Salt Leakage Crisis: Fault Tree Analysis (FTA) and Stepwise Troubleshooting Manual for the Decline in Desalination Rate of Reverse Osmosis Systems

December 15 2025, by WANGZEYU, 7 min reading time

A sudden or gradual decline in the salt rejection rate of a Reverse Osmosis (RO) system is more than a performance hiccup; it is a direct threat to product water quality, process integrity, and operational economics. In industries ranging from seawater desalination to ultra-pure water production for photovoltaics and petrochemicals, "salt leakage" can lead to downstream corrosion, product contamination, and costly system shutdowns.

Diagnosing the root cause is often a complex puzzle, as the symptom—rising product water conductivity—can stem from a dozen interrelated failures. This article provides a systematic engineering framework to solve this puzzle. We will employ a Fault Tree Analysis (FTA) to map all potential failure pathways, from simple instrumentation errors to catastrophic membrane damage, and pair it with a field-tested, step-by-step troubleshooting manual. Our goal is to transform a reactive, costly crisis into a structured, efficient diagnostic procedure.

Understanding the Symptom: What Does "Declining Salt Rejection" Really Mean?

Salt rejection (SR) is the fundamental metric of an RO membrane's performance, calculated as (1 - Product TDS / Feed TDS) * 100%. A decline indicates the membrane's selective barrier is compromised. However, the key is to distinguish between an apparent decline and a true membrane failure. An apparent decline is caused by factors external to the membrane that affect the measurement or the system's hydraulic conditions, while a true decline results from physical or chemical damage to the membrane polymer itself. The speed of the decline is a critical clue: a sudden drop (over hours or days) often points to mechanical damage or acute chemical attack, whereas a slow, continuous decline (over months) typically suggests scaling, fouling, or gradual degradation.

The Master Diagnostic Tool: Fault Tree Analysis (FTA) for Salt Leakage

Fault Tree Analysis is a top-down, deductive failure analysis method that starts with the undesired event—"High Product Water Conductivity"—and logically maps all possible underlying causes. The following FTA chart provides a complete visual roadmap for the troubleshooting process.

Interpreting the FTA: The logic is clear. Before dismantling pressure vessels, the FTA guides you to eliminate the simplest and most probable causes first. Instrumentation errors are the most common cause of an "apparent" rejection drop. A drifted conductivity meter or a contaminated sample line can create a false crisis. The next branch, hydraulic changes, involves the operating conditions that affect the driving force for salt passage. For instance, a significant drop in system recovery rate can lead to lower feed pressure and higher salt passage, even with perfectly healthy membranes. Only after these are ruled out should investigation proceed to the most serious branch: actual membrane damage.

The Step-by-Step Troubleshooting Manual: From Field Checks to Autopsy

Following the FTA logic, this manual provides a sequential action plan.

Phase 1: Validate the Data (Eliminate Apparent Causes)

Calibrate and Verify: Calibrate all conductivity meters (feed, product, concentrate) using standard solutions. Check the temperature compensation setting on the meter.
Inspect Sample Points: Ensure product water sample lines are flushed thoroughly and are not contaminated by feed or concentrate water through improper valving. Compare a grab sample analyzed by a handheld meter to the online reading.
Review Datalogger Trends: Check for correlated changes in feed pressure, flow rates, pressure drop (ΔP), and recovery rate at the time the rejection drop was first noted. A simultaneous change points to a hydraulic cause.

Phase 2: Analyze Hydraulic Performance

Calculate Normalized Values: Use membrane manufacturer software to normalize salt passage and flows. This removes the influence of temperature, pressure, and salinity, revealing the true membrane condition.
Check Pressure Drop (ΔP): Measure and trend the ΔP across individual stages and across the entire system. A significantly increased ΔP, especially in the first stage, indicates fouling that can alter flow patterns and concentrate polarization, leading to higher local salt passage.
Verify System Recovery: Confirm the actual system recovery rate matches the design. A lower-than-designed recovery often means the membranes are operating at a lower average pressure, reducing driving force for permeation and effectively increasing salt passage.

Phase 3: Investigate Membrane Integrity (The Final Frontier)
If Phases 1 and 2 yield no answers, the problem likely lies with the membrane elements themselves.

Individual Vessel Profiling: Isolate and test each pressure vessel individually. Measure the conductivity of the product water from each vessel's sample port. A single vessel with significantly higher conductivity (a "hot vessel") pinpoints the location of the problem.
Vacuum Decay Test: For systems where a broken O-ring or glue line is suspected, a vacuum decay test on individual elements or vessels can identify gross leaks without needing an immediate tear-down.
Membrane Autopsy (The Definitive Diagnosis): Remove the lead element from a "hot vessel." This step-by-step autopsy is critical:
- Visual Inspection: Check for telescoping (physical damage from water hammer), broken fiberglass shells, or damaged interconnectors.
- O-ring and Brine Seal Inspection: Examine every O-ring and brine seal for cuts, rolling, or improper installation.
- Dye Test: Under controlled conditions, a specific dye applied to the feed side can visually identify the location of a breach if it appears on the product side.
- Laboratory Analysis: Send membrane samples to a specialized lab for analysis. Fourier-Transform Infrared Spectroscopy (FTIR) can detect chemical changes like oxidation, while Scanning Electron Microscopy (SEM) with Energy Dispersive X-ray Spectroscopy (EDS) can identify scaling elements and physical damage.

Case Study: The Phantom Salt Leak in a Coastal Power Plant

A coastal combined-cycle power plant using a two-pass RO system for boiler feed water noticed a 0.3% gradual decline in overall salt rejection over six months in the first pass. Normalized data confirmed the trend. Initial hydraulic analysis showed a 15% increase in first-stage ΔP.

The Fault Tree in Action: Following the FTA, instrumentation was verified (Phase 1). Phase 2 analysis pointed to fouling as the increased ΔP correlated with the rejection decline. However, an acid cleaning improved ΔP but did not fully restore rejection. This led to Phase 3.

The Root Cause Revealed: Individual vessel profiling identified two "hot vessels." An autopsy of the lead element from one vessel revealed no mechanical damage. However, lab SEM-EDS analysis showed a thin, uniform layer of silica scaling on the membrane surface, invisible to the naked eye. The scaling was not severe enough to cause massive flow loss after cleaning but was sufficient to disrupt the membrane's surface chemistry, providing a pathway for enhanced salt diffusion.

The Solution and Outcome: The root cause was identified as inadequate anti-scalant dosage control during periods of high seawater silica concentration. The dosage was optimized, and a specialized silica-specific cleaner was used in a subsequent cleaning cycle. Salt rejection was restored to 99.6%, and a monitoring protocol for silica and anti-scalant residual was implemented, preventing recurrence. This case highlights that not all "membrane damage" is permanent physical damage; some reversible surface conditions can cause significant rejection loss.

Conclusion: Transforming Crisis into Controlled Process

A decline in RO salt rejection should trigger not panic, but a disciplined investigative protocol. The Fault Tree Analysis provides the mental model, and the step-by-step manual provides the actionable path. By methodically eliminating possibilities from the simplest to the most complex—from meter calibration to membrane autopsy—plant operators and engineers can accurately diagnose the root cause, apply the correct corrective action, and prevent costly, unnecessary membrane replacements. In the world of industrial water treatment, the most valuable tool is not just the membrane itself, but the systematic knowledge required to maintain it.

Is your RO system experiencing unexplained salt leakage or performance loss? Don't guess. Our team of specialists can provide remote performance analysis, on-site troubleshooting support, and membrane autopsy services. Contact us today to download our comprehensive RO System Troubleshooting Checklist or to schedule a diagnostic review of your plant data.