Breaking The Bottleneck of Ozone Application: Enhanced Mechanism of Micro-nano Bubble Technology in Dissolution, Mass Transfer And Oxidation

Breaking the Bottleneck of Ozone Application: Enhanced Mechanism of Micro-nano Bubble Technology in Dissolution, Mass Transfer and Oxidation

In the field of water treatment, ozone, as a commonly used oxidant, has certain effects, but it has problems such as low utilization rate and poor stability. However, micro-nano bubble technology has gradually emerged and been widely applied in recent years. When ozone is combined with micro-nano bubble technology, it shows many advantages and has become a new technological hotspot in the control of water environmental pollution.

Background of Combined Use of Technologies

Ozone has always held an important position in water treatment, with an oxidation-reduction potential of 2.08V, effectively oxidizing and removing organic matter, thus receiving extensive research. However, the mass transfer efficiency of ozone in water is generally low, and the oxidation is incomplete, leaving much room for improvement. To enhance its treatment effect, many researchers have made various attempts. On one hand, ozone is combined with other processes to form advanced oxidation processes such as ozone/hydrogen peroxide, UV/ozone, or ultrasound/ozone; on the other hand, the oxidation capacity of ozone is increased through catalysis, such as ozone oxidation technology catalyzed by metal ions or metal oxides. However, the former has problems such as low ozone solubility, high cost, complex and immature technology, while the latter requires control of the addition ratio, and metal ions are difficult to separate and the oxidation rate is not fast. These factors limit the application of these advanced technologies.

Due to their tiny characteristics, micro-nano bubbles possess special properties, such as long existence time, high gas-liquid mass transfer efficiency, generation of more free radicals, and high interface ζ potential. After years of research and development, micro-nano bubbles have begun to be applied in aquaculture, in-situ groundwater remediation, mineral flotation, biopharmaceuticals, wastewater treatment, and other fields. In recent years, researchers have found that micro-nano bubbles have a reinforcing effect on ozone oxidation, and the combined use of ozone and micro-nano bubbles has good effects. Microbubble generators also have the advantages of small footprint, low energy consumption, easy operation, and high treatment efficiency, making ozone micro-nano bubble technology a feasible water treatment technology, and related research has gradually become a hot topic.

Technical Characteristics and Advantages

Ozone has a strong oxidation capacity and performs well in decolorization, deodorization, odor removal, organic matter degradation, and COD reduction. The use of ozone oxidation to remove organic matter is fast, simple to operate, does not cause secondary pollution, and operates under mild conditions. However, it has certain selectivity for organic matter and cannot completely mineralize pollutants. It has a low solubility in water, poor stability, and is prone to decomposition into oxygen. The generation equipment is complex and the operating cost is relatively high. Micro-nanobubbles usually refer to tiny bubbles with a diameter of less than 50 μm. Bubbles with a diameter greater than 1 μm are called microbubbles, and those with a diameter less than 1 μm but greater than 1 nm are called nanobubbles. Due to their small size, they have special properties such as a long existence time, high gas-liquid mass transfer efficiency, high interface ζ potential, and the generation of more free radicals. The ozone micro-nanobubble technology combines the advantages of both, featuring enhanced ozone dissolution, prolonged residence time, and increased oxidation capacity.

In terms of promoting ozone dissolution, the Laplace formula can be used to calculate the internal pressure of micro-nanobubbles based on their diameter. A microbubble with a diameter of 1 μm has an internal gas pressure approximately three times that of atmospheric pressure, and the internal overpressure of a nanobubble with a diameter of 20 nm is even higher. The internal high pressure facilitates better mass transfer and dissolution of ozone in water. Research shows that small and stable bubbles have a large surface area, which can increase the mass transfer coefficient and the concentration of dissolved ozone. Domestic comparisons have found that the ozone absorption rate in the microbubble contact method is much higher than that in the traditional contact method. Foreign research by Fan also indicates that the ozone solubility and mass transfer coefficient in micro-nanobubbles are 2.03 to 3.68 times that of traditional large bubbles. In studies on the degradation of drugs using ozone micro-nanobubbles, the ozone utilization rate is 2.8 times higher than that of ozone treatment alone.

The removal effect of organic matter

Ozone micro-nano bubbles have a stronger oxidation capacity compared to ozone alone. The ζ potential at the interface of microbubbles is high, and their adsorption performance is strong. They can be used to remove refractory organic matter and treat complex industrial wastewater. For industrial wastewater with high pollutant concentration, strong biological toxicity, poor biodegradability, complex water quality conditions, and large fluctuations, they can effectively adsorb and remove suspended solids and oil substances, and also have good removal effects on COD, ammonia nitrogen, and total phosphorus, demonstrating a promising application prospect in wastewater treatment.

From related studies, the application of ozone micro-nano bubble technology in treating different types of wastewater has shown significant results. For example, in textile industrial wastewater, this process is efficient and cost-effective, with TOC, COD, and SS all meeting the standards for industrial reuse water. In acrylonitrile manufacturing wastewater, under the same ozone dosage, the removal rates of COD, NH3-N, and UV254 by ozone micro-nano bubble technology are much higher than those by macro bubble ozonation. In biochemical effluent from coal chemical wastewater, it can effectively degrade and remove refractory nitrogen-containing heterocyclic aromatics, generating NH3-N and other small-molecule organic substances, and improving the biodegradability of the wastewater. In industrial wastewater from chemical parks, the COD in the effluent is reduced to below 20 mg/L, the inhibition rate of wastewater luminescence is decreased, and the biodegradability is increased. In TCE-contaminated groundwater, the total removal rate reaches 99% after continuous treatment. In atrazine solution, under different pH conditions, the removal rate by ozone micro-nano bubble technology is better than that by large bubble aeration systems. In simulated aquaculture water containing antibiotics, the removal rates of high manganese acid index, trace metronidazole, and sulfadiazine in the water are also considerable.

Microbial Inactivation Effect

In water treatment, disinfection is a crucial step aimed at eliminating microorganisms. In China, chlorination is commonly used to treat pathogenic microorganisms in water and wastewater, but it has some drawbacks, such as the formation of undesirable by-products related to the removal of coliform bacteria, poor inactivation of spores, cysts, and some viruses at low doses, and increased dechlorination costs to reduce toxicity. Therefore, more advanced disinfection processes are needed for natural water bodies. Ozone, with its strong oxidizing properties, can effectively kill bacteria and viruses, especially protozoan parasites that are resistant to most other disinfectants. However, due to its low disinfection efficiency, high related costs, and lack of continuous disinfection effect, ozone disinfection has not been widely applied in most water treatment plants.

After comparison by researchers, the ozone micro-nano bubble technology has a faster reduction rate, more complete inactivation, smaller reactor size, and lower ozone dosage requirements compared to traditional ozone disinfection processes. When applied in disinfection processes, it shows excellent microbial inactivation effects. For example, in the secondary effluent of wastewater reuse, after 600 seconds of contact time, the ozone consumption is reduced by 3 mgO₃/L, 99% of the microorganisms are inactivated, the dissolution rate increases, and the ozone half-life is extended by 1.6 times. In plant nutrient (hydroponic) solutions, after different contact times, 99% of the microorganisms can be inactivated. In laboratory-cultured spores, under a specific inlet O₃ concentration, the logarithmic inactivation rate significantly increases. In tomato airborne disease suspension, it has a good dissolution effect on two airborne pathogens.

Color removal and sludge reduction

Most colored pollutants are toxic and can disrupt aquatic ecosystems. Decolorization only removes color and does not decompose complex dye molecules. Color removal does not mean the degradation of organic matter in water. The wastewater discharged from the textile and dye manufacturing industries is highly colored and many of the dyes are not biodegradable. Although ozone oxidation is used to remove color, there are some problems, such as the formation of organic acids from ozonation products, which lowers the pH around the dye molecules, and the reaction mechanism may shift towards selective direct oxidation. Advanced oxidation in acidic environments may generate undesirable and toxic by-products, and the process is costly. Therefore, researchers have begun to use micro-nano bubble ozone oxidation to improve these problems. Its larger gas-liquid interface area, less ozone requirement, and the generation of a large number of hydroxyl radicals have enhanced the efficiency of the ozone oxidation process.

From relevant studies, it is known that the ozone micro-nano bubble technology has a remarkable effect in removing color. In the simulation of dye wastewater, after a specific working time and removal time, the removal rate of industrial dye RB5 reached 99%, with an extremely high ozone utilization rate. For the original printing and dyeing wastewater, compared with the traditional air flotation, the decolorization rate increased by 110%. For melanoidin water solution, the decolorization was mainly caused by the direct oxidation of ozone. In terms of sludge reduction, many sewage treatment plants in China adopt the activated sludge method, which generates a large amount of excess sludge, with high treatment costs and the excess sludge containing harmful substances. If not properly treated, it will cause environmental pollution. Compared with traditional ozone treatment, the ozone micro-nano bubble technology can significantly improve the efficiency of ozone treatment and enhance the effect of carbon source release from sludge. In different studies, this technology provides an effective and low-cost approach for sludge reduction. For instance, after the treatment of sludge from different sewage treatment plants with this technology, there were obvious effects in terms of COD release, TN and TP release, increase in dissolved chemical oxygen demand, improvement in the reduction rate of suspended solids in the mixed liquid, increase in protein and polysaccharide content, reduction in sludge particle size, and enhancement of the porosity on the surface of sludge flocs.