In normal operation, the membrane in reverse osmosis elements can become fouled by suspended solids, microorganisms, and mineral scale. These deposits build up during operation and cause loss in water output or salt rejection, or both. Regular cleaning of the membrane elements minimizes the loss of performance and extends membrane life. Membrane elements should be cleaned whenever the normalized permeate water output rate drops by 10% from its initial flow rate (the flow rate established during the first 24 to 48 hours of operation), when salt passage in the product water increases over 5-10%, or when normalized pressure drop across the membrane increases by 10-15%.
What Type of Membrane Cleaner to Use
Whether the membrane needs acid cleaner or alkaline cleaner will depend on the type of foulant suspected. If calcium carbonate scaling is the known problem, acid cleaning alone may be sufficient. Otherwise both kinds of cleaning are needed and it is recommended to start with the alkaline cleaning then follow with the acid cleaning after the system has been flushed.
Type of Fouling
Type of Cleaner
Membrane Cleaning Chemical
The RO Membrane will likely exhibit low permeate flow, but salt rejection will usually be as good if not better than original test. The membrane may have strong odor, possible mold growth on scroll end.
It is recommended to start with the alkaline cleaning then follow with the acid cleaning after the system has been flushed. 1. Alkaline Cleaning - AM-22 (if required) 2. FLUSH 3. Acid Cleaning - AM-11 4. FLUSH Note: Acid cleaning may be performed alone, but alkaline cleanings should always be followed by an acid cleaning after the system has been flushed.
RO Membrane Fouling and Scaling
The concentration of all materials in the feed water - dissolved and suspended - is highest near the membrane surface. As permeate is removed through the membrane, all impurities are left behind near the membrane surface. The layer of water next to the membrane surface (boundary layer) gets more and more concentrated in the dissolved and suspended materials. These concentrations reach a certain steady level depending on feed velocity, element recovery and membrane permeate flux (gallons per square foot of permeate produced per day).
It is important to follow membrane manufacturers' recommendations on minimum feed flow, maximum element recovery and maximum element flux. These recommendations are based on the element size and quality of feed water being treated. The concentrations of the dissolved and suspended solids in the boundary layer control the performance of the membrane. Higher concentrations mean higher osmotic pressure, higher tendency of suspended solids to coagulate and coat the membrane surface, and higher likelihood of scaling to take place. Maintaining proper operating conditions for the membrane is the key preventative step to minimize membrane fouling.