Chemical cleaning of reverse osmosis membranes-Hezhong Environment (Beijing) Co., Ltd. was established in 2003

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Chemical cleaning of reverse osmosis membranes

Published Time:

2022-09-14

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As everyone knows, membrane elements must be cleaned before a large amount of fouling occurs. If cleaning is delayed too long, it will be very difficult or impossible to completely remove the fouling from the membrane surface and restore the membrane performance to its initial state. So what are the signs that membrane cleaning should be performed? That is, when the standardized pressure difference between the feed water and the concentrate water increases by 15%, or the standardized permeate water production decreases by 10%, or the standardized salt permeability increases by 5%, we should perform membrane cleaning!

Selection of Chemical Cleaning Agents

It is very important to determine the type of fouling on the membrane surface before cleaning. The best way to determine the type of fouling is to perform chemical analysis on the residue collected on the SDI test membrane to determine the main type of pollutants for targeted chemical cleaning. If chemical analysis cannot be used, the color and density of the residue on the test membrane can be tested according to the SDI determination, and then the fouling can be classified. For example, brown residue leads us to judge whether it is iron fouling; white residue may be silicon, sandy clay, calcium scale, etc.; crystalline shape is a characteristic of inorganic colloids and calcium scale; in addition to judging from the smell, biofouling or organic fouling can usually be seen as viscous pollutants.

Once the pollutants on the membrane surface have been identified, the correct cleaning procedure must be selected. If the fouling is considered to be a metal hydroxide, such as iron hydroxide or calcium scale, citric acid cleaning can be used; if the main fouling is determined to be organic matter or microorganisms, an alkaline cleaning method is recommended.

Below is a table to help you understand how to choose chemical cleaning agents:

The percentages in the above table are all the mass percentages of their active ingredients; SHMP is sodium hexametaphosphate; Na-SDS is sodium dodecyl sulfate; Na ₄ EDTA is tetrasodium ethylenediaminetetraacetate.

The chemicals used for cleaning interact with the pollutants, removing them from the membrane surface by dissolution or separation. This method is usually used after rinsing. Regular chemical cleaning and preventive maintenance before major system failures are good practices. After chemical cleaning, use pretreated raw water or permeate water (preferably) to thoroughly rinse the pollutants out of the RO system.

Matters Needing Attention in Chemical Cleaning

(1) For bacterial pollutants, cleaning with disinfectants and cleaning agents is the most effective. Disinfect first, then clean with a detergent.

(2) For the coexistence of multiple pollutants, multiple cleaning methods are usually needed to achieve good results. The cleaning order is generally to perform iron removal cleaning first, then acid cleaning, and then alkaline cleaning.

(3) If the desalination rate after cleaning is not ideal, the membrane surface can be rinsed again with an acidic agent, and finally the cleaning solution can be thoroughly rinsed with pretreated water (preferably RO product water).