ozoneis generated by a numberof methodsdepending on the concentration
required. Lowconcentrations(0.03ppm)can be produced by the exposure of air
to radiationwitha wavelengthof 185 nm fromUV lamps.Higherconcentrations
can be produced by the coronadischarge method.A highvoltagealternating
current is applied across a discharge gap in the presenceof air or oxygen.This
causes electronexcitation,whichinducessplitting of oxygenmolecules. Ozone
is thenformedby the recombination of the free oxygenatoms.
Theconcentrationof ozone produced depends on many factors such as
voltage, current frequency, dischargegap,dielectric material and thickness.
Oxygen or an oxygenconcentrator can be usedinsteadof air to increasethe
yieldbut if air is used,it should be driedto prevent the formation of nitricacid
(Kimet al.1999).
Ozonatedwatercan alsobe produced by a number of different methods.
Ozone is 12 timesmoresolublein waterthanoxygenbut it has a lowerpartial
pressure,making it difficult to obtain high concentrations. Under normal
conditionsof temperatureand pressure, no morethana few milligramsper litre
can be dissolved. To obtainthe maximum oxidative effect, conditionsmust be
created wherethere is maximumtransfer acrossthe gas/water interface. Normal
methodsof dissolving oxygenin waterare unsuitablefor ozonebecause of its
highrate of decomposition.Alternative methods include allowing a large
number of small bubbles containingozone,produced by a fine porousdiffuser,
to rise througha column of water. The rate of rise of the bubbles and the volume
of the liquidcolumn are adjusted to maximizeabsorptionin relationto the rate
of decompositionof ozone.Injecting ozoneinto waterthrougha seriesof water
jets or violentlymixing ozoneand waterin emulsionturbines can also achieve
ozonation. Sonicationcan be employed in conjunction withthesemethodsas it
formsextremely fine bubbles, whichaids dissolutionof ozonein water.
31.1.2 Modeof action
It is thought that ozoneinitiallytargetsthe bacterialmembrane glycoproteins,
glycolipids or amino acids, causing cell death through a change in cell
permeabilityand cell lysis.It can also reactwiththe sulphydrylgroupsof some
enzymes causingdisruptionof normalcellularactivity.Ozone will also affectthe
nucleicacid componentsof the cell by modifyingthe pyrimidineand purinebases.
Whenozoneis dissolvedin waterit decomposesto formmolecular oxygen
and other highlyreactivefreeradicals(HO 2 ,OH andH). The hydroxyl
radicals, whichare formedas a productof ozonedecomposition,can havea
powerful biocidal effect(Choet al.2003).The sitesof actionof ozonedissolved
in waterare similar to those of gaseous ozone.Gurley(1985)highlightsthe
primarysources of attack as the cell membrane, possiblythroughozonolysis of
the carbon±carbondoublebondsin the membrane lipids,leadingto cell lysis.
Otherpossible sitesof actionare the aminoacidsand nuclides of the cell
membrane or enzymes. The actualmethodof microbialinactivationis probably
a combination of boththeseprocesses.
508 Handbookof hygiene controlin the foodindustry