What could cause fouling of membranes?

QFS designs, manufactures, installs, commissions and maintains membrane plants and general water treatment equipment. The company recently experienced differential pressure increases due to fouling of the membranes in one of its desalination plants.

A2 Mℓ/day seawater ultrafiltration unit (SWUF) that abstracts water from a river estuary near the mouth entering the Indian Ocean experienced differential pressure due to fouling of the membranes. This reduced the plant production by increasing the daily downtime required for chemical cleaning of the membranes.

To identify the root cause of the fouling, QFS took samples from the raw water feed (upstream river) and fibre from the fouled membrane. The raw water samples were sent for analysis to two independent laboratories while the fouled membrane fibre samples were sent to the Central Analytical Facilities of Stellenbosch University for analysis with a scanning electron microscope (SEM).

FIGURE 1 SEM images at 100 µm show a flaky morphology, which is a typical characteristic of clay

FIGURE 2 The overall spectrum shows a high concentration of sodium and chloride. Other spectrums of specific blotches show significant weight percentage of aluminium, silica and oxygen, which is typically found in aluminium silicates FIGURE 3 The analysts noted that the clay-like samples show a presence of diatoms, which are algae with silica cell walls. The observed diatoms have a size of 5 µm to 30 µm based on the SEM images. The presence of diatoms would suggest the presence of diatomaceous earth, which is the diatoms’ fossilised remains, with the characteristics of a fine, highly porous and siliceous powder used in the ceramics industry. Diatomaceous earth generally has a size of 10 µm to 200 µm

Fouled membrane

Conclusion

A combination of aluminium silicates and diatomaceous earth is contributing to the formation of clay layering in between precipitated salt layers. A large presence of diatoms generally coincides with nutrient-rich water. It is known that the river from which the plant abstracts is subject to sewer run-off and sewage dumping.

Although the feedwater turbidity, total suspended solids and total dissolved solids (TDS) are all within design specifications, it is evident that the combination of the abovementioned solids creates a clay-like surface coating. This coating proved difficult to remove from the membrane fibres that standard backwashing and chemical cleaning could not successfully remove.

A combination of various detergents was tested and resulted in the successful cleaning of the membrane samples. This combination was recreated using industrial chemicals with the active ingredients of the abovementioned detergents, and used to successfully clean the UF membranes, bringing it back to a baseline state.

Detergent-cleaned membrane fibre

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