Although it is not popular or widely known, a reverse osmosis home water filter is the most effective home water filtration system. A reverse osmosis system will remove virtually any impurity and contamination from your water supply. After all, this technology was originally developed in the 1970s as a cheap method to desalinate sea water. Today, this is one of the main technologies used for kidney dialysis, desalination and removing contaminants from industrial waste water.

More recently, reverse osmosis (RO) home water filters have started showing up in the market, but have yet to make headway because of their price - they cost at least $1000. Compare this to high-end systems using 0.5 micron carbon block water filters which cost $300 to $500 or low-end 50-micron GAC filters costing under $50.

Reverse osmosis works by using high pressure to force water molecules across a polymer membrane. The contaminated water molecules are flushed from the inlet while the pure water molecules exit from the outlet. There is more fancy physics behind the action, but since it sounds like gobbledegook to most people (including me), I will not bother explaining further. Suffice to say that RO water filters will remove everything, including:

  • heavy metals, including lead, copper, mercury and much, much more
  • bacteria
  • fertilizer
  • herbicide
  • pesticide
  • dirt
  • rust

While a reverse osmosis home water filter is very powerful and very effective, there has been a lot of disinformation about them. This is natural and expected, given the entrenched self-interest of the many companies still relying on GAC and Carbon block filter technology for their water filters. The following are the two most commonly heard lies about RO:
  • Myth: They do not remove organic contaminants like pesticides and herbicides because these molecules are smaller than water. Fact: RO removes pesticides and herbicides even more effectively than carbon. Water molecules are very, very small - they only have 2 hydrogen atoms (there is no smaller atom) and one oxygen atom. Modern herbicides and pesticides are made from hydrocarbons, long chains of many carbon atoms and hydrogen atoms. 3 atoms versus many atoms - which is larger? So water will pass through the RO membrane, but herbicides and pesticides will not.
  • Myth: Water purified using reverse osmosis becomes acidic (or alkaline, depending on which guru you listen to). Fact: Pure water is always neutral. Water only becomes acidic when it becomes contaminated with positively charged hydrogen ions, for example by adding hydrogen chloride to make hydrochloric acid. Water only becomes alkaline when it becomes contaminated with negatively charged hydroxide ions, for example by adding sodium hydroxide. This is basic chemistry. If you stick a strip of pH paper, blue litmus paper or red litmus paper into pure water (regardless of whether you purified it using reverse osmosis or distillation), the color will not change. This shows that RO-treated water is neutral.

This is not to say that a reverse osmosis home water filter has no flaws. Beyond the relative lack of publicity, there are good reasons why more people have not installed them:
  • They are expensive. The entire water filtration system is beyond the budget of many middle-class families, especially when the economy is still in the doldrums. The ongoing replacement of water filter cartridges will blow the budget of most families, especially since the practical lifespan of the RO filter works out to be shorter than the lifespan stated by most manufacturers.
  • RO, like distillation, removes everything from your drinking water, including essential minerals like fluoride. In practice, this is not a problem for most people. Most of us get enough nutrition from our daily diet and multi-vitamins. However, someone on a restricted diet might face additional problems.
  • RO filters have very small holes, so they choke up very easily when filtering water with a lot of sediment. That is why you need to make sure to get a home water filtration system with sediment pre-filters.
  • RO filters degrade and rot quickly in the presence of chlorine. Since the water companies of most civilized countries use chlorine to disinfect water, this is a serious problem, especially given the high cost of replacement RO water filter cartridges. You'll need to make sure there is a pre-filter to get rid of the chlorine. Any decent multi-stage home water filter should already have this in place.
  • Reverse osmosis needs high pressure from the water inlet to work. If you live somewhere with leaky pipes and low water pressure, this is a serious problem.
  • RO home water filters are expensive to operate. They waste a lot of water, having an efficiency of only 5% to 15%, i.e. out of every 100 gallons of water that flows into the system, up to 95 gallons is discarded.
  • RO is slow, much slower than filters using granular activated charcoal or powdered carbon blocks.

At this point, there are cheaper and more convenient ways to get pure drinking water than to use a reverse osmosis home water filter, for example bottled water or faucet-mounted water filters using carbon supplemented with a resin ion exchange system. Unless you know your water supply is badly polluted, or the price drops and efficiency improves, there are better ways for you to spend your money.