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.
Page 2
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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