There are many techniques developed to seperate substances based on the sizes of the particles. Some of these techniques are widely applicable. Some of them are only applicable for a particular size. An uncomplete list is shown below for different particle sizes with their applicatable technique.

• Diameter between 1 µm and 1 mm
   Drum filter
   Sand filter
   cartridge filter
   Bag filter

• Diameter between 100 nm - 1000 nm (0,1 µm - 1,0 µm)
   Microfiltration
   Ultrafiltration

• Diameter between 1 nm - 100 nm
  Ultrafiltration

• Diameter smaller than 1 nm ( 0,001 µm)
  Reverse Osmosis
  Ion exchange

The choice between different techniques depends on several factors like;
The amount of pollution
 The amount of fluid to be treated
The properties of the substance which is to be removed this in order of the damage and the choking of the filter
Quality of the filtration
Investement

Some of the techniques are only suitable for one time. Other techniques the filter will be cleaned and used again, after a while (this can be several years) the filter eventually should be replaced.