When you use the term "membrane technology", it encompasses a lot of technologies that are centered around various membrane-based components. Aussiemike refers to one, reverse osmosis. I too am not aware of any reverse osmosis systems that work well in the presence of iron and manganese. However, there are several others that do.
Hollow-fiber and tubular ultrafiltration systems work well when configured for high velocity-tangential flow. This is typically referred to as a "feed and bleed" operation. The high shear velocity keeps the metal oxides from plugging the boundry layer at the membrane surface. This results in substantially higher flux rates over single-pass configurations. These polysulfone membranes are then cleaned periodically with a mild acid solution, in order to restore flux. My experiences are based on the Romicon hollow-fiber membranes manufactured by Rohm & Haas. They are no longer available from Rohm & Haas, but I believe that Koch (Wilmington, DE) purchased the rights to make them.
Another method, involves using dynamically-formed membranes on a tubular ceramic substrate. As with the above, this method is typically set up using the feed and bleed configuration for high velocity tangential flow. In my personal experience, we were primarily removing boron from a nuclear reactor cooling water system, however, iron and manganese were present and easily removed by the process. As with all dynamically-formed membrane processes, the flux rate is re-established by dissolving the old membrane and then forming a new one.
About five to ten years ago, Smith & Loveless had purchased a technology from overseas (Denmark, I think). It is a membrane technology that they were promoting for reverse osmosis pretreatment to remove iron, manganese and to reduce silt. They also recommended it for municipal drinking water, particularly iron and manganese. As I recall, it was based on hollow-fibers and it appeared to be single-pass configured. It had a quirky name (not Zee-Weed, but something like that). It used to appear in trade magazines, but I haven't seen it lately.
And finally, electro-dialysis reversal is another membrane technology that is proven to work well on very high iron and manganese bearing waters. I don't have any direct experience with this technology, but I have read numerous testimonials to it's success with a wide range of typical membrane foulants, including iron and manganese. There are a number of communities in Texas that have extremely high TDS coupled with iron and manganese and, after failing miserably with reverse osmosis, electro-dialysis reversal was the only method that worked.
For obvious reasons, if you are just trying to remove iron and manganese from water, the conventional filtration-based or sedimentation-based methods are the least expensive ways to go. But, if you need to remove other contaminants that can only be removed by membrane, as well as iron and manganese, then the membrane approach can be worth the cost.
S. Bush
