Ozone treatment of a cooling tower can reduce the quantity
of blowdown. Actual reductions will be system specific, but
can range from 50% to 90%. Although actual elimination of
blowdown altogether is unlikely, this is a very effective
technique used to reduce water consumption. Ozone decomposes
into oxygen, and therefore imparts no undesirable chemical
residue into the system. Therefore, blowdown from cooling
towers treated with ozone only will be free of biocide, scale,
or corrosion control chemicals.
Ozone is an unstable
gas that will occur naturally during a thunderstorm. There
are also several man made sources such as electronic copying/printing
equipment, electric motors. For industrial use, it is generated
and it is to be used. The inherent instability of ozone makes
it impractical to package for use and shipment.
Ozone can be
generated from dry air, with a dew point of -51oC
or less, or dry oxygen with a dew point of -51oC
or less. An air-fed ozone generation system includes an air
filter, air cooler, compressor and dryer. Ozone reactions
in cooling water are as follows:
1) Biofouling
Control
Ozone will disinfect
more rapidly than any other commercially available oxidant.
This feature makes ozone a viable technique for biofilm control
and to attack organic binders holding scaling materials to
cooling water surfaces. The mucilage is usually generated
by bacteria in the system.
2) Organics
Ozone will destroy
many organics commonly encountered in recirculated cooling
systems, some rapidly, some slowly, and some no reaction.
Ozonation of some organics will result in by-products that
will be difficult to oxidize further. Thus, although ozone
is effective in destroying many organics, all organics will
not be completely converted to carbon biocide and water.
3) Algae
Algae are readily
oxidized by ozone. However, algae will reappear on exposure
to sunlight unless a small ozone residual is left in the system.
Ozone levels in the 0.01 to 0.05 mg/l range, maintained for
a few minutes, are usually sufficient for algae control.
4) Corrosion
Control
Current data
on corrosion control benefits that are obtained by using ozone
is conflicting. In some cases, corrosion rates are slow and
others, more rapid. Mild steel and aluminum corrode slower
with ozone presence, whereas galvanized steel has an increased
corrosion rate versus water containing oxygen. It seem reasonable
that some corrosion is expected, but can be minimized by maintaining
water cleanliness.
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