Water disinfection has been a concern since the 1800s, when typhoid and cholera were the major recognized waterborne health threats. Since then, attention has broadened to other contaminants and to non-chemical disinfection methods that don't pose additional health risk.

 The result is increasing interest in ozone, an allotrope of oxygen that has been used commercially for a century -- most notably as a primary disinfectant for Europe's potable water treatment plants and public swimming pools, and in Olympic competition pools in the United States.

 Ozone's use in commercial swimming pools, cooling towers and drinking water treatment plants has accelerated since 1991. Companies that use it are often able to reduce their use of chemicals, cut operating costs, increase their reuse of water and meet Environmental Protection Agency (EPA) requirements to reduce levels of chlorine byproducts. Yet to get the most out of ozone, you must get the most ozone into water.

Ozone Transfer

 Ozone supplied by an ozone generator must be transferred from the gas phase to the liquid phase. Maximizing ozone transfer is critical to reduce the difficulties of handling undissolved ozone and to keep ozone production expenses at a minimum. 

Ozone can be used as an oxidizer, a disinfectant or a flocculating agent. Consequently, mixers and contacting systems are used for varying applications. The systems used to dissolve ozone gas into the liquid phase can vary in terms of type, design and operating conditions. The type of application determines both the type of ozone system used and the mixing system for the ozone generator.

 For instance, some ozone generators operate with positive pressure on the output line. An ozone generator of this type is most effective when installed in conjunction with a positive-pressure mixing system. Ozone generators operating with a vacuum on the output line are most efficient when installed with negative-pressure mixing systems such as aspirating turbines or injectors.

Specific methods to dissolve ozone into water include:

 1. Bubble diffusers. A bubble diffuser is made from a porous material such as ceramic or titanium. Ozone gas from the generator is pushed through the small pores in a diffuser installed at the deepest point of a contact vessel. This produces minute ozone bubbles at the surface of the diffuser.

 Small bubbles have a larger surface area than fewer, larger bubbles and thus increase ozone mass transfer. Fine-bubble diffuser contactors require a positive pressure feed from the ozone system. They depend on the height of the water column above the diffuser to provide contact time. Water columns near 20 feet high are required to achieve greater than 90 percent mass transfer of ozone.

 These systems have no moving parts, provide more efficient mass transfer of ozone and create no additional energy requirements, but they require cleaning and deep tanks.

 2. Injectors. Water flowing rapidly through a small orifice in the injector creates a partial vacuum to pull ozone gas into the water stream. Injectors can use either negative or positive pressure, but they're very effective when operating at a negative pressure and with low gas-to-water flow ratios.

 To select the right injector, consider the available water pressure, the volume or flow of water to be treated and the ozone dosage required. The injector should also be constructed of ozone-resistant material.

 Venturis, like diffusers, have no moving parts and don't require cleaning. They require energy, in the form of differentail pressure across the injector, to operate. They may be installed in full- or side-stream configurations to attain efficiencies of greater than 90 percent mass transfer.

 3. Packed columns. These are tanks packed with a material that causes turbulence in water flowing through them. This turbulent flow causes ozone bubbles introduced into the column to be broken up and thoroughly mixed, leading to improved ozone gas transfer.

 When seeking a packed column, consider the hydraulic pressure losses associated with it and whether it's constructed of ozone-resistant materials. Packed columns contain no moving parts and require little maintenance, but require increased energy to achieve desired flow rates. Packing material may also attract a chemical scale build-up, resulting in head loss.

 4. Reverse flow columns. These inject ozone gas into a column through which water flows in the opposite direction. They increase the amount of ozone contact time and ozone transfer. They can also be used as degassing chambers, but require the right water velocity to operate properly.

 5. Spray chambers. A spray chamber pressurizes the liquid and sprays it into an ozone-rich atmosphere. These systems are suitable for applications involving rapid reactions, such as oxidation of iron and manganese. They provide a relatively short contact time.

 6. Static mixers. These create water turbulence by directing water flow over a series of baffles. They're sometimes used in conjunction with an injector. To select a static mixer, consider the acceptable pressure drop and available water velocities. The systems have no moving parts, but create pressure drop.

Don't Jump to Conclusions

The key to applying an ozone-based system is to view the application in its entirety. Use reliable equipment from a reputable organization, establish rigorous system monitoring and preventive maintenance procedures, and provide responsive service and maintenance. 

Benefits can vary based on the application. Ozone isn't industry-specific; it's compatible with a broad range of operational requirements. While the means by which water quality goals are achieved may vary from one job to the next, the end result will be the same: expanded capacity, financial savings and better overall water quality.

Paulette Scoville is consumer education coordinator and marketing manager for DEL Industries, San Luis Obispo, CA. Daniel Erickson is the company's head of engineering and designer of its commercial/corona discharge water treatment systems.