When it comes to membrane filtration and fouling—specifically for microfiltration (MF), ultrafiltration (UF), reverse osmosis (RO), and nanofiltration (NF)—diagnosing system performance issues can be complicated. Even the slightest change to operating procedures, stream composition, pH, temperature, etc., can alter the corrective measures needed to keep membranes working properly. But it’s also important to stay ahead of fouling because when it does happen, production levels decrease, energy consumption increases, and damaged equipment often needs replacing, which can all add unnecessary expenses to your facility’s bottom line.
In short, it’s always best to ensure membranes are properly cared for, staving off fouling in the first place and guaranteeing optimal membrane performance and longer lifespans.
Since fouling is inevitable, however, you might be wondering what are the signs a membrane is fouling, and once fouling begins, how can you clean it? The following is an overview of some of the most common membrane fouling issues industrial facilities experience, including some possible solutions.
Signs your membranes are fouling
Since there are several variables to consider when assessing membrane fouling (what are the system dynamics? operating conditions? solute properties? etc.), we always recommend consulting your water treatment or membrane specialist before making any assumptions. However, these are generally the common signs:
Increasing transmembrane pressure
The amount of pressure required to push solvent through a semipermeable membrane is known as transmembrane pressure. The more concentrated the stream is, the more pressure is needed to force the permeate through. Similarly, if foulants adhere to the membrane, even more pressure will be required to force the liquid through. When the transmembrane pressure increases sharply or in increments over time, fouling, whether colloidal, biological, or organic, can often be suspected and cleaning measures taken.
Water quality decline
If membranes exhibit poor salt rejection, sometimes in conjunction with a decrease in permeate flow, fouling can be to blame. Sometimes this can be a symptom of poor pretreatment methods, so be sure to check the foulant’s composition, as this can sometimes lend some clues. If it’s colloidal in nature, cleaning and enhancing pretreatment might help. If the poor salt rejection is accompanied with an increase in transmembrane pressure, the membrane or system could be experiencing scaling, which might require lower system recoveries, adjusted antiscalant treatment, or better pretreatment measures.
Strong odors and mold
Usually, if the membranes begin to form an odor, biological growth could be the culprit. In this case, the filters might have slime gathering at the surfaces or evidence of mold along the ends. An adjustment in temperature is sometimes required to prohibit future biological growth, and some type of biocide or alkaline cleaner might be used in this case, either once to clean the biological foulants or repeatedly to help keep them from growing again.
Accumulations along membrane edges
If you notice solids forming on the edges of the membrane, it could be an early sign that something isn’t right. Occasionally solids and elements like silt or rust will start to collect at the scroll ends, but if it begins to occur more than usual, it can be an indication to investigate before these foulants become a bigger issue. Depending on the foulant in question, you may need to run an acid cleaner through the membranes in addition to regular flushing or backwashing. If you see this type of accumulation, be sure to flag it and consult with a specialist to ensure it’s not indicative of a larger issue.
Ways to clean a fouling membrane
Membrane fouling is sometimes reversible—but not always. That’s why it’s best to implement preventative measures to avoid or minimize membrane fouling in the first place. A systematic cleaning regimen can help to prevent foulants from building up on the membrane. Cleaning cycles should be scheduled monthly or at other regular intervals to provide the greatest benefit.
Below, we’ve outlined some common cleaning methods used with membrane filtration, which will vary depending on the type of foulant, system configuration, and operating conditions.
- Mechanically: involves the use of physical force to loosen contaminants from the membrane and flush them out of the system. Typical approaches include vibration of the water during cleaning, as well as backward or forward flushing of the brine side, where water or a cleaning solution is run through the unit at a faster speed or higher pressure than in a normal service cycle, resulting in turbulence that removes foulants from the membrane. In a related process known as air scouring, used with UF, air is added to the backwash/forward flush solution to further increase turbulence.
- Chemically: the application of detergents, caustics, acids, antiscalants, or dispersants to loosen and remove foulants from the membrane surface. Biocides are also used for removing microorganisms can potentially foul the units. Cleaning chemicals are selected based on the type of contaminants present, with consideration also given to the membrane material to ensure that the chemicals used do not damage it.
Can SAMCO help?
SAMCO has over 40 years’ experience custom-designing and manufacturing membrane filtration systems for a range of industries and applications, so please feel free to reach out to us with your questions.
For more information or to get in touch, contact us here to set up a consultation with an engineer or request a quote. We can walk you through the steps for developing the proper solution and realistic cost for your MF, UF, NF, or RO treatment system needs.
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