In most cases particulate pollutants are removed from gas streams by making use of one of the following properties:

1. Settlement of the particulates (following reduction in gas stream velocity and or mechanisms to encourage coalescence)
2. High inertia of particulates relative to molecules in flowing gas stream:
   1. Impaction.
      E.g. The smaller particles removed on fabric filters are removed by impaction.
   2. Impingement. - of particulates on liquids or of liquid droplets on particulates.
   3. "Centrifugal processes".
      I.e. cyclones
3. Particle size separation by sieving.
   E.g. Larger particles are separated on fabric filters by this mechanism.
4. Electrostatic properties of particulates.
5. Chemical reactions e.g. combustion.

1. Phase partition processes.
   1. Condensation often used as a pre-treatment process and can often be looked on as a means of product recovery.
   2. Partition between gas and solid phase.
   3. Partition between gas and liquid phase.
2. Chemical reactions
1. Solid phase reactants
   E.g. Hydrated ferric oxide will react with hydrogen sulphide.
   FGD using the lime/gypsum process uses a limestone slurry.
   Note A video on this subject is available, Students should make sure they see it.
2. Liquid phase reactants
3. Gas phase reactants.

Most commonly combustion in air, with or without a catalyst is the method of choice.

It is important to realise that many technologies used for vapour phase pollutant removal are also able to remove particulates but these are often operating by different mechanisms it is thus difficult to optimise for vapour phase and condensed phase removal simultaneously.