I would like to clarify several issues concerning the use of ozone as a
sanitizer. Ozone generators are rated to produce an amount of ozone gas per hour
or per day. This gas must then be efficiently mixed with water in order to
produce ozonated water. Using a coarse sparging stone for example will probably
result in most of the ozone bubbling through the water and then outgassing
before being of any use. The potential for ozone in water to kill bacteria has
the same constraints as chlorine. There is a difference between free chlorine
(chlorine that has not reacted with organic matter and is effective at
killing bacteria) and total chlorine which includes free and bound chlorine (the
latter being chlorine that has reacted with organic matter and is not
effective at killing bacteria).
There must be sufficient ozone or chlorine to overcome the organic demand
and still have a residual. Obviously this is a function of how much sanitizer is
being added and the demand that is imposed by the amount of organic material
in the water. If someone states that they are adding 3-5 ppm of ozone, the
first question is whether this is calculated as the amount of ozone being
produced by the generator divided by the amount of water. Was the assumption that
100% of the gas would be dissolved in the water as this is never the case.
There are test methods and instruments for measuring the net residual in
ozonated water although there are some difficulties in getting good measurements
especially at lower concentrations.
The next problem involves the organic load in the contacting vessel for
ozonated water and the product. Ozone will react quickly with organic matter
before it kills bacteria. If the amount of ozone available is less than or equal
to the amount that will react with the organic load, then little or no
sanitizing effect is possible. And finally, contact time is a consideration for any
sanitizer.
Alan Ismond, P.Eng.
Aqua-Terra Consultants
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