Causes and Solutions for Reverse Osmosis Membrane Fouling

1. Main Causes of RO Membrane Fouling

Depending on the foulants, RO membrane fouling is mainly categorized into four types:

• Inorganic Scaling (Chemical Fouling): During the RO concentration process, sparingly soluble salts such as calcium, magnesium, carbonates, sulfates, and silicates in the feed water exceed their solubility limits. They crystallize and deposit on the membrane surface, forming a hard scale layer.
• Organic and Colloidal Fouling (Viscous Fouling): Natural organic matter (e.g., humic acid), oils, surfactants, or incompletely removed suspended particles and colloids in the feed water adhere to the membrane surface. This forms a viscous fouling layer that not only blocks membrane pores but also adsorbs other impurities.
• Biological Fouling (Reproductive Fouling): When feed water disinfection is inadequate, microorganisms such as bacteria, fungi, and algae multiply rapidly on the membrane surface. They form a tenacious biofilm and secrete metabolites that can degrade the membrane elements.
• Particulate Fouling (Physical Fouling): Suspended solid particles like sand and rust in the feed water typically enter the system due to inadequate or failed pretreatment. They deposit on the membrane surface or at the cross-points of the feed spacers.

2. How to Determine if Cleaning is Required

Cleaning is generally required when the system exhibits the following conditions:

• Decreased Permeate Flow: Normalized permeate flow drops by 10%~15% compared to the initial baseline.
• Increased Pressure Drop: The inter-stage pressure drop increases significantly (e.g., by more than 15%, or the first-stage pressure drop exceeds 0.25 MPa).
• Decreased Salt Rejection: Permeate water quality deteriorates, with normalized salt passage increasing by 10%~15%.

3. Solutions and Cleaning Protocols

The core solution for membrane fouling is Clean-In-Place (CIP), which includes both physical and chemical cleaning.

Physical Cleaning

Mainly used to remove loose foulants from the membrane surface (e.g., mild particulate fouling). It involves flushing with low-pressure, high-flow-rate pure water, utilizing hydraulic shear force to wash away foulants. Air-water mixed cleaning can also be used to enhance the effect.

Chemical Cleaning

Appropriate chemical agents must be selected for circulation and soaking based on the specific type of fouling:

• For Inorganic Scaling: Use acidic cleaners (e.g., 0.2%~1% citric acid or hydrochloric acid, pH controlled at 2-3) to effectively dissolve carbonates and metal oxides.
• For Organics, Colloids, and Biological Fouling: Use alkaline cleaners (e.g., 0.1%~0.5% sodium hydroxide combined with surfactants or EDTA, pH controlled at 10-12) to dissolve viscous dirt and strip off biofilms.
• For Composite Fouling: Alkaline cleaning is usually performed first to remove organic/biological fouling, followed by acidic cleaning to remove inorganic scaling.

Standard Cleaning Procedure

1. Pre-rinse: Shut down the system and flush at low pressure with RO permeate to drain residual feed water from the membrane modules.
2. Solution Preparation & Circulation: Prepare the cleaning solution at the specified ratio. Pump it into the system at low pressure (usually ≤0.2~0.3 MPa) and at half the normal cleaning flow rate. Circulate for 30-60 minutes.
3. Soaking: Stop circulation and let the cleaning solution soak inside the membrane for a period (e.g., circulate for 10 minutes, then stop and soak for 5 minutes) to allow the agent to fully penetrate the fouling layer.
4. Post-rinse: Drain the cleaning solution and thoroughly flush the system with permeate until the effluent pH is neutral and free of foam or odor.
5. Performance Verification: Restart the system for a low-pressure test run to observe whether the permeate flow and salt rejection have recovered to over 90% of the initial values.

4. Prevention and Routine Maintenance Recommendations

• Optimize Pretreatment: Improve pretreatment processes such as coagulation, filtration, and softening to ensure the feed water Silt Density Index (SDI) is <5, reducing foulants at the source.
• Regular Physical Flushing: If conditions permit, perform a low-pressure, high-flow-rate flush daily. This is more effective than waiting for fouling to accumulate before conducting a chemical clean.
• Proper Chemical Dosing: Dose appropriate amounts of antiscalants, dispersants, and specialized biocides based on feed water quality to inhibit scaling and microbial growth.
• Standardized Operation: Avoid overloading the system. Always flush the system before shutdown to prevent foulants from stagnating on the membrane surface.

⚠️ Precautions: Never blindly use high concentrations of strong acids or alkalis, or extend cleaning times, as this will cause irreversible damage to the membrane's active layer. If online cleaning is ineffective or fouling is extremely severe, consider removing the membrane elements for professional offline cleaning or replacing them entirely.