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How does Reverse Osmosis work?

Posted by David Jamieson on August 07, 2014

How does Reverse Osmosis Work?

The Reverse Osmosis Diagram below represents a typical home reverse osmosis system, this together with the notes below will hopefully provide a better understanding of how the purification process takes place and provide a guide in the event that you may have any issues with your RO (reverse osmosis) unit. The diagram represents a four stage undersink RO filter however the same principals apply for five, six or even seven stage RO systems.


Basic Operation of most under the sink RO systems:

  • A Water is fed to system from existing cold water supply
  • B A pressure limiting and check valve is fitted inline before entering the first stage housing at point C
  • The first stage filter is a sediment filter which will remove dirt, rust and sediment protecting the more expensive second stage cartridge
  • The second stage carbon filter removes taste and odours and more importantly chlorine
  • After passing through stage 1 and 2 filters, filtered water enters an auto shut off valve at inlet point the purpose of the auto shut off valve (ASV) is to stop the reverse osmosis process once the holding tank is full
  • Water leaves ASV outlet and goes to inlet of membrane housing G
  • Water enters the membrane housing under pressure, pure water leaves the membrane housing at point goes through a non return valve (NRV) and waste water leaves at point through a flow restrictor
  • After leaving point I, pure water goes through the ASV at points and before entering a tee at point L
  • Pure water flows to holding tankwhich contains an air bladder, N.B. this flow takes place in both directions, once the tank is full it will “push” the water through point through the fourth stage carbon filter which is used to “polish” the pure reverse osmosis water once your dedicated drinking water faucet is opened O

Now that we have an understanding of the basic process let’s look at each component in more detail:

Reverse Osmosis Water Connection tee: the water connection to your domestic RO unit can come in various forms depending on the existing plumbing under your sink. In modern homes the most common connection to your tap will be a flexible hose, options also are available to hook up your system to a dishwasher connection or in the event that your kitchen tap is “hard” plumbed a compression tee is also available.

Reverse Osmosis Pressure limiting valve (PLV): this is a critical component that is often not installed by either non qualified plumbers or by companies trying to cut corners and provide the cheapest possible option. The PLV provides protection against water hammer which can occur due to modern appliances such as dishwashers which use solenoid valves to shut off the water suddenly. It also stops water back-flowing from your system into your existing water supply.

Reverse Osmosis Housings: some RO systems have cartridges which drop into the housings, other have quick change cartridges where the housings and the cartridges are a single unit. It is important that in both cases the housings are AS3497 approved to Watermark Level 1 which guarantees that they will stand up to Australian water conditions. A licensed plumber will not install a system in your home unless it has the Watermark approval.

Sediment filter: not essential however it is advisable to use a sediment filter as they are inexpensive and help protect the more expensive carbon cartridge, the only job that the sediment filter carries out is to remove any dirt or sediment from the water.

Carbon Filter: essential part of the unit, this filter takes out the chlorine before it reaches the membrane, the membrane cannot remove chlorine and is also susceptible to chlorine attack. It is wise to change the sediment filter and carbon filter regularly to help maintain the efficiency of your RO unit.

RO Membrane: the heart of the system, DO NOT buy a membrane unless the retailer can prove that it has been NSF 58 certified. There are two types of membrane Thin Film Composite Membranes (TFC or TFM) and Cellulose Acetate Membranes, the most common in use is TFC which is very susceptible to chlorine attack hence the need to use a carbon pre filter. The membranes come with different flow rates 50 gallons per day (GPD) 75 GPD and 100 GPD are the most widely used, this of course refers to the amount of water that they can produce in 24 hours. It is important to note that it is not a simple case of selecting a 100 GPD membrane just because the flow rate is better, the membrane needs to be “used” to as close to its capacity as possible to ensure proper operation. If the system is maintained properly the reverse osmosis membrane should last 3-5 years.

Flow Restrictor: if the flow restrictor was not fitted the pressure that is required to push the pure water through the membrane would not occur and most of the feed water would go to drain, the flow restrictor is sized according to the GPD rating of the membrane. It can be fitted in the elbow, inline or inserted inside the drain tube.

Auto Shut off Valve (ASV): we believe that a quality made in USA valve is the choice for any RO unit, this valve will stop the water being produced once the holding tank has filled, if this valve fails it could result in water running constantly to drain without the knowledge of the home owner.

Non Return Valve (NRV): This valve stops water back flowing from your holding tank and is critical to the operation of the system, they can be fitted in the elbow or inline. The unit will stop producing water once the pressure of the water in your RO tank is around 2/3 the pressure of the incoming water supply.

Holding Tank: Again come in different sizes, 12, 15 litre etc. It is important to note that the actual litres held by the tank will be less than stated by the manufacturer as a lot of the tank capacity is taken up by air and a bladder. The water does not come into contact with the tank wall as it is stored inside a butyl bladder which is inert, the outlet of the tank should be stainless steel to ensure that your pure water does not come into contact with lead or BPA. The tank has an air bladder inside which is under pressure usually 5 – 10 psi when empty, this can be tested in the same way that air pressure is measured in your tyres. There is only one water connection on the tank this is the connection where pure water both enters and exits the tank. If the air pressure in the tank is either too high or too low the water will not flow fully at your water filter tap.

Post Filter: the purpose of a post filter is to remove any non harmful bacteria that may be present in your holding tank and also to “polish” the pure water produced by the process of reverse osmosis. Post filters can also add minerals and increase the alkalinity of the RO water.