Sterilization and residual chlorine control in the dual-membrane system

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The dual-membrane process (ultrafiltration + reverse osmosis) system is highly sensitive to microbial contamination and membrane oxidation damage; sterilization and residual chlorine control represent the two key challenges in ensuring the stable operation of this system. It is necessary to effectively kill bacteria and algae in water, without allowing residual oxidizing disinfectants to damage the reverse osmosis membrane. I. Necessity of sterilization: The total bacterial count in raw water generally ranges from 10³ to 10⁵ CFU/mL. Even after coagulation and sedimentation, there may still be 10² to 10⁴ CFU/mL of viable bacteria prior to ultrafiltration. If left uncontrolled, microorganisms will form biofilms on the surface of the ultrafiltration membrane, causing the transmembrane pressure difference to increase by more than 50% within 30–60 days, which forces the frequency of chemical cleaning to double. Double-membrane systems typically employ a combination of continuous dosing and shock disinfection: continuous chlorination involves adding sodium hypochlorite to the ultrafiltration feed water to maintain a residual chlorine level of 0.5–1.0 mg/L (based on Cl₂). At this concentration, exposure for 20–30 minutes is sufficient to inactivate over 99% of bacteria and E. coli, reducing the total bacterial count in the ultrafiltrate effluent to below 50 CFU/mL. Shock sterilization: Increase residual chlorine to 3–5 mg/L every 24 hours or when the pressure difference rises, and subject to cyclic immersion for 30–60 minutes. The ultrafiltration system of a municipal reclaimed water project employs shock sterilization three times per week; the interval between chemical membrane cleanings has been extended from 45 days to 120 days, and the SDI of the produced water remains below 2.5 at all times. II. Hazards of residual chlorine: The amide bonds in polyamide composite reverse osmosis membranes are highly susceptible to oxidation by active chlorine, resulting in a weakened membrane structure and an irreversible decline in desalination efficiency. The industry-recognized tolerance limit is free chlorine