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Introduction.

I.A. Background. Oxygen (O₂) is commonly used as a feed gas for generating ozone (O3) gas, which is subsequently used for disinfection or oxidation of water supplies. Oxygen may be generated onsite as a gas or liquid or purchased in bulk as liquid oxygen (LOX). Hundreds of water plants in North America are using ozone for water treatment, with a majority generating ozone from purchased oxygen.

Gaseous oxygen is colorless and odorless while LOX is pale blue and odorless. Oxygen itself is not combustible, but it accelerates combustion to the point where materials, such as some types of clothing that are normally considered nonhazardous, become very flammable. Liquid oxygen may react spontaneously with petroleum products (such as gasoline, kerosene, oils, and greases) and other fuels (such as hydrogen and ethanol). Oxygen is nontoxic under most conditions of use, but LOX or cold gas will freeze tissues and can cause severe cryogenic burns. Breathing high-purity oxygen (greater than 60 percent) may produce a cough and chest pains.

LOX is normally produced through a cryogenic air separation process. This process involves compressing ambient air; cooling the air with a refrigeration unit; and removing residual water, carbon dioxide, and hydrocarbons with a molecular sieve adsorption unit. The clean, cold air is then liquefied and separated into its components, mainly oxygen and nitrogen, by distillation. Final oxygen purity can be controlled by further separating and removing trace components such as argon and krypton.

Ozone can be produced from oxygen in the air or from high-purity gaseous oxygen. This can be achieved by several methods, although the silent electrical discharge process is the most common method. Ozone is produced when a dry oxygen or air gas stream is subjected to a high-voltage/high-density electrical current, which provides the energy to drive the reaction. The oxygen-to-ozone reaction takes place between two electrodes separated by a dielectric, forming a gap across which the energy discharge occurs. Oxygen-fed ozone generators will produce more ozone for a given power input and produce higher ozone concentrations in the product gas compared to ozone generators fed with an air gas stream.