RO membrane cleaning should be performed with high and low pH CIP chemicals. The high pH cleaning should always be performed first to penetrate and disperse biological or organic foulants. Specialty high pH cleaning chemicals more effectively penetrate heavy organic and biological foulants, making them more effective then commonly used CIP chemicals such as sodium hydroxide. While many choose the lowest cost CIP chemicals, their annual cost will end up being significantly higher because of the higher frequency of cleaning; this incurs more labor costs, chemical costs, and downtime. It’s always best to perform a cleaning study to ensure that the membrane performance is fully restored after CIP.
The use of citric acid is recommended prior to high pH cleaning by some companies, due to its chelating properties. We strongly recommend against this practice – Organic foulants lose their anionic charge if a low pH cleaning is performed first, and since some permeate is almost always produced during CIP, the foulants will be compacted into the membrane and become more difficult to penetrate.
What is the Best Method for Cleaning RO Membranes?
Membrane CIP results will be best if each stage is cleaned individually. This will allow for the maximum flow velocity during cleaning. If both stages of a system with a 2:1 array are cleaned simultaneously, each pressure vessel in the second stage will receive twice the flow velocity as the first stage. This would cause membrane telescoping in the second stage if the first stage elements are cleaned at optimal flow velocity. Alternately, by basing the flow velocity on the second stage, the first stage membranes would not receive sufficient surface scouring.
Performing RO Membrane Cleaning
The total volume in the system should always be considered when determining how much CIP chemicals to add. You can use the AWC RO CIP Calculator to help you calculate the correct amount of chemical.
When performing a cleaning of a RO system, the first 20% of the cleaning solution is flushed through the membranes directly to drain, instead of circulating. This will prevent contamination of the cleaning solution with loose foulants and large suspended solids.
The remaining solution is then circulated through the system while carefully monitoring the pressure differential (dP). dP should not exceed 10 PSI (0.69 bar) per membrane element. Every 30 – 60 minutes, the circulation can be stopped to allow the membranes to “soak” for about 30 – 60 minutes. Circulation should then be started again to flush off the disintegrated foulant and to bring fresh cleaning chemical to the membrane surface. pH should be checked every 15- 30 minutes during circulation. If the pH of the cleaning solution has changed, more cleaning chemical should be added to adjust the pH back to the target range.
Soaking and circulating can be repeated alternately until cleaning is complete. Low pH cleaning is determined to be complete when the pH stabilizes, but the length of a high pH cleaning should be based on a predetermined time. The length of time required or optimal CIP results can either be obtained by trial and error, or by performing a cleaning study.
A high pH cleaning is usually performed at a pH of 11 – 12 using a cleaning chemical such as AWC C-236 for silica, AWC C-237 for biofouling or AWC C-227 for heavy organics fouling. More chemical is added during the cleaning every time the pH drops below 11. If the solution becomes very dark or turbid, it should be drained and a new cleaning solution should be prepared.
A low pH cleaning should be performed at a pH of 2 – 3 using CIP chemicals such as AWC C-234 or AWC C-235 (phosphorous free). If the pH of the cleaning solution increases above 3 at any time during the cleaning, more cleaning chemical must be added to reduce the pH to its target range of 2 – 3.