When the above-mentioned cleaning methods are not effective enough to reduce the flux to an acceptable level, it is necessary to clean the membranes chemically.
During chemical cleaning chemicals, such as hydrogen chloride (HCl) and nitric acid (HNO3), or disinfection agents, such as hydrogen peroxide (H2O2) are added to the permeate during backward flush. As soon as the entire module is filled up with permeate, the chemicals need to soak in. After the cleaning chemicals have fully soaked in, the module is flushed and, finally, put back into production.
Cleaning methods are often combined. For example, one can use a backward flush for the removal of pore fouling, followed by a forward flush or air flush.
The cleaning method or strategy that is used is dependent on many factors. In practise, the most suitable methods is determined by trial and error (practise tests).
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