The white filter cartridge is called a sediment cartridge and this is the first stage of RO filtration and is designed to remove solids and particles from the tap water.
 
The sediment cartridge supplied with all D-D RO units is 5 microns which will remove any particles above 5 microns. This produces in effect sediment free water to continue on to the next stage of water purification which is the carbon cartridge. Removal of the sediments extends the life of both the carbon filer and the membrane.
 
The first pod on all D-D RO units is clear and will allow you to see the condition of the sediment filter so that you can see when it looks to be ready for changing.  These low cost filter elements should be changed every 6 months or earlier as necessary.

Suitable for the D-D Reverse Osmosis Kits RO50, RO75 and RO150.