Ozone’s Properties, Functions, and Water Treatment

Ozone has:

I. Strong Oxidizing Property

Ozone has almost the strongest oxidizing power among all known oxidants to date. At the same time, ozone decomposes very rapidly in water. In aqueous solutions containing impurities, it can quickly revert back to oxygen, with a half-life of 5.30 minutes; its stability is further enhanced when the water temperature is close to 0°C. Additionally, studies have shown that the decomposition rate of ozone in water accelerates as both water temperature and pH increase. Due to its strong oxidizing properties, ozone can react with virtually all metals except gold and platinum, and it can oxidize many organic compounds.

II. Easy Degradability

Ozone has relatively active chemical properties and can decompose into oxygen at room temperature, releasing 284 kJ/mol of heat in the process. Expressed in chemical formula, this reaction is: 2O₃ = 3O₂

The decomposition rate of ozone in air is influenced by both temperature and the concentration of ozone itself. Studies have shown that the higher the ambient temperature and the greater the ozone concentration, the faster its decomposition rate. As for ozone's decomposition in water, the rate depends on both water temperature and pH level: the higher the water temperature, the faster ozone decomposes; and the higher the pH—meaning the more alkaline the water—the faster ozone decomposes as well.

III. Highly Corrosive

Because ozone has a high oxidizing power, it can oxidize nearly all metals in the air except platinum and gold. This highlights ozone's corrosive nature. Moreover, ozone also exhibits strong corrosive effects on non-metallic materials. For these reasons, stainless steel is commonly used in the manufacture of ozone-generating equipment in practical production. Furthermore, in both ozone-generating and metering equipment, common rubber cannot be used as a sealing material; instead, corrosion-resistant silicone or acid-resistant rubber must be employed.

Based on the aforementioned properties of ozone, in the fields of wastewater treatment and reclaimed water utilization, we can leverage ozone technology to achieve the following objectives:

I. Ozone Disinfection

1. The mechanism of ozone disinfection

Ozone sterilizes water through two mechanisms: One is that ozone directly attacks the cell walls of bacteria, disrupting them and leading to cell death. The other is that when ozone decomposes in water, it releases reactive oxygen species in the form of free radicals. These free radical oxygen species possess strong oxidizing power; they can penetrate cell walls, oxidize and break down glucose oxidase—enzymes essential for bacterial glucose oxidation within the cells—and also directly interact with bacteria and viruses, damaging their cellular organelles and ribonucleic acid (RNA). They further degrade macromolecules such as DNA, RNA, proteins, lipids, and polysaccharides, thereby disrupting bacterial metabolic processes and reproduction. Moreover, these free radicals can permeate cell membrane tissues, entering the cell interior to act on outer membrane lipoproteins and inner lipopolysaccharides, promoting cell lysis and death. Additionally, they dissolve and denature genetic material, parasitic strains, parasitic viral particles, bacteriophages, mycoplasmas, and pyrogens (metabolic byproducts of bacterial viruses and endotoxins) contained within dead bacterial cells. Some scholars also suggest that ozone acts on the cell surface, altering the permeability characteristics of the cell membrane and ultimately causing cellular components to leak into the surrounding medium.

The inactivation of bacteria by ozone always occurs very rapidly. Unlike other disinfectants, ozone can react with the double bonds in lipids of bacterial cell walls, penetrating deep into the bacterial cells and targeting proteins and lipopolysaccharides, thereby altering cellular permeability and ultimately leading to bacterial death. Ozone also acts on intracellular nucleic acids, such as purines and pyrimidines in nucleic acids, causing damage to DNA. As for viruses, ozone first targets the four polypeptide chains that make up the viral capsid proteins, damaging the RNA—particularly disrupting protein synthesis. After phages are oxidized by ozone, electron microscopy reveals that their outer shells have been fragmented into numerous pieces, releasing large amounts of ribonucleic acid in the process. This interference prevents the phages from attaching to host cells, ensuring thorough sterilization by ozone.

2. Factors Affecting Ozone Disinfection

Ozone exhibits extremely high bactericidal efficiency when used for drinking water disinfection; however, in wastewater disinfection, it often requires a larger ozone dosage and a longer contact time. The main factors affecting disinfection effectiveness are summarized below.

a. The effect of the raw water's pH

Through kinetic experiments conducted under fixed gas flow conditions (q = 10 L/h), we examined the disinfection effectiveness of ozone on experimental water at different pH values. As an example, we considered pH values of 6.7 and 8.0. The experimental results showed that, under the same reaction time, ozone persisted longer in acidic water than in slightly alkaline water. Consequently, the disinfection effect was better in acidic water than in alkaline water.

b. The influence of other substances in water

The main contaminants in water include COD, NO₂⁻-N, suspended solids, and color. These substances can consume ozone in the water. Sometimes, after wastewater is disinfected with ozone, the COD level actually increases. This phenomenon is mainly due to ozone oxidizing refractory, inert substances in the water into smaller molecular compounds or breaking open the rings of certain cyclic organic compounds, thereby increasing the original BDOC. It is therefore essential to pre-treat the raw water thoroughly before ozone disinfection to remove organic matter as completely as possible.

c. The effects of ozone dosage and residual ozone concentration

The required ozone dosage varies depending on water quality; the higher the ozone dosage and the longer the contact time, the better the effluent water quality will be. In a study conducted by Mpetala, secondary effluent treated by conventional activated sludge process was subjected to advanced treatment followed by ozone disinfection, resulting in effluent that met the U.S. EPA’s reclaimed water quality standards.

d. The impact of ozone contact methods

Ozone decomposes rapidly in water. According to the principles of gas-liquid mass transfer, enhancing the mass-transfer efficiency of ozone can improve its utilization rate. Therefore, different dosing methods result in varying ozone utilization rates.

II. The Decolorization Effect of Ozone Technology in Wastewater Treatment

The strong oxidizing property of ozone can also be used to reduce BOD, COD, decolorize, remove odors and unpleasant smells, kill algae, and remove iron, manganese, cyanide, phenol, and other substances [15]. Currently, the ozone oxidation process is mainly applied in the treatment of dyeing and printing wastewater, among which decolorization is particularly important.

According to Clause 4.1.2.1 of the "Emission Standards for Pollutants from Urban Wastewater Treatment Plants" (GB 18918—2002), officially implemented by the state on July 1, 2003, when effluent from wastewater treatment plants is used for urban landscape water and general recycled water applications, the effluent must meet the Class I Standard, Grade A, with a colority ≤ 30. It is required that the reclaimed water undergo decolorization, deodorization, and sterilization treatments.

Ozone has an oxidizing and decomposing effect on colored organic substances in water bodies, and even trace amounts of ozone can achieve excellent results. These colored organic substances are typically polycyclic organic compounds containing unsaturated bonds. When treated with ozone, the unsaturated chemical bonds are broken, causing molecular fragmentation and thus clarifying the water.