Types and Causes of RO Membrane Fouling

Reverse osmosis (RO) membranes are widely used in water treatment. By applying external pressure to the concentrated feedwater side to overcome osmotic pressure, they allow solvent to pass through to the dilute side, achieving separation of solvent and solute. During RO operation, concentration polarization and membrane fouling are inevitable, with fouling being the primary factor limiting membrane performance and service life.

As RO membranes handle feedwater, they are exposed to a variety of contaminants. Over time, this inevitably leads to clogging and fouling, causing a decline in water flux, reduced treatment capacity, lower salt rejection and recovery rates, and shortened membrane lifespan.

RO membrane fouling can generally be categorized into physical fouling, chemical fouling, and biological fouling.

Physical Fouling:
Caused by suspended solids and colloidal particles (e.g., silica) in the feedwater depositing on the membrane surface and blocking membrane pores.

Chemical Fouling：
Caused by inorganic and organic compounds adsorbing onto or within the membrane pores.

• Inorganic fouling: Mainly from metals and metal oxides, or sparingly soluble salts such as calcium salts, barium salts, and magnesium salts.
• Organic fouling: Caused by humic acids, proteins, carbohydrates, and other organic matter forming a film on the membrane surface.

Biological Fouling
Caused by the growth and reproduction of microorganisms such as fungi, molds, and algae. Microorganisms can settle on the membrane surface, release organic matter to form a gel layer, and secrete substances that promote the crystallization of inorganic compounds and adhesion of organic matter. This often accelerates the occurrence of combined organic-inorganic fouling.

Fouling formation is closely related to feedwater quality and operating conditions, and is mainly influenced by:

• Ion concentration, pH, and temperature of the feedwater
• Deposition of contaminants: As most feedwater permeates through the membrane to the product water side, the remaining concentrated flow facilitates the adsorption and deposition of contaminants onto the membrane surface.
• Concentration polarization: During operation, solute concentration near the membrane surface increases, creating a concentration gradient and boundary layer. When local concentrations exceed solubility limits, precipitation occurs, blocking water passage and reducing flux.

If fouling is not addressed in its early stages, it can lead to significant and often irreversible membrane performance deterioration.

The fouling process typically follows this sequence:

1. Organic matter deposits first on the membrane surface.
2. As feedwater becomes more concentrated, inorganic compounds precipitate on top of the organic layer, forming organic–inorganic composite fouling.
3. Biological fouling may develop further, exacerbating blockage.

Fouling distribution often shows distinct patterns:

• Lead elements (inlet section): More prone to organic fouling, metal oxide fouling, and colloidal fouling.
• Tail elements (outlet section): More likely to experience scaling and silica deposition.

Additionally, the design, materials, and manufacturing process of membrane elements can influence how fouling develops.