3030226 Rev. D 5-12 Routine Maintenance
5.3 Cleaning Indications
Foulants such as minerals, silt and organic substances may collect on the RO membrane
surface and cause the RO membrane to foul and diminish in its function. Many of these
foulants are prevented by proper selection of pretreatment equipment, while others
are treatable only with more sophisticated designs. Regardless of the foulant, it is
essential to clean the membrane promptly once cleaning is indicated (typically loss of
flow or decreasing rejection). The longer a foulant is allowed to remain in contact with
the RO membrane, the more difficult it will be to remove. In severe cases, repeated
cleanings may be required.
NOTE:
Some foulants CANNOT be removed with cleaning and membrane performance
may not be fully restored. Observe the appearance of the cleaning solution
before discarding it. If the solution is heavily soiled, repeat the cleaning
procedure with fresh solution of the same type (high or low pH).
Routine cleaning of the RO membranes in conjunction with regular
disinfection/sanitization will keep bacteria counts at a minimum and the RO membrane
performance at its best. Cleaning removes the silt, organic and mineral deposits that
collect on the RO membrane surface, which if left untreated, encourages bacterial
growth and will eventually impair the RO membrane’s ability to perform.
Sanitization/disinfection kills most of the bacteria that may be present in the system
but it does not remove the biofilm from the membrane.
There are a variety of indications for cleaning membranes. The most common is a
reduced product flow rate. Remember that a reduction in inlet water temperature can
also result in a reduced flow rate. Refer to Technote 113, "Temperature Correction
Factors", to determine the reduction in flow that can be attributed to reduced water
temperature. Clean the membranes whenever the % rejection drops by 5%. A loss in
the membrane performance (percent rejection) or increase in TDS/µS of the product
water may also be an indication to clean the membranes. If the current cleaning
schedule is not effectively maintaining acceptable readings, more frequent cleaning
may be required.
CAUTION:
Membranes that have not been cleaned for a long time may actually incur
damage during cleaning as perforations are exposed that were masked by
accumulations of foulant. Regular maintenance cleaning is the best assurance of
long membrane service.
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