During the operation of reverse osmosis systems for industrial pure water and purified water, deterioration of RO permeate water quality and decline in salt rejection rate are the most common critical faults. They directly affect the effluent stability and water supply quality of the entire water treatment system.
Based on various on-site fault cases, abnormal RO membrane effluent and reduced salt rejection mainly stem from two root causes: leakage of seals or damage to membrane sheets, and oxidative degradation. All membrane integrity tests are carried out targeting these two primary causes, to accurately identify fault types and locate failure positions.
Below are six standard testing methods widely adopted in the water treatment industry, together with accurate judgment criteria, operational prohibitions and a complete set of implementable troubleshooting procedures.
1. Vacuum Decay Test (Offline / Shutdown Mode | Highest Accuracy)
Application Scenarios: Re-inspection of new membranes in warehouse, factory quality inspection, overhaul during system shutdown, troubleshooting of tiny hard-to-detect leaks. This method delivers the highest sensitivity in the industry.
Test Principle: Drain all accumulated water from membrane elements and pressure vessels, dry pipelines. Apply negative pressure above -80 kPa on the permeate side and maintain pressure. Leakage is determined according to vacuum decay volume. It can accurately detect pinholes on membrane sheets, minor leaks of O-rings and center connectors.
Judgment Criteria (Standard 8040 Membrane):
Evacuate to -90 kPa and hold pressure for 5 minutes.
Pressure drop ＜ 3 kPa: Pass
Pressure drop ＞ 10 kPa: Obvious leakage confirmed.
Key Operation Notes:
Membrane elements shall be fully wetted before testing. Dry membranes may lead to misjudgment due to pore air permeability. All test instruments must be calibrated; the air tightness of pipelines shall be verified in advance.
2. Pressure Decay Test (On-line during Shutdown | Seal Inspection for Single Pressure Vessel)
Application Scenarios: For suspected faulty pressure vessels, quickly identify failed seals or damaged membrane sheets.
Test Principle: Isolate and drain residual water in the suspected pressure vessel. Feed clean nitrogen or compressed air from the permeate side to maintain stable pressure. Leakage status is judged based on the magnitude of pressure decay.
Judgment Criteria:
Pressurize to 1.0 bar and hold pressure for 10 minutes.
Pressure drop ＜ 0.1 bar: intact and qualified.
Rapid pressure drop indicates leakage points.
Operation Prohibitions:
Never apply pressure reversely from the feed or concentrate side. Reverse pressure may crush membrane sheets and peel off the desalination layer, resulting in permanent damage to membrane elements.
The test gas source must be oil-free and water-free to prevent secondary contamination. This method is less sensitive to tiny pinholes compared with the vacuum decay test.
3. Single Vessel Permeate Conductivity Profiling Method (On-line Operation | Rapid Preliminary Screening)
Application Scenarios: Non-destructive testing without system shutdown; the preferred method for routine troubleshooting to rapidly locate abnormal pressure vessels.
Test Principle: Under steady-state rated operating load of the system, test the permeate conductivity of each pressure vessel one by one. If the reading of a single vessel is 50% or higher than the average value of all vessels, it is judged that there exists seal cross-flow or desalination failure of membrane sheets.
Key Operation Notes:
Flush stagnant water in pipelines before sampling; record operating parameters simultaneously. Avoid working conditions of start-stop cycles and chemical dosage fluctuation to prevent erroneous data judgment.
4. Probe Test Method (On-line Operation | Precise Location of Single Membrane Element)
Application Scenarios: After identifying the faulty pressure vessel, accurately locate the single defective membrane element without disassembling the entire vessel.
Test Principle: The probe performs progressive depth sampling through the permeate central tube of the pressure vessel, and monitors conductivity in real time. The failed membrane element can be precisely located according to the position where the conductivity value changes abruptly.
Key Operation Notes:
The operation requires two personnel working together. The probe shall be cleaned and wetted in advance and pushed forward smoothly to avoid pipeline scratching. This is currently the only non-destructive on-line method capable of locating a faulty single membrane element.
5. Dye Stain Test (Offline Disassembly | Definitive Verification of Membrane Sheet Damage)
Application Scenarios: Ultimate verification method to distinguish seal leakage and physical damage of membrane sheet after disassembling faulty membranes.
Test Principle: Seal the permeate end of the membrane element, and circulate dye solution (Methylene Blue / Rhodamine) under low pressure. Intact membrane sheets can block the dye. Visible stains will remain at positions with cracks, damage or delamination of desalination layer.
Judgment Criteria: Visible stained spots on the membrane sheet indicate defects. The damage severity can be quantified based on the distribution and quantity of stains to decide whether to scrap the membrane element.
Key Operation Notes:
Dye concentration: 0.01% ~ 0.05%; test pressure ≤ 1 bar.
This is a destructive test. Membrane elements cannot be reused after testing. Waste liquid must be collected and treated centrally.
6. Particle / Turbidity On-line Comparison Method (Continuous On-line Monitoring | Advance Early Warning)
Application Scenarios: 24-hour continuous on-line monitoring to provide advance warning of membrane sheet breakage and particle penetration failures.
Test Principle: Intact RO membranes generate permeate with extremely low and stable particle count and turbidity. Once membrane sheets are damaged, particles will penetrate the membrane, resulting in abrupt rises in monitored readings.
Limitation: This method only serves for global early warning and cannot locate specific fault points. When abnormal data appears, other testing methods shall be adopted for targeted troubleshooting.
Standard On-site Fault Troubleshooting Procedure (Directly Implementable)
Early Warning Screening: Monitor total permeate conductivity, particle count and turbidity to confirm abnormal water quality.
Vessel Preliminary Screening: Adopt conductivity profiling method to rapidly identify the abnormal pressure vessel.
Precise Positioning: Use the probe test to locate the single faulty membrane element.
Shutdown Qualitative Verification: Disassemble the membranes and verify via vacuum decay test or dye stain test to distinguish failure types.
Minor seal leakage: Replace O-rings and reuse the membrane element.
Membrane sheet damage or oxidative degradation: Scrap and replace directly.
Summary
The six testing methods work progressively, each with distinct responsibilities:
On-line particle monitoring delivers advance warning; conductivity profiling enables rapid preliminary screening; the probe test achieves precise positioning; vacuum decay test and dye stain test provide definitive qualitative confirmation.
Standardized troubleshooting effectively mitigates water quality risks, reduces unnecessary maintenance costs caused by blind membrane replacement, and ensures stable operation of semiconductor ultrapure water systems.
