Citric acid is an organic acid that is often used for removal of calcium carbonate scale and iron hydroxide. Citric acid is not very effective at removing phosphate salts such as calcium and iron phosphates. It cannot be used to dissolve sulfate scales or silica, and is ineffective for biofilm removal.
Although it is often recommended for organics removal, it is not very effective due to the protonation of the carboxylic acid functional groups in natural organic matter (NOM) in the low pH environment created by citric acid. For this reason, attempting to clean at low pH when organics are present can compact the NOM into the membrane, resulting in a foulant that is more difficult to clean, or even irreversible.
Certain anti-caking agents present in citric acid can act as membrane foulants, and it is therefore always safer to use specialty RO sanitizing agents. Furthermore, in the United States, citric acid prices have been inflated due to anti-dumping duties imposed on imports. For this reason, it makes little sense to use citric acid in place of specialty cleaning chemicals which are more effective over a broader spectrum and are of a similar cost.