1. By Form of Fouling:

Membrane Pore Blocking: Caused by small molecular substances.

Gel Layer on Membrane Surface: Formed due to concentration polarization of large molecular substances.

Sludge Cake (Filter Cake): Deposition of particulate matter.

Floating Matter Entanglement/Blockage: Clogging by suspended particles.

2. By Cleanability (Recoverability):

Reversible Fouling / Temporary Fouling: Usually refers to the sludge layer deposited on the membrane surface, which can be removed by physical cleaning.

Irreversible Fouling / Long-Term Fouling: Usually refers to the gel layer on the membrane surface and pore blocking, which can be removed by chemical cleaning.

Irrecoverable Fouling / Permanent Fouling: Usually caused by irreversible adsorption and cannot be removed by any cleaning methods.