Membrane fouling in reverse osmosis (RO) systems is as all pervasive and inevitable as the
common cold. Avoidance is just as hit-or-miss: wash your hands, don't get your feet wet and
hope for the best. The many unseen factors influencing whether a person becomes ill parallel
the many variables that determine the onset and severity of fouling in a membrane system.
Design, operation, and maintenance all play a role. Accepting that fouling is inevitable is the first
step in effectively anticipating, compensating for and counteracting the fouling phenomena.
Ultimately, this mindset can help operators keep their systems as "healthy" as possible.
Fouling is a broad, generic term used to identify a multitude of time-dependent phenomena,
which, singly or in combination, impact membrane performance. Just as illness can take many
forms -- from a relatively mild cold to life-threatening cancer-- fouling also can have a range of
Some degree of fouling has already been accounted for in any properly designed system. From
a practical standpoint, fouling is of interest only when it negatively affects a system's
Calculations such as the Langelier saturation index (LSI) or silt density index (SDI), found in
ASTM standards D4189-94 and D3739-94, respectively, can provide insight into the fouling
problems that are inherent in any membrane system. Ideally, these calculations are used in the
original design of the system. However, operators may find it useful to work through the
calculations to anticipate the potential effects of a system upset.
While overall flux performances will decline for all membrane systems, the actual time between
cleanings will vary, depending upon membrane type, system design and feedwater quality.
Recognize the signs
Knowing ahead of time that your system is susceptible to scaling will help you diagnose
recurring problems like persistent flow loss, which occurs near the end of each softener cycle.
Perhaps your system is running close to a positive LSI, and a relatively small increase in
hardness is enough to cause scaling.
For proper diagnosis, it is crucial to have a reasonable baseline available. A 50-year-old man
should not expect the results of his physical exam to compare favorably to the physical he had
at age 18. Over time any membrane's performance will deteriorate (figure 1). In addition,
fluctuations in temperature, feedwater composition and other factors are common and can
affect the apparent membrane performance.
Ongoing tracking of system performance-- and compensation for the variations in pressure,
temperature, feed composition and recovery-- provide the operator with useful comparisons.
The chart on below lists some common fouling symptoms and possible diagnoses. Also listed
are some reactive, troubleshooting "cures" and long-term, preventive steps to take for each
condition. Again, each system comes with its own health history and predispositions. There is
no substitute for a thorough, ongoing understanding of your system's operating norms and
Membrane Fouling Symptoms and Treatments
Symptoms Likely "Disease" Diagnostic Steps How to
Significant increase Scaling Check water analysis. Acid Clean
Calcium, Recalculate LSI. Reduce recovery.
Moderate increase in magnesium or
DP salts (most Adjust pH.
Slight to moderate Use antiscalant.
flow loss Barium or
strontium (less Increase softener
All effects usually
most noticeable in
last stage or element Metal hydroxides
(for example, iron)
Significant increase Dissolved Run SDI or particle High pH Clean
in DP Organics/ sizing to determine if
Suspended pretreatment is effective. (Chelants,
Moderate flow loss Colloids Detergents)
Check for visible foulant
Moderate increase in Clay or silt in membrane elements Improve
passage particles, often and housings. prefiltration.
inorganic Compare total organic
All effects usually Increase crossflow.
constituents compound (TOC) levels
most noticeable in
first stage or in last in the feed with baseline
element Humic and fulvic measurements.
Rapid and significant Suspended Check fluid-membrane Often Hard to
flow loss Organics compatibility. Clean
No or slight to Fats and oils Check for contamination Change membrane
moderate increase in of system. type.
compounds Remove organics
Generally no or slight with granular
increase in DP Synthetic activated carbon
coagulation (GAC) upstream.
High pH soaks may
Significant, steadily Biofouling Check micro-counts Sanitize
increasing flow loss (cfu/mL) in
Biofilm concentrate/permeate. Eliminate deadlegs.
Slight to moderate
increase in passage "Free-floating" Check for visible Do not allow
microorganisms evidence in system system shutdowns,
Moderate but rapidly (not likely without (slime layers). unless biostat used.
increasing DP concurrent biofilm)
Check sampling valves Sanitize all system
for contamination. components,
Because many factors play a role in fouling, it is nearly impossible for operators to calculate the
individual impact of each, much less determine the interrelated effects.
Pretreatment, be it chemical adjustment or physical filtration, can compensate for your system's
specific weaknesses. Knowing your system's potential frailties ahead of time will allow you to
balance its need for sufficient pretreatment with economic realities.
For example, use of high-flux membranes increases the probability of particulate fouling. By
increasing the flux rate, you increase thc likelihood of particles contacting the membrane
surface. Because more water is moving through the membrane, the drag due to filtration is
increased and more foulants may be carried to the
Increased fouling is obviously more likely in this situation, but to mathematically determine
the absolute effect is an extremely complex undertaking (figure 2).
Biofouling is very much the "Cancer" of Membrane Systems
Everything from the roughness of the membrane to the fluid viscosity must be considered. A
more common-sense approach is to recognize this general predisposition and increase the
crossflow, or use media or depth filtration to minimize the particulate loading.
Similarly, persons with a family history of hypertension are well-advised to take reasonable
precautions without insisting that each specific risk factor be calculated for every possible
combination of lifestyle choices.
A Membrane´s Biggest Threat
Biofouling is perhaps the most difficult fouling threat to overcome. It is very much the "cancer" of
membrane systems. Micro-organisms are present in every feed stream, and no system operator
can afford to ignore the potential for biofouling. Just as with human cancer, prevention is
preferred. Eradication of the growth always requires aggressive measures.
Once corrective measures have been taken, consistent follow-up treatment and monitoring are
vital to ensure that the biofilm is in remission. In most cases, a "cure" is not possible, and control
is all that can be achieved.
The key to biofouling control lies in a sanitization approach very much like chemotherapy. All
aspects of the system must be contacted with the sanitizer, and the treatment must occur on a
regular basis. Abbreviated or weak sanitizing brings with it the same effects as discontinuing an
antibiotic treatment prematurely: the surviving micro-organism population continues to grow and
may eventually become resistant to the treatment.
The trend in healthcare today is to manage and prevent. A similar approach to membrane
system fouling can provide many benefits for operators, including time and cost savings and a
healthier, relatively stress-free work environment.
With technological advances in system design and control, it is now much easier to predict,
compensate for and counteract fouling conditions. Approaching the problem through a holistic
combination of system design, operation and maintenance will reduce the service interruptions
and quality problems which can prove to be so unhealthy to a company's bottom line.
November 1995 Water Technology 69