Membrane and Desalination Technologies - TCE Moodle Website

586 J.P. Chen et al.
Chlorination of feed water prior to membrane pretreatment may extend membrane run
times between cleaning. Over 90 h of MF operation was achieved with prechlorinated feed
water compared with 42-h operation reported when feed water was not chlorinated (59).
Similar observations were reported with dosage of chloramine prior to MF pretreatment (53).
It was speculated that preoxidation resulting from chlorination altered the chemistry of EPS
produced by the microorganisms in the feed water. This could weaken the attachment of the
EPS on the membrane and thus offset the detrimental effect on the membrane flux. However,
care must be taken to verify the compatibility of the membrane with chlorination as some
membranes are not tolerant to the aggressive action of chlorine.
4.3. Membrane Cleaning and Regeneration
Fouling is almost an inevitable consequence of the nature of the RO process itself even
when good pretreatment is employed. Furthermore, the high operating pressures compress the
membrane and cause further loss of membrane properties. During operation, symptoms of
fouling would include one or all of the following conditions:
(a) Decrease in normalized water flow by 10–15% from startup (reference) conditions.
(b) Increase in pressure drop over a stage or the system by 10–15%.
(c) Significant decrease in salt rejection over time.
The challenge is, therefore, to reduce and control fouling sufficiently to minimize the rate
of RO flux decline and prolong membrane lifetime. This can be accomplished by a combination
of good feed pretreatment as aforementioned and membrane cleaning programs.
Periodic cleaning is essential to improve the performance and to prolong the life of RO
membranes. Cleaning methods used for RO membrane restoration could be broadly categorized
into two types: chemical and physical.
4.3.1. Chemical Cleaning Methods
Most membrane manufacturers recommend chemical methods for membrane cleaning and
regeneration. Chemical cleaning methods depend on chemical reactions to weaken the
cohesion forces between the foulants, and the adhesion forces between the foulants and the
membrane surface. Chemical reactions involved in cleaning include hydrolysis, peptization,
saponification, solubilization, dispersion, chelation, sequestering, and suspending. The
choice of chemical cleaning agents not only depends on the type of foulants present in
the membrane system but also on the chemical resistance of the membrane material and
the whole system. Caution must be taken in applying these chemicals as the aggressive nature
of these chemicals may adversely affect the system or membrane integrity when not applied
properly. This is especially true for aromatic PA membranes, which are less robust than CA
and polysulfone membranes. For example, some cationic and nonionic surfactants may
adsorb onto PA membranes and cause a flux decrease. PA membranes are also not resistant
to the strong oxidizing actions of disinfectants such as hydrogen peroxide and hypochlorite.
Acid used to clean the membrane system must be rinsed out thoroughly before application of
hypochlorite for disinfection, because hypochlorite at low pH can cause corrosion in stainless