Stages  of  Reverse  Osmosis image
1   The first stage of filtration is the sediment filter, which reduces suspended particles such as dirt, dust and rust.
2 The second stage of filtration is the carbon filter. This filter reduces volatile organic chemicals (VOCs), chlorine, and other compounds that might impact the taste or odor of the water.
Chlorine   should   be   removed from the water before the water enters  the  membrane.  This  will help   preserve   the   life   of   the membrane.
3 The center and third stage of a reverse  osmosis  system  is  the  semi-permeable   membrane.   It is responsible  for  rejecting  up to 98%  of  the  total  dissolved solids   in   the   water.   This   is where   the   purification   takes place.