If you’ve ever cleaned your RO membranes and the performance did not recover, odds are that you overlooked one or more of the “cardinal rules” of membrane cleaning. There are ten (10) common mistakes that can give you less than desirable results, so to determine what went wrong, we offer the following checklist:

1. Improper flow rates. The goal is to achieve high velocity at a low pressure during the cleaning. Remember, an 8” diameter system needs to be cleaned at a flow rate of 35-40 gpm per vessel, no matter how many membranes are in the vessel. A 4” diameter system needs to be cleaned at a flow of 10-12 gpm per vessel. The pressure during cleaning should never exceed 60 psig.

2. Not separating arrays. If you attempt to clean the whole RO system at once, proper flow rates cannot be achieved, foulants will be redeposited on lower flowing membranes, and pressure drop across the RO will cause foulants to be driven down to the membrane surface.

3. Wrong choice of chemistry. The cleaners need to be suited to the foulant being removed – low pH for metals and carbonate scales, high pH for sulfate and silica scale, a combination of low and high pH cleaners for colloidal fouling, biological and organic fouling. In severe cases, you can also consider cleaning membranes in reverse flow from concentrate to feed (but not permeate to feed!!!).

4. Wrong chemical concentrations. When diluting your cleaners, don’t forget to take into account the volume of water in the system: tank volumes, pipe and/or hose volumes, and membrane vessel volumes. These can really add up, diluting your cleaner to the point where it is ineffective.

5. Cleaning too late. Waiting until the membranes are severely scaled or fouled can render the best cleaning techniques ineffective. Data collection is key! Your normalization program will let you know when it’s time to clean.

6. Not enough or too much heat. We all know that heating the cleaning solution helps to speed up the reaction of the cleaners with the foulants. However, you need to keep a close eye on the temperature to prevent damage to your membranes. Most membrane manufacturers recommend cleaning at temperatures between 95°F and 110°F. Compatibility is based on the combination of temperature and pH. If in doubt, contact your membrane manufacturer for advice.

7. Attempting to clean damaged membranes and expecting improved performance. Cleaning will only improve the performance of your membranes if the membranes are not damaged. Increased product flow, decreased salt rejection, or increased silica leakage are all indicators that the membrane has been either mechanically or chemically damaged.

8. Cleaning outside the proper pH range. pH excursions can also damage your membranes, even in short durations, so it’s imperative to monitor pH throughout the cleaning cycle and adjust your chemical concentrations accordingly. High pH excursions will damage the permeate carrier; pH below 2 or greater than 12 will cause hydrolysis of the membrane. Both of these are permanent conditions.

9. Foaming in CIP tank. Foam will rise to the top of the RO pressure vessels and does not allow the cleaning solution to flow at the proper velocity. Hence, the foulant does not get properly wetted or removed.

10. Experience. What works with one system is not guaranteed to work on another – so if it works, keep doing what you’re doing.

As many of you know, membrane cleaning is as much art as it is science. If you would like further information, recommendations, or assistance with cleaning your membranes, contact your Process Solutions, Inc. sales representative. We’d be happy to help!