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Presently there are no clear-cut criteria on how to rate the
durability or the useful service life of membrane modules, nor what properties
or metrics to stipulate or measure. Deterioration in filter performance is classified
in two broad categories. One is membrane degradation from mechanical, chemical,
or biological causes. These may be membrane compaction, decomposition or degradation
of the membrane material, or actual membrane rupture. The second is degradation
in performance due to membrane fouling or blocking/occlusion of the module's flow
channels.
1- Material Degradation
Degradation of membrane module materials depends on use and exposure. Factors
such as temperature, water quality, physical loading, as well as cleaning frequency
and the membrane's resistance to the cleaning chemicals employed. Off-line module
storage conditions also impact service life.As a general rule, a three year service
life is a reasonable benchmark for modules with organic membranes. For membrane
modules with inorganic membrane, a seven year service life can be expected.
2- Filter Performance Degradation
With extended processing the membrane filter capacity tends to decline with an
attendant increase in trans-membrane pressure. There is added resistance to permeation
due to formation of a fouling layer at the membrane surface consisting of suspended
solids, dissolved high molecular weight polymers, and compounds having low solubility.
There may also be clogging of the porous support structure of the membrane and/or
blockage of the module flow channels. Filter performance may often be recovered
with hydromechanical techniques such as backwashing and air-scrubbing. Modifying
operating conditions may also help prevent membrane fouling. Chemical cleaning
is used to restore filter performance when the physical techniques prove inadequate.
Eventually the membrane modules are replaced when chemical cleaning becomes too
frequent or is no longer adequate. |
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