Disinfection Using Ozone

Ozone is a very strong oxidizer and has been found to be superior to chlorine in inactivating resistant strains of bacteria and viruses. It is very unstable, however, having a half-life of only 20 to 30 min in distilled water at 20°C. It is therefore generated on site before use.

Ozone may be produced by first refrigerating air to below the dew point to remove atmospheric humidity. The dehumidified air is then passed through desiccants such as silica gel and activated silica to dry to -40 to -60°C. The dried and dehumidified air is then introduced between two electrically and oppositely charged plates or through tubes where an inner core and the inner side of the tube serve as the oppositely charged plates. Passage through these plates converts the oxygen in the air into ozone according to the following reaction:

Typical ozone dosage is 1.0 to 5.3 kg/1000 m of treated water at a power consumption of 10 to 20 kW/kg of ozone. It has been observed that complete destruction of poliovirus in distilled water is accomplished with a residual of 0.3 mg/L of ozone in 3 min. As in the case of chlorination where a chlorine demand is first exerted before the actual disinfection process can take place, an ozone demand in ozonation is also first exerted before the actual disinfection process can take place. The ozone demand can be variable; therefore, the contact time for ozonation is best determined by a pilot plant study. Depending upon the nature of the ozone demand, a contact time of 20 min is not unreasonable. A residual ozone concentration of 0.4 mg/L has been found to be effective (Rice et al., 1979).

The immediate ozone demand parallels that of the immediate chlorine demand. Recall that this demand is due to ferrous, manganous, nitrites, and hydrogen sulfide. The immediate demand reactions with ozone are as follows: with ferrous:

with manganous:

with nitrites:

with hydrogen sulfide:

As indicated in all the previous reactions, an intermediate O- is first produced. This is called nascent oxygen and is the one responsible for the potent property of ozone as a strong oxidant and, hence, as a strong disinfectant.

Note: All in all, a mole of ozone grabs 2 moles of electrons, making it a strong oxidizer. Also, all the previous reactions show that molecular O2 has been produced from the decomposition of ozone.

This is one of the advantages in the use of ozone; the effluent is saturated with dissolved oxygen. Of course, the previous reactions are simply for the immediate ozone demand and have nothing to do with disinfection. As mentioned, these reac. tions must be satisfied first before the actual act of disinfecting commences.

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