Caring for Your Pre-Treatment Equipment: RO Membrane Cleaning

As water and energy costs increase, government regulations tighten and environmental stewardship initiatives progress, facilities must continue to enhance their system efficiencies. Reverse Osmosis (RO) equipment is becoming more common in water treatment systems. Unfortunately, even with high quality feed water, RO membrane scaling and fouling is inevitable. Regular cleaning and servicing of your RO system and its membranes can extend the life of your asset and minimize operational expenses. This article outlines the recommended RO cleaning procedures using acid and caustic cleaning products.

1. Rinse the Clean-in-Place (CIP) tank and lines with RO permeate.
2. Replace the RO cartridge filters (if online during the cleaning), or replace the cleaning skid filters.
3. Fill the CIP tank with RO permeate based on a target volume of 15 to 20 gallons per 8”x40” membrane element in the system being cleaned. If using condensate or some other hot water source to increase temperature, leave room in the CIP tank for any associated additions. Ideal temperature for cleaning is 95°F. For cleaning solutions with a pH from 2.0 to 11.5, the maximum recommended temperature is 107° F. Slowly add the cleaning chemical and mix as required. Note that some CIP systems allow the tank solution to be recirculated using the transfer pump.
   • For removing iron or anaerobic biological materials, cleaning should be performed using pH 3.8 and 4.2.
   • For removing calcium carbonate scale, cleaning should be performed using pH of 3.8 to 4.2.
   • For removing calcium sulfate scale, cleaning should be performed using pH of 1.0 to 1.2 verified using two calibrated pH measurement devices. Usually this problem is isolated to the last stage of the RO.
   • For removing biological materials, clay/silt, silica, or organic solids, cleaning should be performed using pH of 11.7 to 11.9 verified using two calibrated pH measurement devices.
4. Turn off the RO and shut off its inlet water supply. Make all necessary connections and valve adjustments to allow the CIP transfer pump to be able to deliver cleaning solution to the RO system. Some RO systems offer an inter-stage CIP connection, which makes it possible to clean the membrane stages individually. The RO permeate should be diverted to the CIP tank. Throttle the CIP pump outlet value to approximately one-quarter open.
5. Operate the CIP transfer pump with its outlet throttled to displace to drain any raw water from the RO stage/system being cleaned. Continue until there is evidence of the cleaning solution in the outlet by monitoring pH, conductivity, temperature or noting the presence of suds. Disengage the CIP pump and change valve positions as necessary to direct the return solution from the stage/system back to the CIP tank. When circulating through the last stage vessels or through the entire system, open the concentrate flush (bypass) valve or throttling valve if necessary to prevent any restriction on the solution return line.
6. Re-engage the CIP transfer pump with its outlet throttled to send the solution through the stage/system being cleaned while sending the concentrate and permeate to the CIP tank. Slightly open a permeate sample port on one of the vessels being cleaned to verify flow. Slowly open the pump throttling valve as necessary until any one of the following limits is achieved:
   • The permeate sample port shows a vacuum, in which case, immediately reduce the flow or throttle the concentrate return.
   • A maximum stage pressure drop of 60 psi is reached.
   • A flow rate of 45 gpm per (8” diameter) parallel vessel in the stage being cleaned is achieved. For example, if three parallel vessels are being cleaning, the maximum recommended flow rate is 135 gpm.
7. The circulation time required for each stage will depend on the type of fouling and its severity. Thirty minutes may be sufficient if the fouling is light, while more time may be required for heavy anaerobic biological deposits, heavy clay/silt, and for calcium sulfate fouling. Record data initially and every 15 minutes during cleaning. Make adjustments to the cleaning solution pH or its flow rate as necessary to meet guidelines above. If the solution temperature approaches 107° F, stop cleaning to allow the solution to cool. Note: When cleaning fouling solids that have caused the stage/system feed-to-concentrate pressure drop to increase, the cleaning pressure may be used to gauge how well solids are being removed and when the cleaning is completed.
8. Repeat steps 6 and 7 for all remaining stages to be cleaned.
9. After the solution has been circulated through each stage/system, divert the remaining solution in the CIP tank to the appropriate discharge location. Some plants may require that this solution first be pH neutralized prior to discharge. If a source of permeate water is available, rinse and fill the CIP tank. Use this water to displace the contents of the RO to the appropriate discharge or back to the CIP tank.
10. Replace the RO prefilters if they have experienced an increase in pressure drop while cleaning. Reconnect the RO to its normal water source and return all valves to normal, except divert the permeate to drain or to the CIP tank until its quality meets the plant's process water guidelines.
11. When the RO performance has stabilized, record a complete set of its operating data.

We encourage all operators to closely review and follow the RO membrane manufacturer's cleaning procedures prior to conducting a cleaning, as every system is different. For additional information regarding RO membrane cleaning, contact your U.S. Water sales representative.