Reverse Osmosis Water Filters Buyer's Guide
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  We review, analyze and compare the best water filters in the industry including reverse osmosis systems, faucet filters, water distillers, pitcher filters, whole house filters, water softeners, shower filters, bottled waters and more.

Chemistry of Reverse Osmosis


water ionsReverse osmosis membranes are widely used by water companies throughout the world to purify water for human use and consumption. Millions of people a day drink and bathe in water that has been treated by these filters, but yet very few people have even heard of reverse osmosis much less understand how it works. Designed and then perfected over a span of 60 years by the foremost scientists in the field, reverse osmosis membrane technology is indeed a scientific achievement and marvel; however, understanding the basic process of the technology is something that everyone can understand.

water chemistry

Reverse Osmosis Basic Operation

Reverse osmosis (RO) is basically just a very restrictive filter that works by separating and removing a high percentage of contaminants from the water. RO systems use pressure to force water through a pressurized chamber separated by a semi-permeable membrane. Water contaminants down to the absolute smallest particles and microbes are blocked and rejected by the 0.0001 microns membrane, while water molecules easily pass through to the other side to be collected as pure water. When used with sediment and activated carbon prefilters, reverse osmosis systems can effectively remove up to 99% of all contamination making water safe and potable.

Types of Reverse Osmosis Membrane Elements

The earliest membranes created in the 1950s were made from cellulose acetate (CTA) and were asymmetric semi-permeable elements. These membranes were fabricated in a one step process by casting a film or fiber from a solution of cellulose acetate and solvent. CTA membranes were the first commercially operated reverse osmosis filters in the market, and they offered good salt rejection rates of 86-94% with an average 2 year lifespan. In the early ‘70s, a new form of membrane known as Thin-Film composite (TFC) was created by the Filmtec company to address many of the limitations of CTA membranes. These new elements consisted of layers of dissimilar materials joined together to form a single membrane. This multi-layered design benefitted from the use of material combinations that optimize the performance and durability of the membrane. TFC elements were more effective, versatile and long lasting than CTA elements while offering superior rejection rates between 94-99%. The only advantages of CTA membranes were lower cost and chlorine tolerance, therefore TFC quickly became the standard for modern reverse osmosis filtration technology. ro system

Reverse Osmosis Membrane Materials and Fabrication

Modern reverse osmosis TFC membranes normally consist of three layers: a polyester support web, a microprous polysulfone interlayer, and an ultra-thin polyamide barrier layer on the top surface. The combination of these three layers working in synergy is what allows TFC elements to deliver high contaminant rejection, fast filtration rate, and good mechanical strength. The top layer polyamide is the key barrier that is responsible for high rejection as it is highly permeable to water but impermeable to most dissolved impurities. This polyamide is a synthetic polymer material made from 1,3 phenylene diamine and the tri acid chloride of benzene and it is highly chemically resistant and structurally strong. The strength and versatility of these TFC component layers make them ideal for use in spiral wound, crossflow membranes to effectively treat water through a wide range of pH and temperature variations.

Residential RO System Configurations

Most home reverse osmosis systems you see on the market will consist of either 4-5 stages of filtration made up of sediment and activated carbon prefilters, followed by the TFC RO membrane and then a carbon post filter. TFC membranes are always placed behind the prefilters because they are not chlorine resistant and are susceptible to chlorine fouling and damage. As the 4th stage filter, the membrane is further protected from clogging by the frequent replacement of the annual prefilters. Inline carbon and UV light post filters are often installed after the membrane for further quality assurance. This optimized multi-stage configuration increases the life and performance of the membrane while offering the greatest range of contaminant removal for the system.