Fouling and Cleaning Methods of Reverse Osmosis Membranes — Contaminants in Reverse Osmosis Membrane Elements

This article describes common fouling factors affecting composite membrane performance and their cleaning methods, applicable to 4-inch, 6-inch, 8-inch, and 8.5-inch diameter reverse osmosis membrane elements.

Note 1: Under no circumstances should water containing free chlorine come into contact with the composite membrane element. Such contact will cause irreversible degradation of membrane performance. After sterilization of pipelines or equipment, ensure that the feed water to the reverse osmosis membrane element is free of free chlorine. This should be confirmed by laboratory testing. Use an acid solution to neutralize residual chlorine and ensure sufficient contact time for complete reaction.

Note 2: During the warranty period of the reverse osmosis membrane element, it is recommended that each membrane cleaning be performed in consultation with the company. At least for the first cleaning, the company's on-site service personnel should be present.

Note 3: Avoid using cationic surfactants in the cleaning solution, as their use may cause irreversible fouling of the membrane element. 1. Contaminants in Reverse Osmosis Membrane Elements

After a period of normal operation, reverse osmosis membrane elements will become fouled by suspended or sparingly soluble substances that may be present in the feed water. The most common contaminants are calcium carbonate scale, calcium sulfate scale, metal oxide scale, silica deposits, and organic or biological deposits.

The nature and rate of fouling are related to feed water conditions. Fouling develops slowly, and if measures are not taken early, it will damage the membrane element's performance in a relatively short time. Regularly monitoring the overall system performance is a good way to confirm membrane element fouling. Different contaminants will cause varying degrees of damage to membrane element performance.

Contaminant Removal

Contaminant removal can be achieved through chemical cleaning and physical flushing, and sometimes by changing operating conditions. As a general principle, cleaning should be performed when one of the following occurs:

• The product water flow rate drops to 10-15% of the normal value under normal pressure.

• The feed water pressure, after temperature correction, is increased by 10-15% to maintain the normal product water flow rate.

• Product water quality decreases by 10-15%. Salt permeability increases by 10-15%.

• Operating pressure increases by 10-15%.

• The pressure difference between different sections of the RO system increases significantly (perhaps there are no instruments to monitor this).