An apparatus and system for mixing and separating ozonated water is provided. The apparatus is especially useful as a component of a system for cycling sanitizing ozonated water to a body of water such as a pool, spa or pond. The apparatus generally includes a separating vessel including a mixing tower, preferably located substantially entirely within the separating vessel and including a diffuser element for enhancing mixing of ozonated water without increasing turbulence in the separating vessel. Preferably, the apparatus includes a seal mechanism that substantially prevents liquid water from escaping the separating vessel with vented off-gas. The invention further provides an ozone destruct assembly including a condensate collection portion, and may also include a return line for passing a treated off-gas into the body of water.