Ozone disinfection of water

Disinfection of water with Ozone.

Ozone is the strongest chemical disinfectant and its use in water treatment is becoming more and more common. Ozone (O3) is an allotrope of oxygen with three oxygen atoms. The word ozone comes from the Greek ozein, which means "to smell." Ozone in the air has a very characteristic odor, this is detectable by most people in concentrations greater than 0.1 ppm.

Dissolved in water, ozone begins its decay process and forms hydroxyl radicals (HO ·), these also react with microorganisms to inactivate them. However, the action of the direct reactions of ozone with pollutants has a higher disinfection performance than the hydroxyl radical. For this reason, the addition of a residual concentration of this is recommended to ensure the integrity of the water.

The reaction with which ozone inactivates the microorganisms present in water is an oxidation in which oxygen, water and inactive microorganisms are generated:

Reaction with which ozone inactivates microorganisms present in water

Ozone generation.

Ozone is unstable (explosive) in high concentrations (> 23%) and under ambient conditions it degrades easily. Therefore, unlike chlorine gas, ozone cannot be stored in pressurized tanks. This can be generated by photochemical, electrolytic and radioactive methods. The most common method is corona discharge. In this method, oxygen is passed through an electric field, which is generated by applying a high-voltage potential across two electrodes separated by a dielectric material (Figure 1). As oxygen molecules pass through the electric field, they break into highly reactive oxygen ions (O ·), these radicals react with each other to form ozone molecules. The thickness of the gap through which the gas stream (rich in oxygen) passes is 1 to 3 mm.

Ozone generation by corona discharge.

Figure 1. Ozone generation by corona discharge.

Since most of the energy in ozone generation is lost as heat, cooling the ozone generator is necessary to avoid overheating that causes ozone decomposition. The cooling process is accomplished by running cold water close to the ground electrode.

Ozone can be generated from oxygen in the air or from pure oxygen. The most accessible source to generate it is ambient air, which contains about 21% of oxygen by volume. However, this has been superseded by liquid oxygen, except for remote plant systems.

Ozone injection system.

Previously, the most common system for dosing ozone in water was to use a container with a porous diffuser at the bottom, which was responsible for bubbling the ozone and this in turn made contact with the water throughout the entire column. Recently, the injection system and the contact system are designed separately. For the dosing system a venturi injector is used, connected in the line with a bypass (Figure 2), after injecting ozone into the line, it is directed to a contact chamber where disinfection reactions are carried out. These chambers can be pipe systems, coils, or baffles.

Bypass arrangement for in-line venturi installation.

Figure 2. Bypass arrangement for in-line venturi installation.

 

Last update 02/12/2020