Membrane Autopsy
By Ryan Lessing, WATTS Water Technologies
Introductory Note: The following piece, by Ryan Lessing of the Engineering Dept. of Watts Water Technologies, San Antonio, TX, is reprinted from the Winter 2011 issue of H2O Quality magazine, a publication of the Texas Water Quality Association. Ryan’s article is intended for large reverse osmosis membranes, but the same principles can be applied to small residential undersink RO membranes. Residential membranes usually last for several years. When they fail prematurely, applying Ryan’s autopsy procedure may allow you to diagnose and fix the issue to protect subsequent membranes. For example, if it appears the membrane is failing due to hardness scaling, pretreatment to soften or condition the water’s hardness can prevent future failures; if the membrane appears to be damaged by chlorine, you may need to change your carbon prefilter more often; if you cut the membrane open and it’s full of dirt, you definitely need to change your sediment and carbon prefilters more often. –Gene Franks, Pure Water Products.
Membranes can be cleaned by following the cleaning chemical manufacturer’s
guidelines. As a rule of thumb, if the membranes require up to 15% more feed pressure
to make the same amount of permeate as when they were new or make up to 15%
less permeate at the same feed pressure as when they were new they can be cleaned.
Exceeding either of the 15% benchmarks may mean the membranes may not respond
well to cleaning.
Autopsying a membrane is helpful in determining what is causing the scaling or fouling
issue. I follow a simple procedure that can be very telling. Begin by making a shallow
cut to the membrane’s outer wrapping from top to bottom. Remove the tape or fiberglass
wrapping. Again make a shallow cut from top to bottom, this time through the first layer
of the membrane. The membrane should now unroll like a roll of paper towels. Look at
what is on the membrane surface. At this point, given your water analysis results, you
can begin to draw some conclusions.
RO membranes consist of many layers of membrane fabric wrapped around a central core.
Red may indicate the presence of iron or clay, or both. Grey to black could indicate
manganese (gray could also mean silt). Fine loose powder could be silt. Hard grit (with
a caked-on sand paper-like texture) is scale. Dry a section of the scale and put vinegar
on it. If it foams up it is most likely calcium carbonate and this means the softener is
not working properly. If the scale looks like sugar crystals and does not foam when
vinegar is applied it may be a sulfate-based scale. Calcium is still required to make this
form of scale so the softener could still be the problem. Is the membrane slimy? Let the
membrane warm up to room temperature and smell it.
Does it have a fishy smell? Microbiological fouling could be the problem. With each
of these tests, look back at the pretreatment responsible for addressing that issue. Is it
functioning properly, is it sized properly? Then you can make the proper adjustments or
add what component might be missing.
