RO membrane cleaning should be performed with high and low pH cleaning chemicals. The high pH cleaning should always be performed first to remove biological or organic foulants, both of which contain carboxylic functional groups. These weak acids gain an anionic charge at high pH, allowing them to disperse. High pH cleaning chemicals chelate the calcium that bridges foulants and biofilms to each other and to the membrane surface. Commodities such as NaOH are only mildly effective because of their inability to remove the calcium bridging.
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 can end up being compacted into the membrane.
Membrane cleaning 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 an RO Membrane Cleaning
When performing a cleaning, 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 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. Cleaning is determined to be complete when the pH stabilizes.
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 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.