gas-liquid exchange

Water treatment processes include many that involve transferring matter between two phases: liquid and gas.

These mass transfers consist in transferring a constituent from one phase to the other. They can be divided into two sub-categories :

• absorption or dissolution, transferring constituent from the gas to the liquid phase. This involves eitherdissolving a gas(air, oxygen, ozone, chlorine, CO₂ …) in water for the purpose of treating the water : biological purification, iron removal, oxidation, disinfection, pH correction …., or dissolving gas pollution (H₂S, mercaptans, SO₂, NO_x, NH₃ and organic volatile products, HCℓ…) in a liquid solution in order to purify the gas phase :gas scrub (odour control …).

This absorption is often combined with a chemical reaction that fixes the component that has been transferred by precipitation, oxidation ... ;

• desorption, the reverse operation that consists in transferring volatile compounds (CO₂, O₂, H₂S, NH₃, chlorinated solvents) dissolved in water: in these cases, we often refer tostrippingordegassing.

This desorption takes place without any chemical reaction.

In all cases, the liquid/gas system obeys laws governing mass transfer from one phase to the other in order to ultimately achieve a state of equilibrium.

The chapter degasification, odour control, evaporation describes the applications and technologies that use these reactions.