In recent postings we have discussed the components of an ozone water treatment system including gas preparation and the ozone generator. We will now discuss mixing of the ozone with the water.
As noted in the previous posting, ozone is prepared in air or oxygen at a concentration of 2-10% by weight. Ozone is solubility in water is limited by its rather low concentration in the feed gas. A good ozone water contacting device, however, can achieve greater than 95% transfer efficiency under the proper conditions. The ozone transfer efficiency is simply the amount of the gas that ends up in the water.
Since most of the gas is inert, provisions also have to be made for letting this gas escape from system. As noted above, some small percentage of ozone will remain in the gas phase. Depending on the amount of ozone remaining in the gas and the location of the vent, it may be necessary to destroy this remaining amount of ozone.
In terms of the options for mixing ozone and water there are two main approaches with several minor options. In practice, fine bubble diffusers and venturi injectors are the most common methods employed for the contacting of the ozonated gas and water. You can learn more about these methods at www.spartanwatertreatment.com/ozone-mixing-contacting.html.
Fine bubble diffusers employ porous ceramic structures to form relatively small bubbles. To achieve high ozone transfer efficiency the diffusers need to be at a sufficient depth to give the bubbles enough time to diffuse their ozone into the water. Typically these depths are 16-20 feet. An advantage of this approach is that besides being simple without any moving parts, the normal pressure of an ozone generator is sufficient to push the gas through the diffusers. A disadvantage of this approach is that the water from the mixing chamber must be pressurized again, pumped, onto the next process or distribution system.
A venturi injector relies on a device called a venturi which creates a vacuum using the flow of the water to be mixed (see http://en.wikipedia.org/wiki/Venturi_effect). The advantages of this approach is that very small bubbles are created in the venturi aiding mixing, a very small space is required and the liquid remains under pressure during the process. The disadvantages are that you need to add pumping energy to overcome the pressure drop of the venturi, back flow of water to the ozone generator is more complicated to prevent and you may still need a tank to provide contact time between the water and the dissolved ozone. Venturi injection is often done in a side stream to the main flow where the side stream is remixed eventually with the main flow.
Other methods employed include a novel device called a GasTran system that using a spinning porous disk and various packed bed designs.
Once the gas has been mixed with the water, the gases that did not dissolve in the water must be disengaged. In the simple bubble contactors that operate under atmospheric pressure, the gas is simply vented from the tank. In the pressurized systems, special degas devices must be used. One class of device is a hydro cyclone. When combined with a gas relief valve, the system can remain under pressure while venting the excess gas.
In both systems, the vented gas may need to pass through an ozone destroyer to remove traces of ozone that did not dissolve into the water.
The final design of the mixing system will depend on many factors. Spartan Environmental Technologies as a systems integrator can help you decide which approach makes the most sense for a given application.