When reverse osmosis (RO) system performance changes, identifying the root cause is not always straightforward. Multiple issues—such as fouling, scaling, or mechanical problems—can produce similar symptoms, and focusing on a single variable often leads to incorrect conclusions.

The FilmTec™ troubleshooting grid provides a structured way to diagnose RO system performance by linking symptoms to likely causes and corrective actions.

The key is knowing how to use it correctly. We’ll walk you through it below.

➡️ Download the FilmTec™ Troubleshooting Grid (PDF)
(Use this guide alongside the steps below to match symptoms to causes and corrective actions.)

RO troubleshooting begins by identifying which performance indicator has changed—permeate flow, salt passage, or pressure drop—and matching that pattern to known causes and corrective actions.

This approach allows you to narrow potential issues quickly and determine whether cleaning, operational adjustments, or replacement is required.

The three key performance signals

All RO system troubleshooting starts with three primary indicators:

• Permeate flow
• Salt passage (rejection)
• Differential pressure

These metrics should always be evaluated together.

A single change may suggest one issue, but combinations of changes often provide a clearer diagnosis.

How to read the troubleshooting grid

The grid is designed to move from observable symptoms to likely causes and then to corrective actions.

Use it in the following sequence:

1. Identify what changed

Start by determining how each key metric has shifted:

• Has permeate flow increased, decreased, or stayed the same?
• Has salt passage increased?
• Has differential pressure increased?

This pattern is the entry point into the grid.

2. Match direct causes

Once the pattern is identified, locate the corresponding row in the troubleshooting grid.

Each pattern is tied to a set of direct causes, which represent the most likely explanations for the observed behavior.

These causes are based on known relationships between:

• membrane condition
• system hydraulics
• water chemistry

3. Validate indirect causes

After identifying direct causes, review any associated indirect causes.

These may include:

• upstream process issues
• pretreatment performance
• chemical dosing problems
• operating condition changes

Indirect causes help explain why the issue developed and are critical for preventing recurrence.

4. Apply the corrective action category

Finally, apply the corrective action listed in the grid.

These typically fall into a few categories:

• cleaning (to remove fouling or scaling)
• improving pretreatment or upstream control
• adjusting operating conditions
• replacing damaged membrane elements

Importantly, the grid does not require interpretation beyond what is listed—you are mapping symptoms directly to known actions.

Common diagnostic patterns

The grid becomes more intuitive when applied to common performance patterns.

Below are a few representative examples.

Example: Increased salt passage with stable pressure drop

• Likely cause: membrane damage (e.g., oxidation)
• What it indicates: loss of membrane integrity
• Recommended action: replace affected elements

This pattern typically cannot be corrected through cleaning.

Example: Decreased permeate flow with increased pressure drop

• Likely cause: fouling or scaling
• What it indicates: restricted flow paths within the membrane
• Recommended action: cleaning and evaluation of upstream conditions

This is one of the most common performance issues and is often recoverable if addressed early.

Example: Decreased permeate flow with stable pressure drop

• Likely cause: fouling affecting permeability
• What it indicates: accumulation of contaminants on the membrane surface
• Recommended action: targeted cleaning, potentially adjusted based on foulant type

These patterns illustrate how the grid connects measurable system behavior to actionable next steps.

Using the grid with cleaning and replacement decisions

The troubleshooting grid is not a standalone tool—it feeds directly into operational decisions.

Once a cause is identified:

• If the issue is fouling or scaling → cleaning is typically the first step
• If the issue is damage → replacement is required
• If the cause is operational → adjustments may resolve the issue without intervention

➡️ For detailed guidance on whether to clean or replace elements, see Clean or Replace? A Practical Guide for FilmTec™ Elements

When the cause is not clear

In some cases, multiple factors may overlap, or the performance pattern may not clearly match a single cause.

This can happen when:

• more than one issue is present
• fouling progresses over time
• operating data is incomplete

In these situations, additional steps may be required:

• targeted system checks
• improved monitoring
• or membrane analysis to confirm the root cause

Putting it in context

The troubleshooting grid is most effective when used as part of a broader diagnostic process.

It helps you:

• move from symptoms to likely causes
• avoid guesswork
• apply corrective actions more confidently

However, it should always be used alongside:

• operating data
• system inspection
• knowledge of upstream conditions

➡️ For the full decision process after diagnosis, see Clean or Replace? A Practical Guide for FilmTec™ Elements

Quick reference