Citric acid is an organic acid that is often used for removal of calcium carbonate scale and is often mistakenly believed to be effective at dissolving iron fouling. It actually has only marginal impact on iron hydroxide deposits, and is very poor at dissolving phosphate salts such as calcium and iron phosphates. It cannot be used to dissolve sulfate, fluoride or silicate scales, and is ineffective for biofilm removal.
Although it is often recommended for organics removal, it can actually worsen membrane performance due to the protonation of the carboxylic acid functional groups in natural organic matter (NOM) – in layman’s terms, this means that low pH makes the organic foulants lose their negative charges. Since permeate is always produced during CIP, the lack of repulsive negative charges allows organic foulants to compact, making them more difficult to clean, or even irreversible. For that reason, a high pH cleaning should always be performed ahead of a citric acid cleaning when organics are suspected to be present.
Citric acid is therefore only useful for dissolving calcium carbonate scaling or reversing membrane swelling to “retighten membrane pores” after a high pH cleaning. Certain anti-caking agents present in citric acid can act as membrane foulants, and it is therefore always safer to use specialty RO CIP chemicals for membrane cleaning. Furthermore, in the United States, citric acid prices have been inflated due to anti-dumping duties imposed on imports, making it similarly priced to higher performing specialty chemicals.
If you are experiencing iron fouling, consider using AWC C-217, a specialty low pH CIP cleaner with 35X the iron dissolving capacity of citric acid.