Inside Advanced Ozone Oxidation Technology: How It Works and How It Is Applied
The origins of ozone
Ozone, chemical formula O₃, is a triatomic oxygen molecule built from three oxygen atoms and is often called “active oxygen”. Its name comes from its distinctive smell, reminiscent of fish mint. At room temperature ozone spontaneously converts back to oxygen. It is denser than oxygen, has low solubility in water and is chemically rather unstable. Ozone forms when an oxygen molecule binds one extra oxygen atom, giving a metastable structure. In use, the surplus oxygen atom not consumed in oxidation recombines into stable oxygen, so no harmful residue remains. Ozone is generated mainly from nitrogen oxides (NOx) and volatile organic compounds (VOCs) through complex photochemical reactions.
The ozone molecule
How ozone oxidises
Ozone’s outstanding oxidising power comes from the strong electron affinity of the individual oxygen atom in its molecule. When ozone decomposes in water it generates hydroxyl radicals of extremely high oxidising power, which efficiently degrade organic pollutants in wastewater and finally revert to oxygen, leaving no secondary pollution.
Current research generally holds that ozone reacts with organics along two main pathways:
(1) Ozone acts directly, as a molecule, on organics in the water. This pathway is highly selective and preferentially attacks substances containing double bonds, reacting most readily with aromatic compounds and unsaturated hydrocarbons.
(2) In alkaline conditions ozone decomposes in water to produce a range of highly reactive intermediates, hydroxyl radicals among them, which then take part in the oxidative degradation of organics.
The role of ozone in wastewater treatment
Ozone (O₃) is an efficient oxidant, disinfectant, bleaching agent and odour remover; the technology is regarded as a core tool in environmental pollution control and water treatment today and ranks among the four key environmental technologies of this century. It is widely used to degrade organics in air and water, inactivate microorganisms and control odours. Ozone effectively breaks down most colouring substances, and its outstanding oxidising and disinfecting power makes it one of the strongest oxidants in nature, with a redox potential in water second only to fluorine.
The basic principle of ozone oxidation purification
The key to the technology is silent discharge in a specific electric field, which generates ozone in large quantities. In the process, high-energy electrons collide with gas molecules and trigger a series of basic physical and chemical reactions that activate the molecules and generate a variety of highly reactive radicals, which then catalyse, oxidise and break down pollutants and microorganisms into harmless products, achieving deep purification of air and water.
In wastewater treatment, ozone oxidation is used mainly to eliminate bacteria and odours and to remove soluble organics and other pollutants efficiently in various treatment trains.
SINOKLE advanced ozone oxidation industrial unit and its wastewater treatment performance
Characteristics of ozone oxidation technology
Strong oxidising power and fast reaction;
Extremely effective against bacteria, viruses, spores and soft-bodied microorganisms;
Oxidises dissolved organics and lowers BOD and COD in water; ozone is made from oxygen in the air and reverts to oxygen after use, raising dissolved oxygen without secondary pollution;
Improves the physical and chemical properties of water and is good at decolourisation, deodorisation and odour removal;
Disinfection with ozone produces no toxic chloroform or carcinogenic organic halide by-products and leaves no residue harmful to humans or livestock.
Challenges facing ozone oxidation technology
Although ozone, with its high redox potential (2.07 V, second only to fluorine), is widely used for disinfection, deodorisation and decolourisation of drinking water, several technical bottlenecks remain when it is used alone for wastewater treatment:
Ozone reacts rather selectively with organics; at low doses and short contact times it cannot fully mineralise pollutants, and the intermediates it generates inhibit further oxidation.
Ozone generation is expensive and utilisation is relatively low, which makes treatment costly. Research on raising ozone utilisation and oxidising power is therefore a hot topic at home and abroad.
Where ozone oxidation stands in the wastewater industry
In recent years researchers at home and abroad have made marked progress in applying advanced oxidation technologies (AOTs/AOPs). By combining ozone oxidation with other water treatment processes, the technology generates hydroxyl radicals with stronger oxidising power and lower selectivity (redox potential as high as 2.80 V).
Compared with conventional treatment, advanced ozone oxidation offers the following advantages:
Good performance on biologically recalcitrant substances
Fast degradation
Small footprint
High degree of automation
No secondary pollution and no scum
Less sludge production
It has been widely used in the food, pharmaceutical, petrochemical, printing and dyeing, steel and water supply industries.
History of the technology
In 1896 Meritens was the first to use ozone as a disinfectant in drinking water treatment.
The Bon Voyage plant, built and commissioned in Nice in 1906, is regarded as the birthplace of drinking water ozonation.
By the late 1970s ozone was still used mainly for disinfection, chiefly in France and a few other countries.
In recent years ozone oxidation has been used very widely, with clear advantages in disinfection, sterilisation, decolourisation, deodorisation, oxidation of recalcitrant organics and improvement of flocculation. Its good performance in removing colour from dye, printing and dyeing and papermaking wastewater has been put into practice in recent years.
Ozone degrades large chromophore molecules into smaller ones and finally removes them. Papermaking wastewater is characterised by high BOD and COD, strong colour and odour, and contains high concentrations of chlorophenols, chloroacetones and chloroform. Part et al. used ozone to decolourise bleached kraft mill effluent, passing ozone at 32.4 mg/L through a continuously operated column reactor with a 3 min residence time; colour was completely removed.
Domestic application cases
Wang Yu and colleagues treated printing and dyeing wastewater with high-frequency ceramic surface-discharge ozone technology; at a contact time of 10 hours and pH 7–8, COD removal reached 86.6% and decolourisation 98.4%. The study also noted that keeping pH ≤ 7 in decolourisation applications slows ozone decomposition and raises hydroxyl radical yield, improving ozone utilisation.
Jin Lahua’s experiments showed that ozone can improve the biodegradability of mixed wastewater from an automobile plant. Under specific process conditions, two hours of ozone oxidation gave COD removal of up to 60%.
In recent years ozone has also been used for advanced treatment of municipal sewage. A typical dose of 10–20 mg/L with 5–20 minutes of contact achieves 40% COD reduction, 70% BOD reduction, 60% SS reduction, 20% ammonia nitrogen reduction, 80% reduction in carcinogens and 90% colour removal.
A fine-bubble ozone treatment unit developed in Japan has been successfully applied. Its bubbles are only 10–100 μm in diameter, greatly increasing the gas-liquid contact area, raising ozone solubility 5-fold and markedly improving utilisation while enhancing mixing. Because it is efficient and economical, the technology has been extended to wastewater treatment in food, livestock and aquatic product processing plants.
As ozone technology keeps advancing, its research and application have become an industry in their own right worldwide, with broad prospects ahead.
The latest breakthrough in ozone oxidation technology
Building on its successful development of cyclonic dissolved-air flotation (CDFU) technology, SINOKLE has innovatively introduced advanced ozone oxidation to create a highly efficient integrated technology of advanced ozone oxidation plus cyclonic dissolved-air flotation — CDOF. The unit combines ultrasonic catalytic ozonation, chemical flocculation, hydrodynamic cavitation, cyclonic separation and dissolved-air flotation, greatly improving ozone utilisation and accelerating the oxidation reaction.
Ozone oxidation industrial unit
CDOF technology rapidly and cost-effectively reduces all kinds of pollutants in water, including recalcitrant organics, dissolved oil and colour. Through the synergy of ozone oxidation and flocculation, the technology not only enables cyclonic dissolved-air flotation to remove colloidal and dissolved organics, which conventional flotation cannot do, but also greatly improves separation efficiency, achieving efficient integrated treatment of a wide range of complex wastewaters.
Ozone performance in wastewater treatment
CDOF technology, an independently developed patented achievement of a Chinese research team, has reached a globally leading standard. More than ten patents have been granted at home and abroad, including one international patent and three Chinese invention patents.
