The ozone transfer from the gas-phase to the liquid phase in the down-flow tube of the reactor showed high efficiency because of its large value of gas-liquid interface area. The estimated gas-liquid interface area of the down-flow tube was approximately in the range from 60 to 600 (1/m).

The ozone transfer efficiency (O.T.E.) of the down-flow tube was increased with the increase in ozone gas concentration. Therefore, it was clear that a high concentration of ozone gas should be applied when a higher ozone dose was needed for ozonation.

The estimated dissolved ozone concentration and O.T.E. by a mathematical model which was developed in this investigation showed good agreement with experimentally obtained data. As a result, it was shown that the model was able to describe the ozone absorption in the reactor with down-flow injection (DFI) in the wide range of operational variables.

In the case study that was carried out to compare the Cryptosporidium removal of DFI with that of a conventional fine bubble diffusion reactor by numerical simulation, DFI showed a higher performance. This higher performance is contributed to the advantage of DFI which includes the usage of high concentration ozone, greater gas-liquid interface area in the down-flow tube, and the preferable dissolved ozone profile in DFI.