**Physical (mechanical) cleaning**

Physical cleaning is used to remove and loosen the material accumulated on the membrane.

Backwashing is the most common procedure: part of the permeate passes through the membrane in the opposite direction of flow.

This requires a membrane that must physically support the inverted pressure gradient, other practices use pulsed flows.

Air/gas scrubbing: used to inject air or gas into the membrane [47].

If the membrane is not fully restored, chemical cleaning is necessary. **Chemical cleaning**

Chemical cleaning contains two parts of acid and basic cleaning as well as rinsing.

Acid washing is used to dissolve the scale layers of metal oxides and thus prevent the formation of insoluble hydroxides that are difficult to remove [47].

The purpose of alkaline washing is to hydrolyze silica, inorganic colloids and organic and biological matter.

Surfactants are also used as cleaning solutions, they are used to:

1.Move the clogging elements in the surface.


For example for polyether sulfonated membranes Tween 20 is used for cleaning [7].
