Advanced Treatment of Industrial Wastewater: How Does Catalytic Ozone Oxidation Achieve Efficient Pollutant Degradation?

Catalytic ozonation technology efficiently activates ozone through active sites on the catalyst surface to generate reactive oxygen species (ROS), primarily hydroxyl radicals (·OH), converting the selective oxidation of ozone molecules in conventional ozonation into non‑selective free‑radical oxidation, thereby achieving efficient mineralization of refractory organics in industrial wastewater. In advanced treatment of coking wastewater, metal‑catalyzed ozonation improves total organic carbon (TOC) and chemical oxygen demand (COD) removal by 12% to 42% compared with ozonation alone; in one coking plant’s secondary effluent treatment project, COD removal reached 64% to 74%, with a reaction rate constant about 3 times higher than that of ozonation alone, and direct operating costs of only ¥1.67/m³. In petrochemical wastewater treatment, a facility using catalytic ozonation for advanced treatment of biochemical effluent reduced COD from 120 mg/L to below 40 mg/L, with a removal rate of about 67% and operating costs of approximately ¥3.5/ton. This technology has been successfully applied across multiple industries, including coking, petrochemicals, printing and dyeing, pharmaceuticals, and coal chemicals, and has become a key technical pillar for compliance discharge and water reuse in industrial wastewater treatment.

1. From Selective Oxidation to Non‑Selective Oxidation: The Core Logic of Technological Upgrading

Industrial wastewater is highly complex, containing substantial amounts of refractory organics such as benzenes, polycyclic aromatic hydrocarbons, and heterocyclic compounds, which are difficult to completely mineralize by conventional biological treatment. While ozonation alone has some oxidation capacity, ozone molecules exhibit selectivity—they preferentially attack unsaturated double bonds, leaving saturated organics and certain recalcitrant pollutants poorly degraded. Moreover, ozone has limited solubility in water and low utilization efficiency, leading to high treatment costs.

The breakthrough of catalytic ozonation lies in the introduction of a catalyst. Heterogeneous catalytic ozonation uses solid catalysts to partially convert ozone molecules into reactive oxygen species (ROS), significantly enhancing oxidation efficiency and pollutant mineralization. The catalyst's surface active sites (such as surface hydroxyl groups, oxygen vacancies, and metal ions) effectively activate ozone to generate hydroxyl radicals (·OH), superoxide radicals (O₂·⁻), and other ROS. With an oxidation potential as high as 2.80 V—second only to fluorine—hydroxyl radicals react non‑selectively and rapidly with nearly all organic pollutants, gradually degrading them into small molecules and ultimately mineralizing them to CO₂ and water.

Among these, manganese‑based oxides are widely used in ozonation and water purification due to their multiple valence states (Mn(II/III)↔Mn(IV)), low cost, and low toxicity. It is this fundamental shift—from the selective oxidation of ozone molecules to the non‑selective oxidation by hydroxyl radicals—that enables catalytic ozonation to far outperform simple ozonation in treating complex industrial wastewaters.

2. Suspended vs. Fixed‑Bed: How Do the Two Catalytic Processes Affect Operating Costs?

Based on the physical state of the catalyst in the reaction system, catalytic ozonation is divided into homogeneous and heterogeneous catalysis, with significant differences in operating costs and technical feasibility.

In homogeneous catalytic ozonation, the catalyst exists in a dissolved state—typically transition metal ions such as Mn²⁺, Fe²⁺, Cu²⁺, and Co²⁺—which are uniformly dispersed in the aqueous solution, achieving high reaction efficiency. However, a major drawback is the residual metal ions in the treated water, which may cause secondary pollution and require additional post‑treatment steps, often accounting for 15% to 25% of total operating costs.

Heterogeneous catalytic ozonation uses solid catalysts—typically metals, metal oxides, or supported metal/metal‑oxide composites—that remain in the solid phase and are easily separated from water. These solid catalysts can be packed in fixed‑bed reactors for continuous operation, avoiding catalyst loss and secondary pollution. Currently, heterogeneous catalytic ozonation is the predominant approach in industrial advanced treatment, with catalyst types including manganese‑based, iron‑based, copper‑based, and other composite oxides. From a life‑cycle cost perspective, although heterogeneous catalysis requires higher initial capital investment, its long‑term operating costs are generally lower than those of homogeneous catalysis, and it is simpler to operate and maintain.

Regarding catalyst selection, metal‑based catalysts are favored for their excellent activity, structural stability, and tunability. Manganese and iron oxides, in particular, are widely studied due to their abundant availability, strong catalytic performance, and low toxicity. Physicochemical properties such as specific surface area, pore structure, and active‑component distribution directly affect catalytic efficiency. For example, Minstrong Technology produces ozone catalysts for wastewater treatment via chemical synthesis, with active component content exceeding 99%, high specific surface area, and cost‑effectiveness, and these catalysts are extensively used in municipal and industrial wastewater treatment, water purification plants, and chemical industry effluents.

3. Multi‑Industry Applications: Validation and Performance from Engineering Practice

Catalytic ozonation has been successfully implemented in various industrial sectors, with the following representative case studies.

Coking wastewater is one of the most complex and toxic industrial effluents. Pilot studies using the same apparatus on secondary biochemical effluent from five domestic coking plants showed that after 120 minutes of reaction at pH 7–9, COD removal by catalytic ozonation ranged from 64% to 74%, with a reaction rate constant of 0.0101–0.0117 min⁻¹—about 3 times higher than that of ozonation alone. This technology has been applied in a full‑scale project treating coking secondary effluent, achieving direct operating costs of only ¥1.67/m³, with no sludge production and no increase in total dissolved salts. Other studies indicate that COD removal rates for different coking wastewaters vary typically between 55% and 75%, depending on water quality characteristics such as initial COD, salinity, and the presence of coexisting ions. Common Lewis bases like Cl⁻, SO₄²⁻, and CO₃²⁻ mostly inhibit pollutant removal, so engineering designs must adjust parameters based on specific water quality. The optimal conditions are generally temperature 20–25 °C, neutral to slightly alkaline pH, and an ozone consumption ratio (mass of ozone consumed per unit COD removed) of 0.25–0.70 mg/mg.

Petrochemical wastewater: Field surveys and operational data from two industrial‑scale heterogeneous catalytic ozonation units (one treating oily wastewater and the other saline wastewater) confirmed good performance in tertiary treatment. In one petrochemical plant treating 5,000 tons/day of biochemical effluent, catalytic ozonation reduced COD from 120 mg/L to below 40 mg/L, with a removal rate of about 67%, an ozone dosage of 80 mg/L, and operating costs of approximately ¥3.5/ton. For printing and dyeing wastewater, a textile park with highly fluctuating and poorly biodegradable effluent adopted an AO/MBR + catalytic ozonation + biological aerated filter (BAF) process to meet the Class I‑A standard of GB 18918‑2002. Pilot results showed color removal exceeding 95%. In pharmaceutical wastewater, a catalytic ozonation‑BAF combined process was used for advanced treatment of antibiotic production effluent, achieving an average COD removal of 62% with influent COD averaging 125 mg/L.

In coal chemical wastewater, natural dolomite‑catalyzed ozonation in batch experiments improved COD removal by 15.1% compared with ozonation alone, and TOC removal increased by 27.3% at 90 minutes. Long‑term continuous‑flow tests demonstrated that periodic backwashing enabled stable long‑term operation for simulated coal chemical wastewater. This technology has been applied to coking wastewater treatment in steel and coal chemical industries, with over ten demonstration projects at large facilities, producing effluent COD and color that stably meet GB 16171‑2012; it has also been extended to comprehensive steel industry wastewater for deep COD and color removal, with effluent COD below 30 mg/L.

These cases collectively demonstrate that catalytic ozonation delivers stable treatment performance and favorable economics across various water qualities, making it one of the most engineering‑viable advanced oxidation technologies for industrial wastewater treatment.

4. Economic Threshold of Ozone Dosage: The Core Balance Point in Design and Operation

In practical engineering design and operation, the ozone dosage is not simply “the more the better”; there exists an economic threshold that directly governs the optimal trade‑off between treatment efficiency and operating costs.

From a mechanistic perspective, when the wastewater contains large amounts of hydroxyl‑radical scavengers such as bicarbonate (HCO₃⁻) and carbonate (CO₃²⁻), excessive ozone dosage triggers scavenging side reactions, reducing the effective radical concentration instead of increasing it. Moreover, excessive ozone accelerates over‑oxidation of the catalyst’s surface active components, shortens catalyst life, and burdens tail‑gas treatment.

In engineering practice, the standard method to determine the economic threshold is to conduct bench‑scale tests and plot the “ozone dosage vs. COD removal” curve. This curve typically exhibits three stages: in the first stage, COD removal rises rapidly with increasing ozone dosage; in the second, the removal rate gradually slows; in the third, even further ozone addition yields only marginal improvement, or may even decline due to scavenging effects. The inflection point between the second and third stages is the economic dosage point for that specific water quality. Extensive engineering data show that for most industrial wastewaters, the O₃/COD ratio (mass of ozone consumed per unit COD removed) at this inflection point lies between 0.5 and 1.0. Using this as the design basis, the unit treatment cost can usually be controlled within a reasonable range of ¥2 to ¥4/ton. Conversely, if the ozone dosage exceeds the economic threshold by more than 30%, treatment costs may increase by 50% to 80%, while the incremental gain in COD removal is often less than 5%, resulting in significant economic deterioration.

Therefore, in engineering design, it is advisable to first conduct pilot or bench‑scale tests to determine the economic threshold for the specific wastewater, and then install a multi‑level adjustable ozone dosing system to accommodate daily fluctuations in influent quality, thereby achieving the optimal balance between treatment performance and cost.

5. Key Engineering Challenges and Mitigation Strategies

Although catalytic ozonation has demonstrated remarkable advantages in industrial advanced treatment, several practical challenges remain in its engineering application, necessitating targeted mitigation strategies.

Catalyst stability and deactivation are the primary factors limiting long‑term performance. Existing catalysts suffer from loss of active components and declining activity over time. Studies have shown that one major cause of deactivation is the deposition of metal (or non‑metal) salts from saline wastewater onto the catalyst surface, which hinders contact between the catalyst and ozone. Compared with fresh catalyst, the spent catalyst from the reactor shows a decrease in specific surface area of up to 17%, along with a corresponding reduction in pore volume. Fluctuations in pollutant composition, high salinity, and interfering substances can significantly reduce catalyst activity and stability, quench reactive radicals, and impede ozone mass transfer. Furthermore, manganese‑based catalysts face additional issues such as insufficient intrinsic driving force for valence‑state cycling, high sludge production from powdered catalysts, and high energy consumption for recovery and reuse.

Mitigation strategies in practice include: selecting modified catalysts with improved salt‑deposition resistance; installing proper pretreatment units (e.g., coagulation‑sedimentation, filtration) upstream to reduce suspended solids and salt loads entering the catalytic ozonation unit; and establishing a routine monitoring program for catalyst activity, with regeneration or replacement scheduled accordingly.

Low ozone utilization and poor mass transfer are another critical bottleneck. Ozone generation is energy‑intensive, consuming about 15–20 kWh per kg of ozone produced, while conventional bubble‑column or stirred‑tank reactors suffer from limited gas‑liquid mass transfer, with actual ozone utilization typically only 40% to 60%. The reactor configuration directly affects ozone dissolution, mass transfer, and contact with the catalyst.

Mitigation strategies include using microbubble generators to reduce bubble diameter to below 50 μm, dramatically increasing the gas‑liquid interfacial area and raising mass‑transfer coefficients by 3 to 5 times; and employing multi‑stage series reactors to prolong contact time and improve overall ozone utilization. One project case showed that retrofitting a traditional bubble column into a microbubble‑coupled fixed‑bed reactor increased ozone utilization from 52% to 78%, reducing power consumption per ton of water by about 30%.

Water quality fluctuations and coexisting ion interference also cannot be ignored. High salinity, coexisting ions, and pH variations in real wastewater can inhibit catalytic performance. In coking wastewater, common ions like Cl⁻, SO₄²⁻, and CO₃²⁻ mostly suppress pollutant removal. Additionally, pH affects both hydroxyl‑radical generation and catalyst activity.

Mitigation strategies typically involve adjusting the wastewater pH to near the catalyst’s point of zero charge (most metal‑oxide catalysts exhibit higher activity in the pH 6.5–8.0 range) to ensure efficient operation; and installing online pH monitoring and automatic adjustment devices to cope with daily influent variations.

6. Technological Breakthroughs and Future Directions

Addressing the above challenges, current research and industrial practice are advancing along the following fronts:

Catalyst material innovation

Researchers are optimizing catalyst performance through active‑site design, support interface engineering, and pore‑structure control. Manganese‑based catalysts doped with low‑valence metal ions have achieved over 99% removal of typical pollutants under optimal conditions. Studies on monolithic Mn‑Ce dual‑site ozone catalysts indicate that the cerium sites, through electron complementarity from their 4f orbitals, lower the energy required for dual‑site catalytic ozone conversion to hydroxyl radicals and singlet oxygen. Density functional theory (DFT) is playing an increasingly critical role in elucidating ozone activation mechanisms and guiding catalyst design, while artificial intelligence‑accelerated catalyst development also shows great promise.

Reactor optimization

Novel reactor configurations—such as multi‑stage series reactors, internal circulation fluidized beds, and microbubble generators coupled with fixed beds—are being developed to increase gas‑liquid contact area and residence time, thereby enhancing ozone utilization. Computational fluid dynamics (CFD) simulations are used to optimize flow and concentration fields within reactors, eliminating dead zones and ensuring full contact between catalyst and pollutants. Strong‑dissolved‑air‑assisted catalytic ozonation is being piloted for advanced treatment of pesticide wastewater to address the low mass‑transfer efficiency and unstable catalytic performance of conventional systems.

Catalyst regeneration technology

Various regeneration approaches are under investigation. Calcination can effectively remove organic matter from the catalyst surface and pores, partially restoring activity. For catalysts deactivated in high‑alkalinity industrial wastewater, acetic acid washing has proven effective. Liquid‑phase regeneration by eliminating O₂²⁻ from oxygen vacancies also shows good application prospects. In practice, the regeneration method should be selected based on the dominant deactivation mechanism: calcination is preferred for organic fouling, while acid washing is more suitable for salt deposition.

Process integration and intelligent control

Catalytic ozonation is often combined with biological treatment, membrane separation, and activated carbon adsorption to form multi‑stage “physicochemical‑biological‑advanced” treatment trains. At the intelligent control level, researchers are developing models that correlate water‑quality parameters with organic removal in catalytic ozonation, using artificial neural networks trained on fingerprint data and physicochemical indicators from different wastewater types. This direction is expected to enable precise matching of catalysts and process parameters, further improving efficiency and cost‑effectiveness.

7. Conclusion

Catalytic ozonation, by converting ozone molecules into hydroxyl radicals and other reactive oxygen species, achieves a leap from selective to non‑selective oxidation, enabling efficient mineralization of recalcitrant organics that are poorly treated by conventional ozonation. From the 64%–74% COD removal and about 3‑times higher reaction rate in coking wastewater, with direct costs as low as ¥1.67/m³, to the 67% COD removal in petrochemical effluent and over 95% color removal in dyeing wastewater—these data collectively prove that catalytic ozonation is one of the most engineering‑valuable technologies for industrial advanced treatment. In engineering design, determining the economic threshold of ozone dosage through pilot tests (typically with an O₃/COD ratio of 0.5–1.0) can keep unit treatment costs in the ¥2–¥4/ton range, avoiding cost escalation and efficiency loss from overdosing. Although challenges remain in catalyst stability, ozone utilization, and water quality complexity, continuous progress in catalyst materials, reactor design, regeneration techniques, and intelligent control will further cement this technology’s critical role in achieving compliance discharge and resource recovery in industrial wastewater treatment.