Tech Talk 71
Pentair AES has offered ozone generators to the aquaculture industry for over 15 years. Slow to catch on, ozonization is gaining in popularity for the following reasons.
- It is highly effective in removing organics, pesticides, color and nitrates.
- It reverts back to oxygen quickly. Unlike chlorine, there are no detrimental residuals (except in salt water).
- It is produced on site, with no electricity near the water.
- It is economical and non-polluting, when used correctly.
- It can be used as a sterilizer, before during and after water is used for aquaculture.
- Ozonization improves biological filtration and particulate filtration.
- It can remove the biological oxygen demand in the water.
- It oxidizes long chain molecules, which biofiltration cannot do.
Basic Ozone Information
- Ozone is very unstable. It will revert back to oxygen within about an hour, even if there are no organics in the water for it to oxidize.
- Temperature, pressure and shear cause it to revert back to oxygen. When ozone (O3) molecules collide, they recombine as oxygen (O2), so it is virtually impossible to get ozone to the tank above 10% (by weight). When under pressure or traveling a long distance in the tubing, it can revert considerably.
- Use dry air or oxygen to produce ozone. Humidity can reduce ozone production by 70%, form scale in the corona discharge (CD) reactor and produce nitric acid.
- To insure sterile (germ free) water, there is nothing better than ozone. For sterilization, pre-filter to 5 microns and maintain ozone levels above 600 mV for a minimum of 8 minutes.
- OSHA says that it is harmful to breathe ozone above .1 ppm in the air (most people can smell ozone above .05 ppm). Vent gas outdoors or into an ozone destruction device, such a UV light or activated charcoal. Downflow bubble contactors such as cones and saturators are recommended so the gas cannot escape. High levels of ozone may cause a single species biofilter to develop, due to the ozone oxidizing the nitrite. This is not bad unless something goes wrong with the ozone system, causing nitrite to spike!
How It Works
Ozone is generated by passing air or oxygen through a reaction vessel, where either an electric arc, CD or an ultraviolet (UV) lamp "excites" the oxygen. In this reaction, oxygen molecules separate into atoms of oxygen, which then temporarily recombine with each other to form ozone. When ozone oxidizes organics only one atom of oxygen is used, leaving one molecule of oxygen.
Types of Ozone Generators
Ultraviolet lights with a specific ozone-generating wavelength are generally used to produce low levels of ozone. The slower the gas moves through the UV-reaction vessel, the higher the percent of ozone.
The corona discharge (CD) type uses an electric arc similar to sparks or lightning to produce higher percentages of ozone by weight. A small CD reaction vessel can produce a relatively large volume of ozone. The greater the percentage of ozone, the faster the oxidizing reactions take place.
Where To Use It
Ozone can be used in a protein skimmer (foam fractionation device), where it helps the process, while the vessel allows capture of the off gas for venting or ozone destruction. Ozone works very well in oxygen saturators for the same reasons. We do not recommend its use in lakes or ponds, unless the water is pre-filtered and treated in a reaction vessel. Just bubbling it into the water is not effective.
How Much To Use
In a small home aquarium, .1 mg/L may be used (300–400 mV). A sterilizing system for drinking water may need 1 mg/L+ with a 10-minute contact time. In recirculating aquaculture, with high BOD and COD loads, the ozone requirement can be more than 20 mg/L. The dosage is impossible to determine because aquaculture conditions are always changing. Therefore, we recommend either the use of a side stream, where the water is treated as much as possible with ozone before it is mixed back into the main water body, or the use of a redox controller, which will automatically adjust to the changing conditions.
How To Handle It
Ozone is a very strong oxidizer and must be handled with special materials. The best is stainless steel for tubing, valves and other components (certain Sweetwater® air diffusers are made for use with ozone). The second best material is pure Teflon®, then Kynar®, CPVC and HDLPE, in that order. Be careful using vinyl air tubing, as the ozone will leach out the potentially toxic plasticizer (it will look like oil on the inside of the tubing).
A whiff of low concentration ozone will not kill you. If you smell ozone in the air in your building, turn off the ozone generator and vent the air space. Note that OSHA requires an ambient ozone monitor on any generator that produces over 5 g/hr.
No Ozone System Design Service
Due to the complex nature of custom ozone system design, we cannot provide that engineering service.
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