Tuesday, 17 April 2012

The Story of Ozone

Ozone occurs naturally in the environment. Probably the largest short-term concentrations occur during thunderstorms when lightening causes production of ozone. In the office environment, ozone is detected near photocopy machines. Welders are exposed to ozone production by the arc welding process. Urban dwellers in large cities are exposed to ozone concentrations in the 0.5-1.0 ppm range when exhaust gases from cars and industries react with sunlight.

Looking back at the history of ozone, a gentleman named von Marum describes for the first time, in 1785, the characteristic odour at an electrostatic machine. In 1801, a Mr Cruikshank detects the same odour at an anode. In 1840, Schoenbein names the substance ozone, after the Greek word “ozein” which means “reek, smell”. Later, in 1857, Werner von Siemens designs an ozone generator of the cylindrical dielectric type.

The first plant for treating water was installed in The Netherlands in 1906.  Today there are many plants for treating drinking water supplies and much smaller units for treating water during bottling.  The principle of production is the same in most plants. Ozone is produced in an ozone generator. The feed gas can be dry air or pure oxygen. A high voltage is applied to two electrodes producing an arc. In the arc, part of the O2 is converted to O3.

Ozone is very unstable and reverts back into O2 in minutes. That is why ozone must be generated on site. About 1-10 percent of the oxygen flowing past the electrodes is converted into ozone. When air is the feed gas, ozone concentrations between 1-4% are generated. When the feed gas is pure oxygen, the ozone concentrations will be between 4-12% by weight. About 80-95% of the energy will be converted to heat, and must be removed at the ground electrode, usually through cooling water.

Use of ozone in the bottling plant, for terminal disinfection of 19L polycarbonate bottles or for disinfection of the water itself, requires considerable care.  There is a tendency to use concentrations of ozone which are too low for terminal disinfection of 19L bottles (0.4mg/l is often used but 0.8mg/l is required).  Ozonation of the water itself needs special care, to avoid the formation of bromate, which can be carcinogenic at concentrations as low as 10 millionths of a gram/l.

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