Ozone (O3) is an enriched form of oxygen (O2). It is a powerful oxidant and can oxidise organic matter (e.g. algae, humic acids, etc.) as well as dissolved metals (e.g. manganese and iron). Because it is so powerful, ozone is a very good disinfectant and can kill harmful microorganisms like bacteria and viruses.

Ozone is generated on-site at Midvaal by using specialised ozone generation equipment and natural air. In nature, air contains about 18% oxygen. In the ozone generation process, this oxygen is concentrated through a method called Pressure Swing Adsorption (PSA), and then passed over an electrical discharge to form an ozone concentration of 9%.

The ozone in the form of gas is added to the water in specially designed contact reactors, where the ozone dissolutes (dissolves in the water) and reacts with the targeted constituents. At Midvaal, ozone is added at two stages of the process:

Stage 1

Pre-ozonation, where ozone is added at a concentration of maximum 1.5 mg/l to deal with algae in the raw water prior to any other treatment process.

Stage 2

Intermediate ozonation is performed to deal with the more complex constituents, at concentrations ranging between 1.5 and 2.5 mg/l. Intermediate ozonation is applied after most of the algae and suspended particles have been removed through the DAF process.

O3 concentration 9% v/v O3 in air stream
Pre-ozone reactor Radial flow type, contact time 2 minutes Transfer efficiency: approximately 92%
Intermediate ozone reactor Deep U-Tube, contact time 4 minutes Transfer efficiency 98%
U-tube 15 m deep
Control Frequency control; voltage control (±60 Hz)
Generators 3 x 8 kg/h each
Dew point Minimum – 80°C
Oxygen supply PSA method
Energy consumption ± 40 kWh / kg generated