Pre filter: These range in size from 30 microns down to 1 micron. This filter works on the larger particles in the water. The finer the pre filter the faster it will clog up with impurities. You will want to protect the pre-carbon in a thin film composite or TFC system from fouling by sediment. A 5 to 10 micron pre filter is best suited for residential systems. In a cellulose tri-acetate or CTA system the sediment filter is all that stands between the incoming tap water and your expensive membrane. It works as the first line of defense for both types of systems. If you live in an area that has a lot of sediment than you should change this filter more often, typically it is sufficient to change it every 6 months.

Pre carbon: Should only be used with a TFC system it will remove the chlorine before it comes in contact with your membrane this will extend the life of a TFC membrane. If used on a CTA system it will cause the membrane to fail. The reason is because a CTA needs chlorine in the water to protect it from bio fouling.

Membrane: Of coarse is the key to why all RO systems work. There are two types of residential membranes. First is the CTA or cellulose tri-acetate membrane. This membrane works well in chlorinated water but has a lesser rejection percentage than the TFC or thin film composite membrane. The CTA also has a limitation of the pH that it can handle. It will not survive in pH above 8.5. The TFC on the other hand can survive in feed water with pH of up to 11. However the TFC membrane cannot handle more than 1000 hours of contact with chlorine so this is why all residential TFC systems have a carbon filter ahead of the membrane. Many 4-stage systems are TFC systems because of the pre-carbon filter. If you see a 3 stage system chances are you are looking at a CTA system. Always read the label, most membranes are marked TFC or CTA. You should replace a CTA membrane every year and a TFC membrane every 2 years. This will ensure optimal performance. In some parts of the country you can get a longer interval for replacement but you will need a total dissolved solids meter to verify when it is time for replacement.

Check valve: A very important part in the RO system. The check valve stops water from running backwards through the membrane after the shut off cuts off the inlet pressure. A non-working check valve can ruin your membrane in less than 1 month if it is left unchanged. It is inexpensive insurance for your expensive membrane.

Flow control: Also very important to the survival of your membrane. A flow control regulates how much water passes across the surface of the membrane and then to drain. You will want to know how much water is running to drain from your new system so that you will know when it decreases as you will have to replace it. You can do this with a small cup and wristwatch. Measure how much water runs into the cup from the drain line in 15 seconds and then multiply it by 4 to get 1 minutes worth. A typical 15-gallon per day residential RO system uses 100 to 150 milliliters of water per minute. Mark the cup and save it. When you service your system use the cup to check the flow to drain. If the amount of water decreases or increases by any more than half the original amount then it is time for a new flow control.

Post carbon: This is a polishing filter that removes taste and orders. It usually contains smaller more fine carbon than the coarser pre carbon. Most post carbon filters are encapsulated or are in the form of a cartridge. What's inside the filter is simply coconut shells that are burned at 2500 degrees in a furnace. This gives the water a very pleasant taste and should be replaced every 6 months.

Encapsulated filter/membrane: This is a filter that comes sealed inside its own filter housing. The old filter is simply removed and thrown away. The new filter or membrane will not be handled or exposed to outside contamination so it is the best way to ensure a sanitary filter replacement.