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Membrane fouling in reverse osmosis RO systems is as all

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Membrane fouling in reverse osmosis RO systems is as all Powered By Docstoc
					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
effects.

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
productivity.

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
quirks.

Membrane Fouling Symptoms and Treatments


     Symptoms            Likely "Disease"         Diagnostic Steps                How to
                                                                               Clean/Control

Significant increase     Scaling               Check water analysis.        Acid Clean
in passage
                         Calcium,              Recalculate LSI.             Reduce recovery.
Moderate increase in     magnesium or
DP                       salts (most                                        Adjust pH.
                         common)
Slight to moderate                                                          Use antiscalant.
flow loss                Barium or
                         strontium (less                                    Increase softener
                         common)                                            regeneration
All effects usually
                                                                            frequency.
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.
                         organic and
                         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
                                                                            Reduce recovery.
element                  Humic and fulvic      measurements.
                         acids
                         (tannins,"color")

                         Some synthetics,
                         like common
                         herbicides or
                         pesticides

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.
passage                  Hydrocarbon
                         compounds                                          Remove organics
Generally no or slight                                                      with granular
increase in DP           Synthetic                                          activated carbon
                         coagulation                                        (GAC) upstream.
                         /floccing agents
                                                                            High pH soaks may
                                                                            be effective
                                                                            depending on
                                                                              organic type.

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,
                                                                              including
                                                                              pretreatment.




Factors Abound

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
surface.



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

				
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