How to Protect Reverse Osmosis Water Purifier system During Shutdown?

During reverse osmosis unit shutdowns, improper protection exposes its core component—the reverse osmosis membrane element—to risks such as microbial growth, scaling, oxidation, and dehydration aging, leading to irreversible performance degradation.

I. Short-Term Shutdown Protection (≤5 Days)

Low-Pressure Flushing: Immediately after stopping the feed pump, perform low-pressure flushing of the RO pressure vessel using RO permeate or pretreated qualified effluent (never use raw water). (Note: Operating pressure ≤3 bar to avoid backpressure on membrane elements.) This flushes residual concentrate from the membrane surface and displaces raw water within the membrane housings, preventing salt precipitation and scaling.

Fill with Water for Moisture Retention: After flushing, close all feed valves, product water valves, and concentrate valves to fully immerse the membrane elements in the flushed, clean product water. Maintain the wet state of the membrane elements to prevent dehydration and drying, which could cause membrane shrinkage and deformation, damaging the membrane structure.

Routine Maintenance: Repeat low-pressure flushing and water filling every 24-48 hours to inhibit microbial growth. Ensure complete system sealing to minimize oxygen ingress and water quality changes.

II. Long-Term Shutdown Protection (>5 Days)

Thorough Chemical Cleaning: Prior to shutdown, perform a complete chemical cleaning of reverse osmosis membranes (acid cleaning for scale removal followed by alkaline cleaning to eliminate organic matter and microbial sludge).

This removes contaminants deposited on the membrane surface during operation, providing a clean base for long-term storage. Dirty membrane surfaces serve as breeding grounds for bacteria; direct storage will lead to severe biofouling.

Sterilization and Preservation Solution Soak: After cleaning, prepare a preservation solution using a sterilant. The most common and cost-effective preservative is a 1% sodium bisulfite solution.

Sodium bisulfite acts as a reducing agent, effectively inhibiting aerobic bacterial growth. It consumes oxygen in the water, creating an anaerobic environment that further slows microbial metabolism. It protects membrane elements from oxidative damage caused by residual oxidizing agents in water (e.g., chlorine).

After flushing out the cleaning agents, inject the prepared sodium bisulfite preservative solution (prepared using RO permeate) into the reverse osmosis system simultaneously through both the feed inlet and concentrate outlet until all pressure vessels are filled and concentrate is discharged from all outlets. Ensure membrane elements are fully submerged in the preservative solution, then close all valves to seal the system.

Environmental Control and Regular Inspection: Store equipment in a cool, dry location away from direct sunlight, as UV rays degrade membrane materials and high temperatures accelerate bacterial growth and solution degradation.

Maintain ideal ambient temperatures between 5°C and 45°C to prevent freezing.
Monthly inspections should verify the pH and concentration of the protective solution. Abnormal pH readings or solution turbidity indicate potential degradation, requiring solution reconstitution and replacement.
Alternative Preservation Methods: Besides sodium bisulfite, formaldehyde solution (0.5%-3%) may be used for stronger sterilization. However, due to its toxicity and potential carcinogenicity, extreme caution and protective measures are required during handling. Thorough flushing is mandatory before restarting. This is generally not recommended as the primary method.

III. Restart Procedure

Regardless of whether the shutdown was short-term or long-term, the following steps must be performed during restart:

Thorough Flushing: Completely flush and drain all preservative solution from the system. Particularly when using sodium bisulfite or formaldehyde, flush the product water side for at least 30 minutes until no odor remains.

Water Quality Testing: Conduct post-flush water quality analysis (e.g., conductivity, TOC) to ensure treated water meets specifications.

Normal Operation: Only resume normal operation after confirming water quality compliance.