Several applications of ozonation were examined in this study for: i. the treatment of stabilized high strength municipal landfill leachates, ii. the reclamation potential and toxicity reduction of municipal secondary effluents, and iii. the removal potential of phytoplanktons from surface waters. The major parameters examined were the applied ozone dosage and the respective contact time. The application of single ozonation on leachates resulted in the efficient removal of color and organic loading, due to the respective oxidation, induced by ozonation. In addition, ozonation was found to be effective for the removal of the residual organic content of secondary municipal effluents. However, acute toxic effects after ozonation were observed on V. fischeri and were related to ozone concentration and contact time. Furthermore, the surface water used for drinking water production, was subjected to ozonation treatment for the removal of harmful cyanobacteria. Ozonation resulted in the reduction of the number of cyanobacteria species and in the breakage of the chain-type species to cells with a lower number of atoms.