In our post of October 16th, we discussed factors affecting the driving force to move ozone from the gas phase, where it is created, into the liquid phase, where it is needed for water treatment applications. These included factors such as gas concentration, bubble size and the Gas volume/Liquid volume ratio. In this posting, we will discuss some other factors such as temperature, pH and ozone demand of the water.
Ozone solubility increases with decreasing water temperature. This can be expressed by the following equation: In Ha = 22.3 – 4030/T where Ha = apparent Henry’s constant, atm/O3 molar fraction in the liquid.
There is also an effect of pH on the solubility of ozone in water as expressed in the following formula:
Ha = 3.84 x 10-7 x [OH–]0.035 exp(-2428/T). Apparently, the experimental data shows considerable scatter. It should also be noted that at elevated pH ozone decomposes more readily which can make determining molecular ozone solubility difficult.
Ozone demand effects transfer of ozone into the water by consuming ozone and thus shifting the equilibrium. This is not a solubility issue but does effect the design of the ozone mixing system. If the ozone is consumed as it enters solution, it is much easier to transfer the gas to the liquid. In some water ozone transfer efficiency can effectively reach 100% if the compounds in the water readily react with ozone.
The factors affecting the transfer of ozone into water are fairly complicated except in idea systems. As a result, lab and pilot evaluations are almost always needed to assess the design of an overall ozone water treatment system from the total amount of ozone needed to the best method for transferring it into solution given a specific treatment objective.