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.
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.
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.