OZONE DISINFECTION

Wastewater must be adequately treated before disinfection for any disinfectant to be effective. Ozone, an unstable gas (O3) produced by dissociating oxygen (O2) molecules with an energy source, is used for this purpose. Wastewater treatment plants generate ozone onsite by applying a high voltage (6 to 20 kilovolts) across a dielectric discharge gap containing oxygen-bearing gas. Due to its instability, ozone quickly decomposes to oxygen. It is a powerful oxidant and virucide, with disinfection mechanisms including:

Direct oxidation/destruction of the cell wall with leakage of cellular constituents outside of the cell.
Reactions with radical by-products of ozone decomposition.
Damage to the constituents of the nucleic acids (purines and pyrimidines).

When ozone decomposes in water, it forms hydrogen peroxy (HO2) and hydroxyl (OH) radicals, which have strong oxidizing properties and play a key role in disinfection. This process likely destroys bacteria through protoplasmic oxidation, leading to cell wall disintegration (lysis). The effectiveness of disinfection depends on the target organisms' susceptibility, contact time, and ozone concentration. An ozone disinfection system includes feed-gas preparation, ozone generation, ozone contacting, and ozone destruction. The feed gas, either air or pure oxygen, is passed to the ozone generator, where electrical discharge produces ozone. Figure 1 shows a diagram of the ozonation process.

• The control mechanism (either a voltage or frequency unit).

• The physical arrangement of the dielectrics (either vertical or horizontal).

• The control mechanism (either a voltage or frequency unit).

Applicability

Ozone disinfection is generally used at medium to large sized plants after at least secondary treatment. In addition to disinfection, another common use for ozone in wastewater treatment is odor control. Ozone disinfection is the least used method in the U.S. although this technology has been widely accepted in Europe for decades. Ozone treatment has the ability to achieve higher levels of disinfection than either chlorine or UV, however, the capital costs as well as maintenance expenditures are not competitive with available alternatives. Ozone is therefore used only sparingly, primarily in special cases where alternatives are not effective.