Reverse osmosis, as
perplexing as the name appears, simply involves forcing water through a semipermeable
membrane. This highly-advanced filter is the secret sauce behind the
process's high filtration rate. For most solids like asbestos and toxic metals,
the filtration rate is more than 99 percent (but not perfect). For monovalent
ions like sodium, however, the rate is 95 percent.
This secret sauce
usually comes in one of two forms: polyamide thin film composites (TFC) and
cellulose acetate (CA). Here's a rundown of each.
Polyamide TFC
The most widely-used
desalination and filtration membrane, polyamide TFC consists of ultra-thin
films that are durable and can trap impurities better than cellulose-based
materials. Its biggest drawback is chlorine, meaning it's hardly ideal (but not
impossible to use) on filtering pretreated water. Some manufacturers have
developed proprietary alternatives.
Cellulose Acetate
Initially designed as
a substitute to the flammable cellulose nitrate in film reels, CA is the oldest
reverse osmosis membrane in use. This is the polar opposite of polyamide TFC,
as far as pros and cons go. CA is highly-tolerant of chlorine and inexpensive,
but they operate at narrower pH ranges and aren't ideal for desalination.
Cellulose triacetate
is a sub-form of CA, normally a blend of multiple CA.
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