Integrity testing is a core method to ensure the retention performance of ultrafiltration (UF) membranes, relying on physical monitoring to determine membrane integrity and guarantee microbial retention rates (bacteria ≥6 log, viruses ≥4 log).
1. Bubble Point Test
Based on the simplified Cantor formula, the test applies gas pressure to detect the maximum pore size and breakage points of the membrane. The bubble point pressure of an intact membrane is determined by its material and pore size. If continuous bubbles appear on the product side at a pressure far below the bubble point, it indicates membrane fiber breakage.
Implementation Process:
- Fully wet the membrane module, close the feed water valve, and drain the liquid.
- Introduce compressed air into the feed side (not exceeding 1.5 bar) and observe bubble formation on the product side.
- A single fiber break causes a pressure drop ≥40 mbar, triggering an immediate alarm.
2. Pressure Decay Test
Based on gas diffusion laws, the test observes the pressure decay rate after pressurization. Intact membranes allow only minimal gas diffusion, while pressure drops significantly in case of damage.
Parameters and Standards:
- Test pressure: 2.0 bar, hold for 10 minutes.
- Pressure drop threshold: ≤0.2 bar for qualification.
- Sensitivity: Detects a single broken fiber in a membrane module.
Steps:
- Wet the membrane module and pressurize to the set value.
- Seal and start timing after stabilization.
- If the pressure drop exceeds the threshold (e.g., 0.2 bar/10 min) after 10 minutes, the membrane is deemed failed.
3. Airflow Test
Measures the airflow rate required to expel residual water, dynamically assessing membrane integrity. Damaged fibers cause a significant increase in airflow rate.
Parameters:
- Test pressure: 1 bar, duration ≤10 minutes.
- Standard value: 40-180 L/h for 48 intact membrane modules; a single broken fiber increases airflow by 250 L/h, triggering an alarm.
- Advantages: Online rapid detection and damage localization (requires individual valves for each pressure vessel).
Integrity testing accurately identifies membrane fiber defects through physical methods, combining offline and online monitoring to ensure the microbial barrier function of UF systems, meeting high safety standards for drinking water and sensitive industries.
