Introduction to UF Membrane Cleaning Procedure

Ultrafiltration (UF) membranes are the core components of ultrafiltration systems and were among the earliest polymer separation membranes developed for industrial water treatment applications. They are widely used as intermediate treatment units in ultrapure water production systems.

Because ultrafiltration membranes are porous materials that remove contaminants through physical interception, membrane fouling gradually occurs during operation. Therefore, a proper UF membrane cleaning procedure is essential to maintain permeate flow, ensure stable system performance, and extend membrane service life.

The UF membrane cleaning procedure is generally determined according to the membrane material and the characteristics of the feed solution. In most cases, the cleaning process is similar to that used for reverse osmosis systems: hydraulic flushing is performed first, followed by chemical cleaning using suitable cleaning agents depending on the type of foulants present.

For example:

Ionic solubilizing agents may be used for electrophoretic coating materials.
“Bridge-bond” solvents are suitable for water-soluble organic coatings.
Protein fouling in food processing applications can be removed using protease-based cleaners or alkaline detergents formulated with phosphates or silicates.
Inorganic scaling on membrane surfaces can be dissolved using chelating agents such as EDTA, as well as acidic or alkaline cleaning solutions.

A standard UF membrane cleaning procedure generally consists of the following seven steps:

Preparation of Cleaning Solution

Prepare the cleaning solution according to the type of membrane fouling and the membrane manufacturer’s recommended cleaning protocol.

Low-Flow Introduction of Cleaning Solution

First, circulate and mix the cleaning solution using the cleaning pump. During preheating, the cleaning solution should be circulated at a low flow rate.

Then, displace the feed water inside the membrane elements using the lowest possible cleaning pressure. The pressure should only be high enough to overcome the pressure loss between the feed and concentrate sides of the system. It must remain low enough to avoid significant permeate production.

Low-pressure displacement minimizes the risk of redepositing foulants onto the membrane surface. If necessary, discharge part of the concentrate stream to prevent excessive dilution of the cleaning solution.

Circulation

Once the feed water has been fully displaced, cleaning solution should be visible in the concentrate piping. Continue circulating the cleaning solution back to the cleaning tank while maintaining a stable cleaning temperature.

Soaking

Stop the cleaning pump and allow the membrane elements to soak completely in the cleaning solution.

In some cases, soaking for approximately one hour is sufficient. However, severe fouling may require extended soaking times of 10–15 hours or overnight soaking.

A very low recirculation flow may be used during soaking to maintain solution temperature.

High-Flow Recirculation Cleaning

High-flow circulation helps remove contaminants loosened by the cleaning chemicals.

For heavily fouled systems, operating at flow rates approximately 50% higher than the standard recommended cleaning flow may improve cleaning effectiveness. However, excessive pressure drop must be avoided during high-flow operation.

Maximum allowable pressure drop for a single membrane element: 1 bar (15 psi)
Maximum allowable pressure drop for a multi-element pressure vessel: 3.5 bar (50 psi)

The lower operating limit should always be respected first.

System Flushing

Qualified pretreated permeate water can be used to flush residual cleaning chemicals from the membrane system, unless corrosion concerns exist. For example, stagnant seawater may corrode stainless steel piping.

To minimize precipitation risks, the flushing temperature should not be lower than 20°C.

Notes for Acid Cleaning

During acid cleaning, the pH value of the cleaning solution should be monitored continuously. As inorganic scales dissolve, acid is consumed, causing the pH to rise.

If the pH increases by more than 0.5 units, additional acid should be added to the cleaning tank.

The total circulation time for acidic cleaning solution should not exceed 20 minutes. Beyond this period, the solution may become saturated with dissolved inorganic salts, potentially causing redeposition of contaminants onto the membrane surface.

If this occurs:

Discharge the first cleaning solution completely.
Flush the membrane system with qualified pretreated water.
Prepare fresh cleaning solution.
Perform a second-stage acid cleaning cycle.

If the UF system will remain shut down for more than 24 hours, the membrane elements should be preserved in a 1% (by weight) sodium bisulfite solution.

Before cleaning large-scale membrane systems, it is recommended to remove one membrane element from the system and conduct a single-element cleaning performance evaluation test.

Cleaning of Multi-Stage Systems

For multi-stage UF systems, the flushing and soaking steps can typically be carried out simultaneously for all stages.

However, high-flow recirculation cleaning should be performed separately for each stage to ensure proper cleaning flow distribution. Otherwise, the flow rate in the first stage may become insufficient while the final stage experiences excessive flow.

This can be achieved by:

Cleaning each stage individually using one pump, or
Installing separate cleaning pumps designed for the required flow rate of each stage.

Conclusion

A proper UF membrane cleaning procedure is critical for maintaining membrane permeability and extending membrane lifespan. Timely cleaning not only improves operational efficiency but also reduces the risk of membrane damage during the cleaning process.

Different membrane module configurations may require different cleaning methods. For example:

Tubular membrane modules may be mechanically cleaned using sponge balls.
Hollow fiber membrane modules may utilize backwashing procedures.

For food industry membrane systems, disinfection treatment is also required, commonly using chemicals such as NaOH and H₂O₂.