The Technology

To explain on the reverse osmosis (RO) process, firstly we need to understand the osmosis process.

The osmosis process is the moving of water from a less concentrated side to a more concentrated side through a semi-permeable membrane until it reaches equilibrium. This natural flow is called osmotic pressure. A semi-permeable membrane is a porous membrane that only allow moving of water through it and blocks the passing of solutes (dissolved particles).

Therefore, the reverse osmosis process is the moving of water from a more concentrated side to a less concentrated side through a semi-permeable membrane. As we have known that there is a natural osmotic pressure, so there is a need to apply external force to fight against the natural pressure so that the water can flow on the opposite direction.

Components of a reverse osmosis water filter:

  • at least two stages of particle filters to filter sediment from more coarse to finer one

    • how fine it is, is important to reduce the membrane loss of performance at it is supposed to be,
    • the first stage particle filtration normally at 5 or 10 micron and is called pre-filter,
    • it has better care for the membrane (which filter at 0.0001 micron) if the second sediment filter can filter at least at ultrafiltration level (0.01 micron);
  • carbon filter

    • to remove chlorine,
    • size of chlorine is smaller than water and it can pass through RO membrane,
    • depends on the type of membrane material used, carbon filter is to be installed either before RO membrane or after RO membrane (explained below) or both before and after RO membrane for better safety,
    • besides filtering chlorine, carbon filter is important to filter other chemicals that smaller size than water;
  • reverse osmoses membrane
  • ultra-violet lamp (optional)

    • to kill any microorganisms that may escape from RO, membrane, as not all membrane are perfect during manufacturing,
    • however, most dangerous bacteria have size bigger than 0.5 micron, and it unlikely to present in RO water filter which has gone though multi-stages of filtration;
  • high pressure pump

    • to force water moving through semi-permeable RO membrane;
  • storage tank

    • to store filtered water,
    • RO process is too slow for you to immediately collect water for consumption,
    • generally, a reverse osmosis water purification system comes with a tank that can store 2 to 5 gallons of water.

Membrane of RO System

Not all reverse osmosis water systems are the same. The water quality difference is depends on:

  • the types of membrane

    • Cellulose Triacetate (CA/CTA) – organic membrane
    • Thin Film Composite (TFC) – synthetic membrane
  • pore sizes of membrane

    • 0.001 micron
    • 0.0001 micron

Thin Film Composite (TFC) is more effective in filtering contaminants than Cellulose Triacetate (CTA).

TFC membrane needs chlorine free water to avoid from getting damaged. However, CTA membrane needs to have chlorine in the water to avoid the membrane material from rotting by micro-organisms. Therefore, carbon pre-filter is required for TFC membrane and carbon post-filter is for CTA membrane. TFC membrane is bacteria resistant.

As the size of filtration is very small, the membrane tends to get clogged easily. To reduce the chances, pre-filtration prior entering reverse osmosis water filter system needs to be as small as possible, more effective with several stages from bigger sediment removal to smaller size of filtration.

Some reverse osmosis systems come with automatic flushing to reduce chances of blockage and thus, reduce loss of capacity.

Benefits of Reverse Osmosis Drinking Water System

  • Because of the filtration size is very small, it removes almost all of the impurities that present in the water regardless they are harmful chemicals, micro-organism or beneficial minerals,
  • although none of the water filtration systems can filter all the chemicals, reverse osmosis system is considered the most effective technology.

Disadvantages of Reverse Osmosis Water Filter

  • Water produced is more acidic than normal, without the present of minerals;
  • water is tasteless (flat taste) without the present of minerals, you may think this point is not important but what if you subconciously drink less water from what you will drink normally?;
  • minerals like calcium, magnesium and potassium which are necessary for bone development and cardiovascular function, are being removed;
  • as the water filter needs external force for the reverse osmosis system to run, pressure pump is required which means additional maintenance cost on the pump breakdown and running cost on the electricity;
  • as the membrane is under high pressure, the pore size gradually increase over time and thus reduce about 20% effectiveness in about 6 months time;
  • to produce filtered water for consumption, it will waste 2 to 19 times of water into the drain. The water wastage is depend on the pump pressure. The lower the pressure, the water wastage is higher. This will put additional cost to you;
  • RO is a very slow process, thus it needs a tank to slowly collect the filtered water which means additional maintenance work on the hygiene of the tank;
  • any chemicals that has smaller size than water such as pesticides, can pass through the RO membrane;
  • the units are bulky with pump, storage tank, etc. occupy your kitchen space.

The above listed benefits and disadvantages of reverse osmosis water filters are general guidelines for you when choosing a drinking water filter. You may check out the details from relevent manufacturers of the RO drinking water system on the specific items mentioned.

Remember that not all reverse osmosis water filter systems or reverse osmosis membrane are created equally.

Some examples of water purifiers in Malaysia that use reverse osmosis technology are from Elken and Coway.

How would we know whether a particular brand of reverse osmosis water filter can filter harmful chemicals as per claim? Check it out here. However, if you could not find the brand listed there, you could check with the manufacturer if any of the water filter components are certified for the claim of the particular chemicals reduction.