Causes of Membrane Fouling in Ultrapure Water Systems

Reverse osmosis membranes in ultrapure water systems are easily fouled. Once fouled, they can affect the operation of the ultrapure water system and damage the reverse osmosis membrane. So, what causes membrane fouling in ultrapure water systems?

1. Ion Scaling

During the reverse osmosis process, salts with low solubility, such as CaCO3, CaSO4, and BaSO4, in the feed water are concentrated. This causes the concentration of these salts in the concentrate to exceed their solubility, resulting in precipitation and deposits that remain on the membrane surface and inside the membrane pores, forming scale. This fouls the membrane, leading to a decrease in the permeate flow rate, a significant increase in pressure differential, and a rapid decrease in efficiency.

2. Silica Precipitation

The solubility of silica is affected by many factors, such as temperature and pH value. The maximum solubility of silica is typically 150 mg/L, while the silica content in many raw waters exceeds 20 mg/L. Once silica precipitates on the membrane surface, there are few effective methods for removal. Therefore, for safe operation, the concentration of silica in the concentrate should be strictly controlled, typically using 100 mg/L as the control index. Silica must not be allowed to precipitate and deposit on the reverse osmosis membrane during operation.

3. Deposition of Metal Oxides

Groundwater often contains free ions such as iron and manganese. Upon contact with air or oxidants, these ions can be oxidized into insoluble colloids, clogging the membrane pores and causing an increase in the reverse osmosis membrane pressure differential and a decrease in permeate flow.

4. Suspended Particle and Colloidal Fouling

When the security filter in the reverse osmosis feed water pretreatment system short-circuits or malfunctions, it loses its filtering function. Suspended particles and colloids in the feed water cannot be removed, and the turbidity and clogging index of the reverse osmosis feed water far exceed the standard range. This causes suspended particles and colloids to form amorphous precipitates on the membrane surface, clogging the reverse osmosis membrane feed channels and causing membrane fouling.

5. Microbial Growth

Reverse osmosis membranes are organic materials, which inherently promote the growth and reproduction of microorganisms. The presence of high levels of organic matter (CODMn) in the feed water for reverse osmosis promotes biological growth. Adding phosphorus-based scale inhibitors to the feed water, coupled with suitable feed water temperatures, further accelerates microbial growth. When a large number of vigorous microorganisms proliferate on the membrane surface, they not only damage the desalination layer of the reverse osmosis membrane, reducing its desalination capacity, but also clog the membrane pores with their slime, leading to increased pressure differential and decreased permeate production during operation.

This concludes our explanation of the causes of membrane fouling in ultrapure water systems. Reverse osmosis is a key component of ultrapure water systems, and the membrane module is of paramount importance. Replacing damaged or fouled membrane modules inevitably involves cost and convenience considerations, thus requiring careful attention.