Even with all the preventative care given to a reverse osmosis system, some fouling of the membranes will take place. Cleaning of the membranes can improve membrane performance.
All membranes lose their performance with time. One of the major causes for the loss of performance is due to substances that deposit on the membrane surface. Although the term fouling is used for deposit of any materials on the membrane, the coating of the membrane surface can be due to the following:
Fouling of membranes is due to the suspended or emulsified materials that may be present in the feed water to the reverse osmosis system. Examples of such materials are: silica, oil, clay, iron, sulfur and humic acids. These substances can be present in a very fine or colloidal form. Even the typical 5 micron cartridge filters used upstream from a reverse osmosis system may not completely remove these foulants.
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