Reverse Osmosis Water Purification - How the Process Works

Reverse osmosis, also known as hyperfiltration or ultra-filtration, was developed with U.S. government funding as a means of desalinating ocean water. A reverse osmosis water purifier uses a semi-permeable membrane that allows pure water to pass through it, while contaminants are trapped by the tiny pores in the membrane.

The process requires that the water be pressurized to help force it through the membrane. Most standard residential water systems have sufficient pressure. Like activated carbon filters, reverse osmosis also uses charged particles to help filter out impurities.

The effectiveness of a reverse osmosis water purifier depends on the density of the membrane. A membrane with larger pores will obviously let larger impurities pass through. It is also important that the membrane be cleaned regularly for proper functioning.

Higher quality reverse osmosis water filters use a process known as crossflow to allow the membrane to continually clean itself. Crossflow directs some of the water downstream from the membrane, sweeping the rejected contaminants away so they do not build up and block the membrane.

There are two types of membranes commonly used in reverse osmosis water filters --Thin Film Composite (TFC) and Cellulose Triacetate (CTA). TFC membranes are noticeably more effective the CTA membranes but they tend to be less resistant to the deteriorating effects of chlorine. This problem can be avoided by using an activated carbon pre-filter to remove the chlorine first.

Reverse osmosis water purifiers come in different sizes and styles including countertop and whole house water filters. They are capable of filtering out a long list of contaminants including:

• chlorine and other treatment related products
• bacteria
• salts, sugars, proteins
• dyes
• heavy metals

One drawback of reverse osmosis water filters is that they are more expensive than carbon filter treatment systems, with the average home system costing around $300. Also, the purification process is usually somewhat slow due to the use of pressurized water. It can take an entire day to produce 15 gallons of purified water. And a typical reverse osmosis water purifier may need up to 8 gallons of untreated water to generate a single gallon of purified water.