Can You Replace Only the Poor-Performing RO Membrane

Introduction

In industrial RO system operation, membrane performance degradation is inevitable over time.
A common question from operators and plant managers is:

Can we replace only the poor-performing membrane instead of the entire membrane set?

From a cost perspective, partial replacement appears attractive.
However, RO systems are hydraulically and chemically balanced systems, and replacing a single membrane element can introduce unexpected risks.

This article explains when partial membrane replacement is possible, what risks it brings, and how to manage those risks properly.

1. Is It Technically Possible to Replace Only One RO Membrane?

Short Answer: Yes, But With Conditions

From a mechanical standpoint:

•RO membrane elements are modular

•Individual elements can be removed and replaced

However, from a system operation perspective, replacing only one membrane changes the internal balance of the pressure vessel and the entire array.

So the real question is not “Can you?”
It is “Should you?”

2. Why Partial Membrane Replacement Is Risky

2.1 Performance Imbalance Within the Pressure Vessel

A new membrane typically has:

•Higher permeability

•Higher salt rejection

•Lower fouling resistance initially

When installed alongside aged membranes:

•More flow passes through the new membrane

•Older membranes carry higher salt load

•Local recovery becomes uneven

This accelerates fouling and scaling on the older elements.

2.2 Increased Risk of Local Over-Recovery

Partial replacement often causes:

•Uneven flux distribution

•Excessive local recovery on downstream membranes

Consequences include:

•Scaling

•Concentration polarization

•Reduced membrane lifespan

This effect is especially severe in:

•High-recovery systems

•Seawater RO

•Systems with marginal pretreatment

2.3 Misleading Performance Data

After partial replacement:

•Overall system performance may appear improved

•Conductivity and flow readings become less representative

This can:

•Mask deterioration of remaining old membranes

•Delay necessary maintenance decisions

2.4 Accelerated Degradation of Old Membranes

The “new + old” combination often leads to:

•Higher differential pressure increase

•Faster fouling on old membranes

•Earlier need for another replacement

In many cases, the cost savings disappear within months.

3. When Is Partial Membrane Replacement Acceptable?

Partial replacement may be acceptable if all of the following conditions are met:

•The system has low overall recovery

•Only one element is physically damaged (e.g., oxidant attack, glue line failure)

•Remaining membranes are:

Similar age

Similar performance

Similar fouling history

•The replaced membrane is:

Same brand and model

Same salt rejection class

Same nominal flow

Even in these cases, performance monitoring must be intensified.

4. Best Practices If Partial Replacement Is Unavoidable

If full replacement is not feasible, follow these risk-reduction strategies:

4.1 Install the New Membrane in the Lead Position

•Reduces fouling stress on older membranes

•Minimizes local over-recovery downstream

4.2 Avoid Mixing Different Membrane Types

Never mix:

•Different manufacturers

•Different rejection rates

•Different membrane generations

4.3 Reduce Operating Recovery Temporarily

•Lower recovery by 5–10%

•Reduce concentration stress

•Allow system to re-stabilize

4.4 Closely Monitor Key Parameters

Track daily:

•Differential pressure

•Individual vessel flow (if available)

•Salt rejection trends

•Cleaning frequency changes

5. When Full Replacement Is the Better Choice

Full membrane replacement is strongly recommended when:

•Membranes are near end of life

•Fouling or scaling is widespread

•System recovery is high

•Water quality requirements are critical (electronics, pharma, boiler feed)

In these cases, partial replacement often results in:

•Shortened membrane life

•Higher operating cost

•Increased risk of unplanned shutdowns

Conclusion

While replacing only a poor-performing RO membrane is technically possible, it is rarely the optimal long-term solution.

The risks include:

•Hydraulic imbalance

•Accelerated fouling

•Misleading system data

•Reduced overall membrane lifespan

For most industrial RO systems, strategic full replacement or staged vessel-level replacement provides better reliability and lower lifecycle cost.