Booster Pump: In order to the raw water through the pretreatment filter, we install a raw water pump to increase the water pressure.
Sand Filter: to use quartz sand media down to 0.5-1mm. which filter size can be down to 200micron. some big suspended solids like colloids, rust, mud, imparity etc can be traped by media bed. and be removed from water.
Activated Carbon Filter: can absorb some organism, odor & color matter etc.
Softener filter: use cation exchange resin to remove Calcium, Magnesium etc to make soft water. The principal is ion exchange.
Micron Filter: use 5 micron pp sediment filter cartridge to remove the passed suspended solid from pre-treatment. so that more safety for entrance of RO.
High pressure Pump: To increase the water pressure, let the water through reverse osmosis membrane.
Reverse Osmosis: the pore of the reverse osmosis membrane is 0.0001 micron,most of ions,virus and bacteria are trapped by RO membrane. to de-mineralize/desalination etc. it go out PURE WATER from output. 

EDI system: the principle is ion-exchange.