Membrane and Desalination Technologies - TCE Moodle Website

Membrane Technologies for Point-of-Use and Point-of-Entry Applications 617
Treated
water
(dilute)
Storage tank
Water flow
Semipermeable
membrane
Feed water
under pressure
3.3. Types and Configuration of Reverse Osmosis
Waste stream
to drain
Rejected
contaminants
(concentrate)
Fig. 14.6. Water across the membrane from the concentrated solution into the more dilute solution in
reverse osmosis (30).
Normally, the central part of the reverse osmosis for POU/POE system is the module
which is a pressurized container that houses the semipermeable membrane. Several square
feet of this membrane are arranged to form the module. There are two types of membranes,
one for chlorinated supplies and another for nonchlorinated supplies. It is important that the
proper membrane is used. Chlorinated water can damage the membrane used for nonchlorinated
water.
Reverse osmosis membranes are generally manufactured from cellulosic types (cellulose
acetate, cellulose triacetate, or blends) or polyamide materials (and their respective derivatives).
Very thin membranes are made from these synthetic fibers. However, there are various
advantages and disadvantages associated with each. The advantages of cellulosic membranes
are their lower cost and ability to tolerate chlorine, which curtails the growth of microorganisms
in the system. However, cellulose membranes are susceptible to biodegradation and
must be operated within a relatively narrow pH range of about 4–8; however, they do have
some resistance to continuous low-level oxidant exposure (66). In general, for example,
chlorine doses of 0.5 mg/L or less may control biodegradation as well as biological fouling
without damaging the membrane. Polyamide (PA) membranes, by contrast, can be used under
a wide range of pH conditions and are not subject to biodegradation (66). Although PA
membranes have very limited tolerance for the presence of strong oxidants, they are compatible
with weaker oxidants such as chloramines. PA membranes require significantly less
pressure to operate and have become the predominant material used for RO applications (66).
The membrane made from cellulose acetate, cellulose triacetate, or blends can produce an
adequate capacity for most households, but thin-film composites (TFC) should be used if