An injected air stream containing ozone gas was used in separating microalgae from high rate oxidation pond effluent by ozone flotation. Use of ozone-induced flotation for algae recovery and effluent treatment was studied (Betzer et al., 1980). The ozone gas promotes cell flotation by modification of algae cell wall surface and by releasing some surface active agents from algae cells. The ozone-flotation process has been studied in numerous applications (Jin et al., 2006; Benoufella et al., 1994).

Elimination of a Microcystis strain of cyanobacteria (blue-green algae) by the use of ozone flotation was investigated in a pilot study (Benoufella et al., 1994). The oxidizing properties of ozone and the physical aspects of flotation were exploited in the flotation process. A specific ozone utilization rate of Microcystis was calculated, and ozone concentration and contact time curves were plotted versus algal removal. The study found that use of ozone in pretreatment leads to an inactivation of the algal cells. A prior coagulation stage was necessary for satis­factory cyanobacteria removal, and use of ferric chloride as a coagulant produced the best performance. Preozonation was also of influence on enhancement of the coagulation efficiency. Coupling ozone flotation with filtration can improve water quality, with treated effluent indicating low turbidity and low organic content.