Practical Hydroponics & Greenhouses Magazine
First Published: September/October 1997 (issue 36)
 
 

Introduction To Ozone Generation

 

 Ozone is a powerful disinfecting and oxidising agent,
successfully used in thousands of water treatment applications.
STEVEN CARRUTHERS reports on its use in hydroponics.


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High Efficiency Injectors

When designed correctly, this injection method will provide high diffusion efficiencies (>95%), however additional energy is required to produce a partial vacuum. Injector selection and design are important in achieving maximum efficiencies with minimum energy.

"This type of injection system has been used throughout the world with both small and large ozonation facilities," said Philip, "however its use is certainly more common in small and medium size applications."

With this technique, a side stream of water is normally taken from the main plant flow, where the pressure is boosted to supply a vacuum injector. This involves the water flowing rapidly through a small orifice, creating a venturi effect (partial vacuum), that pulls the gas from the ozonator into the water stream. When designed correctly this provides very effective efficiencies. The ozonated side stream is then injected into the main plant flow, normally via a low head loss static mixing device. Alternatively, this side stream of ozonated water can be injected into a degassing/contact tank which operates at atmospheric pressure.

"With this design, the dissolved ozone is effectively mixed with the main body of water, after which a contact tank would be required to achieve the necessary contact of ozone with the water to be treated," Philip explains.

In the Fancyleaf system, the ozonated water stream is led along a 20mm pipe located at the bottom of each contact tank. The PVC pipe is riddled with tiny holes to allow the fine bubbles to escape and diffuse into solution.

The following advantages and disadvantages are applicable for this injection technique:
 

Advantages
ïBoth injectors and static mixers (when used), are simple to operate, with low maintenance requirement, due to the lack of moving parts.
ïExcellent mixing and mass transfer can be achieved. Depending on the desired detention time, contact/reaction tank depths and volume may often be smaller than those required for bubble diffusion.
ïThis type of diffusion system may be adapted to any hydraulic plant design.
ïCorrectly designed counterflow or vortex static mixers impart low hydraulic head loss to the plant.
 
 
Disadvantages
ïAdditional electrical energy is required to efficiently operate the vacuum injector, however this is often counteracted by the increase in diffusion efficiency.
ïThese systems have a turndown capability limited to the capacity of the specific injection device. This problem is minimised where a side stream from the main flow is fed to the injector.

Figure 4 details two typical methods of applying an injector ozone diffusion system. Diagram A would be used for an inline system operating under pressure, whereby Diagram B shows a typical design for an atmospheric tank system.
 
 

Fig 4. Injector Ozone Diffusion Methods



 

1. Water Feed 4. Static Mixer 7. Ozone Destructor 10. Bubble Diffuser 2. Injector Boost Pump 5. Ozone Contact Tank 8. Treated Water 11. Contact Tank 3. Ozone Fed under Vacum 6. Off gas Vent Valve 9. Off Gas

Ozone Off-Gas Destruction

Ozone is an oxidative and reactive gas which is harmful to humans above certain concentrations. Ozone gas is heavier than air, therefore any undissolved ozone must be removed from the system and disposed of correctly. Several methods are available for ozone destruction, but the simplest one used for hydroponic applications is by ozone adsorbtion and reaction with Granulated Activated Carbon (GAC)
 

Absorption and Reaction with GAC

Activated carbon adsorption is extensively used for small applications where air is the ozonator feed gas. This is an adsorption process whereby the reaction consumes the carbon media. The material used is elemental carbon which has been steam activated to provide a large internal surface area.

"Carbon is a strong reducing agent, therefore upon contact with ozone gas, the carbon is oxidised to carbon monoxide and carbon dioxide, resulting in destruction of the ozone molecule," Philip explains.

This reaction degrades or powderises the granular activated carbon, therefore it has a finite life. To this end, the ozone consumes the carbon by slow-rate combustion. Due to this consumption of carbon, the media must be replaced regularly. When the carbon is saturated with water and washed with sprays, the reaction becomes partially catalytic, requiring a larger volume of GAC to be used.

"For ozone installations where oxygen is used as the feed gas, carbon adsorption destructors must not be used, due to the dangers of combustion," Philip warns.
 

Environmental and Health Aspects

Because ozone has a very short half life in aqueous solutions, and its degradation predominantly results in either oxygen or oxygenated by-products, waters treated with ozone will be less of an environmental hazard than the water left untreated or treated with halogens or related compounds, such as chlorine. Ozone accelerates the natural oxidation process of both atmospheric and biological oxygen take up. Water treated with ozone can generally be recycled in the environment without fear of releasing toxic substances.

Although ozone in the gaseous form is both toxic and reactive, it represents no safety handling problems in properly designed operating systems. Unlike most other oxidants which are stored on site in bulk form, ozone is produced on site in low concentrations and immediately consumed. Consequently, any accidental leakage can be easily controlled, as evidenced by ozone's long safety history in many applications around the world.

Breathing traces of ozone in air for a few minutes is of little public health concern. Even though throat and lung irritation plus oedema have been observed after extreme exposures to ozone, it is important to recognise that during more than 100 years of commercial use, no deaths related to ozone exposure have ever been reported.
 

Conclusion

The design and operation of ozone generating and ancillary equipment is both a detailed and complex subject, however this article is intended to give a very brief overview of current thinking and techniques for safely producing and using this unique oxidant for hydroponic applications.
When designed correctly, modern generation equipment will provide a reliable and safe source of ozone, however one must still remember that ozone is a toxic gas, therefore equipment and installations, if incorrectly designed and operated, can present significant dangers to both plant and personnel. No ozone installation should be considered without following the recommendation of organisations who are familiar with and experienced in the generation, use and operation of complete ozonation facilities.

If used correctly, ozone is a valuable tool for disinfecting and improving water quality in hydroponic applications, at the same time addressing the rapidly growing requirements for environmentally friendly or suitable products.
 

Further Reading

Handbook of Ozone Technology and Applications (Vol 1), Ann Arbor, Science Publishers, 1982.
Ozone in Water Treatment - Application and Engineering, Lewis Publishers, Editors Bruno Langais, David A. Reckhow, Deborah Brink, 1991.
Design Guidance Manual for Ozone Systems - IOA Pan American Committee, Editor M.A. Dimitriou, 1990.
Ozone in Swimming Pools - Facts and Fallacies, Philip J. Barlow, Proceedings of 11th World Ozone Congress, San Francisco, 1993.
High Concentration Ozone Generation, D. Moras, P. Uhlig, J.F. Petitimbert, C.H. Henery & L. Ayad, Proceedings of 11th World Ozone Congress, San Francisco, 1993.


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