Reverse‑osmosis membranes serve as core components for pure‑water and reclaimed‑water reuse systems. Long‑term operation tends to bring about inorganic salt scaling, biological slime and organic fouling. These problems will raise differential pressure between membranes, reduce permeate flow and lower salt rejection rate. Proper and standardized chemical cleaning is critical to restore membrane performance and extend its service life. Poor cleaning outcomes or even damage to membrane elements are mostly caused by failure to identify fouling types, reversed cleaning sequence and improper parameter settings.
Today I sort out a widely‑accepted industry‑standard cleaning process (acid‑first then alkaline). It covers fouling diagnosis, pre‑preparation, chemical cleaning, flushing and system restart, which can be directly put into practice.
01 Before cleaning: diagnose fouling types for targeted cleaning
Fouling types must be accurately identified before cleaning. Blind cleaning will produce counter‑productive results. Below are two simple and reliable judging methods:
1.Weight‑check method (for inorganic‑salt scaling)
Take out a single heavily fouled front‑end membrane element and weigh it. If an 8‑inch membrane element gains over 15 kg in weight, it is generally confirmed to suffer from inorganic‑salt fouling, and acid cleaning shall be the main procedure.
2，Visual inspection method
Remove the end caps of the membrane housing to check contaminants:
·Rough, whitish, yellowish and hard deposits mean inorganic scale; acid cleaning should be prioritized.
·Slippery, sticky, foul‑smelling and strongly adherent contaminants point to biological slime and organic fouling, which mainly requires alkaline cleaning.
02 Pre‑Cleaning Preparation Work
After confirming the fouling type, you can start preparation work with strict safety protection and parameter control throughout the whole process.
1.Safety Protection
Operators shall wear acid‑alkali‑resistant gloves, goggles and protective suits. An emergency shower, eyewash station and plenty of fresh‑water shall be available onsite to prevent chemical splash‑induced burns.
2.Pretreatment of Cleaning Solution
Fill the dedicated cleaning tank with RO permeate water and turn on the heater. Keep the water temperature steadily between 30‑35℃, which is the optimal temperature for cleaning. Cleaning efficiency drops sharply below 25℃, while temperatures over 40℃ will damage the membrane separation layer.
3.System Isolation
Slowly close the permeate valve, feed‑water valve and concentrate‑water valve. Fully isolate the membrane section under cleaning from the running system so that cleaning fluid cannot enter production pipelines.
4.Pipeline Connection
Connect the outlet hose of the cleaning pump to the main inlet of the membrane section. Link the total permeate outlet and concentrate outlet back to the cleaning tank via return hoses to build a closed‑loop circulation pipeline for cleaning.
03 Acid Cleaning (Removal of Inorganic‑Salt Scaling)
Strictly follow the principle of acid‑first‑then‑alkaline cleaning to remove inorganic fouling such as calcium‑magnesium scale and metal oxides firstly.
1.Pre‑rinsing with clean water
Start the cleaning pump and circulate clean water at low‑pressure and high‑flow rate for 5‑10 minutes to thoroughly drain concentrated‑water trapped inside membrane elements. Empty residual water from the tank afterwards.
2.Prepare acid‑washing solution
Refill the cleaning tank with fresh RO permeate water. Slowly add citric acid to make a 2%‑by‑weight solution and stir fully until dissolved completely. Adjust the pH value to a steady range of 2.0‑2.5 with ammonia water and keep the temperature between 30‑35℃.
3.Circulating acid cleaning
Turn on the cleaning pump and set the pressure at 0.3‑0.4 MPa to maintain adequate circulation flow without damaging membrane sheets. Keep circulating cleaning for 60‑90 minutes. Test pH value every 15 minutes. If the pH rises by over 0.5 units, supplement citric acid timely until pH remains stable without fluctuation. (Stable pH serves as the core judgment standard for scale removal.)
4.Static soaking plus secondary circulation
Shut down the cleaning pump and keep static soaking for 1‑2 hours to fully dissolve deep‑seated stubborn scale inside the membrane layers. After soaking, restart the pump for another 15‑20‑minute circulation to flush off loosened scale deposits, then drain all acid‑cleaning liquid finally.
04 Thorough Flushing after Acid Cleaning (Indispensable Step)
Once the acid solution is drained out, fill clean water right‑away and flush the membrane system under low pressure. Continue flushing until the outlet‑water pH returns to neutral.
Important warning: Never start alkaline cleaning while acid liquid remains inside membranes. The mixture of acid and alkali will produce massive heat from neutralization and secondary salt precipitates, which will directly damage membrane elements and worsen fouling.
05 Alkaline Cleaning (Removal of Organic Matter and Biological Slime)
After acid‑based scale removal is finished, alkaline cleaning strips biological slime, oil stains and colloidal organics off membrane surfaces to restore permeate flux.
1.Prepare alkaline cleaning solution
Fill the cleaning tank with fresh permeate water. Add 0.1% sodium hydroxide plus 0.02% sodium dodecyl sulfate (SDS), stir thoroughly for full dissolution. Keep solution pH steady between 11.5‑12 and maintain water temperature at 30‑35℃.
2.Circulating alkaline cleaning
Set cleaning pressure at 0.3‑0.4 MPa and run circulating cleaning for 60‑90 minutes. Monitor pH value all through the process. If pH drops by more than 0.5 units, add alkaline solution timely to maintain cleaning effectiveness.
3.Static soaking and secondary circulation
Stop the pump and keep soaking for 1‑2 hours to fully hydrolyze organics and peel off stubborn biological slime. Afterwards restart the pump and circulate for 20 minutes to thoroughly flush out contaminants, then drain alkaline solution completely.
06 Final Flushing and System Restoration
1.Thorough flushing
After draining the alkaline solution, flush the membrane system repeatedly with clean water for 30‑60 minutes until the pH value and conductivity of permeate and concentrate water return fully to baseline levels measured before cleaning.
2.System restart
Check opening‑closing status of all valves one‑by‑one before startup. Open feed‑water valves and concentrate‑water valves slowly to avoid pressure shock. Run the system at 50% low‑load water‑production rate for 30 minutes. After confirming stable and qualified permeate conductivity, gradually increase the system to full‑load operation.
3.Evaluation of cleaning performance
Compare three core indicators before and after cleaning: differential pressure across membranes, permeate flow and salt‑rejection rate.
·Qualified standard: The differential pressure drops obviously, and permeate flow recovers above 90% of its initial value.
·Abnormal condition: If membrane performance hardly improves, severe irreversible fouling or incorrect earlier fouling diagnosis is highly probable. You need to open membrane housings for re‑inspection and adjust plans for a second cleaning.
07 Summary of Key Safety & Operational Prohibitions
Reverse‑pressure loading is strictly forbidden: Under any working condition, the pressure on the permeate side shall never exceed that on the concentrate‑water side to prevent membrane sheet debonding, rupture and scrappage.
Strictly control cleaning temperature: The optimal temperature ranges from 30‑35℃. Too low temperature will greatly reduce cleaning efficiency, while temperatures above 40℃ will cause permanent damage to the membrane separation layer.
Adhere to the cleaning sequence: Always carry out acid‑first‑then‑alkaline cleaning under standard conditions. Thorough flushing is required between the two steps to avoid mixing acid and alkaline solutions.
Dynamic parameter monitoring: Keep monitoring pH value, pressure and temperature throughout cleaning. Add chemicals or make adjustments promptly once parameters deviate.
RO‑membrane cleaning does not require high frequency but high accuracy. Correct identification of fouling types and compliance with standardized procedures can quickly restore equipment performance, maximize the service life of membrane elements and cut operation‑and‑maintenance costs.
