The reverse osmosis filtration process was originally developed more than 60 years ago as an affordable way to desalinate sea water and today’s recognised market leader in this type of drinking water filtration system is the Hydrotech System.

It works by means of a semi-permeable membrane that looks like cling-film plastic wrap and is made out of the same sort of polymers. This membrane first filters out the tiniest molecules of practically all solid particles including bacteria, asbestos, micro-organisms and organic material. This filter is so fine that its pores measure only two ten-millionths of an inch or 0.0005 microns and can barely be seen even under an excellent microscope. The same filter also removes toxic metals, mineral salts and other types of dissolved impurities by the process of reverse osmosis in which the impurities are repelled by the membrane’s surface while the purer water molecules are able to diffuse through it.

Reverse osmosis uses practically no energy other than the water pressure itself and is the only known process other than distillation which can separate pure water from molecules of:

  • Sediment, turbidity, and particulate matter
  • Dissolved solids
  • Colloidal matter
  • Radioactive elements
  • Toxic metals
  • Fluorine and fluoride
  • Microorganisms
  • Asbestos
  • Herbicides and pesticides
  • Heavier organic molecules (MW>300)

How Reverse Osmosis works inside the water filter

In a reverse osmosis filter, numerous sheets of the semi-permeable polymer membrane are wound in a spiral around a hollow tube that runs down the centre. The pressure of the water as it enters the filter container forces the water molecules through the membrane into the hollow central tube leaving the impurities behind on the outside. This hollow central tube is connected to the outlet on your water dispenser and it is this purified water that flows into your glass.

Meanwhile, the flow of the unfiltered water as it enters the container also forces the trapped impurities down and out of the filter by a separate drainage tube so that the membrane is being continually flushed clean when the filter is in use. It is this constant flushing action that gives the reverse osmosis membrane its longevity and prevents the build-up of accumulated impurities.

Reverse osmosis filters come in a range of sizes with diameters of 2-8 inches and lengths of 10-48 inches, depending on how much filtered water you need.

Types of Filters

The type of reverse osmosis filter you use depends to a great extent on the source of your water supply. If you are connected to municipal, chlorinated water then it is best to use cellulose triacetate (CTA) and cellulose acetate (CA) filters which are resistant to chlorine and oxidizing chemicals. However, if your water is drawn directly from a well or none-disinfected source this type of filter material – particularly cellulose acetate – is susceptible to the eventual growth of bacteria on the surface of the membrane and so will need to be changed more frequently.

The best type of membrane filter material for water supplied from wells and other none-chlorinated sources are made from thin film composite polyamides (TFCs) which are completely bacteria resistant, but will deteriorate over time in the presence of chlorine and other oxidizing chemicals. TFCs have the added advantages of better repelling impurities, being more resistant to adverse levels of water pH and can filter larger quantities of water more quickly.

Shortcomings of Reverse Osmosis

Reverse osmosis is unable to remove the more toxic, smaller, ligher weight volatile organic molecules such as carbon tetrachloride, vinyl chloride, TCE and THMs because they pass easily through the filter membrane. The only way to remove these from your filtered water is to pass it first through a carbon filter which is best installed after your reverse osmosis filter for increased longevity of the carbon filter. Some clients who have an unpredictable water supply install carbon filters both before and after their reverse osmosis filter since the carbon filter is more economical to change if your water has large quantities of impurities. We will discuss these more complex, engineering design aspects with you when you enquire about setting up a reverse osmosis system in your home or business.