OZONISERS

Description

Ozone is a very powerful oxidising agent and biocide. It is used to oxidise impurities in process waters and to disinfect potable and waste waters. Ozone is fast acting, effective, nonselective and versatile. Contaminants are oxidised to smaller molecules and ultimately to carbon dioxide and water. Dangerous by-products are not generally formed.

Ozone is the triatomic form of oxygen; oxygen gas has two atoms (O2), ozone has three (O3). Ozone is manufactured from oxygen but is unstable and tends to decompose back to oxygen. It is this instability that makes ozone a powerful oxidant, but it also makes storage impractical, so ozone is manufactured at the point of use.

Commercial ozonisers produce ozone by passing dried air or oxygen between electrodes separated by a dielectric such as glass. A pulsed potential difference of 7000-18,000 volts is maintained between these electrodes producing a silent corona discharge in which oxygen molecules split. A proportion of the resultant oxygen atoms reassemble as ozone.

To be effective, the ozone must be dissolved in the liquid containing the microorganisms or the pollutants. Ozone does not dissolve easily, so the contact tanks are up to 5 m deep and incorporate efficient mixing and gas transfer systems to minimise wastage. After oxidation of the pollutants, residual ozone has to be destroyed in a thermal oxidiser, a catalytic incinerator or with activated carbon, to prevent it coming into contact with operators or the environment.

Application

Ozone is used in water supplies to oxidise and destroy pesticides and other pollutants and to assist in flocculation. Ozone is used to treat toxic pollutants and oxidise intractable discharges from the chemical and textile industries.

Ozone is used to disinfect potable water, and is effective against the pathogens Cryptosporidium and Legionella. Ozone can also be used to disinfect swimming pools, and water for recycling in fish farming, horticulture and aquatic zoological exhibits.

Ozone is non-selective and expensive. Therefore, easily oxidisable material should be removed before employing ozone against the most intractable compounds. Ozone can produce partial oxidation products that, although not toxic, increase the BOD of an effluent.

Sizing

Typical doses for potable water treatment could be 1-7 mg [O3]/l and for waste water treatment 5-15 mg [O3]/l. Small units are used for water recycling applications, mid-sized units for industrial discharges and the largest units for water supply. The production of 1kg of ozone requires about 20kWh. An ozone installation large enough to treat about 3000 m3/hour of water would have an installed load of about 250 kW.