates concentration polarization and scaling risk.

Membrane Compaction

What it is

Long-term exposure to excessive pressure or temperature can compress the membrane structure. Permeate flow decreases, while salt rejection may temporarily improve slightly because the membrane has become denser.

Why it happens

• Feed pressure remains too high for an extended period.
• Feedwater temperature exceeds the membrane’s design limit.

Membrane Surface Scratching

What it is

Hard particles or deposits can damage the membrane’s functional surface layer. Salts then pass through the damaged areas, directly lowering rejection.

Common causes

• Hard particles: sand, activated-carbon fines, and rust can act like small cutting tools.
• Scale crystals: rough crystalline deposits can rub against and damage the surface.
• Water hammer: sudden startup or shutdown creates pressure waves and internal movement.
• Chemical attack: incompatible chemicals can weaken or erode the membrane surface.

How to protect the membrane

• Replace cartridge security filters on schedule and confirm that they are intact.
• When cleaning a scaled system, begin with a low flush flow and increase it gradually so loosened debris does not scour the membrane.
• Vent air at low pressure before starting the high-pressure pump.
• Use a variable-frequency drive or soft starter so system pressure rises gradually.
• Verify membrane compatibility before introducing any chemical.

Oxidation Damage

What it is

Chlorine and other strong oxidants can attack the functional layer of a polyamide RO membrane. A damaged element commonly shows higher permeate flow together with a sharp decline in salt rejection.

Why it happens

Free chlorine or other oxidants, such as ozone or permanganate, reach the membrane above its permitted exposure limit.

How to prevent it

• Use an appropriate reducing agent, such as sodium bisulfite, to neutralize chlorine upstream of the RO system.
• Keep free chlorine below 0.1 ppm as stated in this operating guide, or follow a stricter limit if required by the membrane manufacturer.
• Install an ORP monitor or free-chlorine analyzer with an alarm to track oxidant breakthrough continuously.

Scaling on the Membrane Surface

What it is

Calcium, magnesium, silica, and other sparingly soluble substances become concentrated inside the RO system. When their solubility limits are exceeded, they precipitate and form crystalline deposits on the membrane surface. Permeate flow and salt rejection decline, and scaling often appears first in the final stage at the concentrate end.

Why it happens

• Incomplete design data: insufficient feedwater analysis leads to an unsuitable recovery target or pretreatment design.
• Ineffective antiscalant control: the product is unsuitable, incompatible, or under-dosed.
• Changing feedwater quality: the water composition changes without corresponding operating adjustments.

How to prevent and address it

• Obtain a complete water analysis before designing the system.
• Select a proven, membrane-compatible antiscalant and control its dose carefully.
• Adjust recovery, chemical dosing, and other parameters when feedwater quality changes.
• For very hard feedwater, consider softening upstream to remove calcium and magnesium.

Prevention Checklist

Most RO membrane problems are easier and less expensive to prevent than to repair. Three operating habits provide the strongest protection.

• Use data as an early-warning system. Record normalized permeate flow, salt rejection, differential pressure, and temperature every day and investigate sustained drift.
• Treat pretreatment as the first line of defense. Reliable filtration, dechlorination, and scale control prevent most avoidable fouling and damage.
• Operate smoothly. Avoid sudden changes in pressure and flow, and use controlled startup and shutdown procedures.