Membrane fouling refers to a phenomenon that occurs during the membrane separation process, where solid particles, colloidal particles, macromolecular solutes, inorganic ions, and microorganisms present in the wastewater interact with the membrane physically, chemically, or mechanically. This leads to adsorption and deposition on the membrane surface or within its pores, causing pore size reduction or blockage. As a result, the membrane's resistance to water permeability increases, hindering the dissolution and diffusion of the solution on the membrane surface, ultimately causing an irreversible change in the membrane – a decrease in its flux and a deterioration of its separation characteristics.

Membrane fouling can be classified into four main categories: fouling due to microbial adsorption and growth, fouling due to organic matter adsorption, fouling due to colloidal and particulate aggregation, and inorganic fouling. In actual operation, the fouling mechanisms are quite complex, and various fouling types often occur simultaneously.

Scaling: Occurs when cations and anions reach a saturated state, forming crystals that adsorb onto the membrane surface. Common scales include sparingly soluble substances such as calcium carbonate (CaCO3), calcium sulfate (CaSO4), barium sulfate (BaSO4), strontium sulfate (SrSO4), etc. It often occurs at the end of the membrane system.

Organic Fouling: Formed due to the adsorption of organic molecules. Common pollutants include polyelectrolytes, fats, oils, etc. It is present at all stages of system operation.

Colloidal Fouling: Formed due to the dehydration and polymerization of colloids, electro-adsorption, and the precipitation of particles. Main pollutants are compounds of metals such as iron (Fe), aluminum (Al), silicon (Si), etc. It often occurs at the front end of the membrane system.

Biofouling: Formed due to the adhesion and growth of microorganisms. This type of fouling is present at all stages of system operation.