RO Membrane Scaling: Causes, Warning Signs and Prevention

Reverse osmosis (RO) is widely used to reduce dissolved substances in water. However, RO membranes require suitable operating conditions and appropriate pretreatment to maintain performance. One common challenge is membrane scaling.

Scaling occurs when dissolved substances become concentrated near the membrane surface and precipitate or form deposits. If scaling is not controlled, it can affect membrane performance and increase maintenance requirements.

What Is RO Membrane Scaling?

During reverse osmosis, water passes through the membrane while many dissolved substances are retained. As water passes through the system, the concentration of certain dissolved substances increases in the concentrate stream and near the membrane surface.

If the concentration of a sparingly soluble substance exceeds its solubility limit, deposits can form. These deposits are commonly referred to as scale. The exact type of scale depends on the water chemistry and operating conditions.

What Causes Scaling?

Several factors can contribute to scaling.

1. High Hardness

Calcium and magnesium are common components of water hardness. Under appropriate chemical and operating conditions, certain calcium-containing compounds can form deposits.

2. High Dissolved Solids

Higher concentrations of dissolved substances can increase the potential for precipitation as water recovery increases.

3. Excessive Recovery

RO recovery represents the proportion of feed water converted into permeate. Increasing recovery can increase the concentration of dissolved substances in the concentrate. Higher recovery is not automatically a problem, but it needs to be compatible with the feed-water chemistry and system design.

4. Inadequate Pretreatment

If pretreatment does not sufficiently control the factors contributing to scaling, the RO membrane may be exposed to a greater scaling risk.

5. Changes in Water Chemistry

Feed-water conditions can change over time. Seasonal changes, source-water changes or changes in upstream processes can alter the chemistry entering the RO system. This is one reason periodic monitoring can be important.

What Are the Warning Signs?

A change in RO performance does not automatically prove that scaling is the cause. However, certain changes may justify further investigation. These can include:

• Reduced permeate flow
• Increased pressure requirements
• Changes in salt rejection
• Increased differential pressure
• Increasing frequency of cleaning
• Changes in normal operating conditions

The correct diagnosis should consider operating data and water chemistry rather than assuming that every performance problem is caused by scale.

How Can Scaling Risk Be Reduced?

Analyse the Feed Water

Water analysis provides information needed to understand the chemical conditions entering the RO system. Important parameters depend on the water source and system.

Review System Recovery

Recovery should be evaluated against the water chemistry and membrane-system design. Higher recovery is not necessarily better if it creates unacceptable concentration conditions.

Use Appropriate Pretreatment

Pretreatment can help control substances that contribute to membrane fouling or scaling. The correct configuration depends on the feed water.

Consider Antiscalant Treatment

Antiscalants are chemicals designed to reduce the tendency of certain scale-forming substances to precipitate under appropriate conditions. However, antiscalant selection and dosing should be based on the specific water chemistry and system conditions. More chemical does not automatically mean better performance.

Monitor System Performance

Maintaining baseline operating data can make it easier to identify changes over time. Useful information may include:

• Feed pressure
• Concentrate pressure
• Permeate flow
• Feed conductivity
• Permeate conductivity
• Recovery
• Temperature
• Differential pressure

Comparing current performance with established baseline conditions can help identify trends.

What Happens If Scaling Is Ignored?

Persistent scaling can affect membrane performance and may increase cleaning requirements. In severe cases, deposits may become more difficult to remove. The appropriate response depends on the type and severity of the deposit.

Cleaning should therefore be based on the membrane manufacturer's recommendations and the characteristics of the deposit rather than using a generic chemical programme.

Scaling Isn't the Same as Fouling

These terms are sometimes used interchangeably, but they are not identical.

Scaling generally refers to precipitation and deposition of sparingly soluble inorganic substances.

Fouling is a broader term that can include deposits caused by organic matter, biological material, colloids and other substances.

Correctly identifying the problem matters because the appropriate treatment approach can differ.

Final Thoughts

RO membrane scaling is generally easier to manage when it is considered during system design and monitored during operation. Water analysis, appropriate pretreatment, sensible recovery, suitable chemical treatment and operating-data monitoring can all contribute to better control.

When an RO system begins showing performance changes, the first step should be diagnosis rather than immediately increasing chemical dosing.

Wolgan works with water treatment systems and chemical treatment programmes according to application and project requirements.