Ozone Catalytic Oxidation: Engineering Answer for Leachate

Landfill leachate typically exhibits COD concentrations in the range of 500–5,000 mg/L, with a BOD₅/COD ratio often below 0.1, placing it firmly in the category of recalcitrant wastewater. Although conventional “biological treatment + membrane separation” processes can achieve compliant effluent, the secondary treatment of membrane concentrate remains a significant challenge. Ozone catalytic oxidation technology addresses this by using catalysts to promote ozone decomposition into hydroxyl radicals (·OH), which break down large, refractory organic macromolecules into smaller, biodegradable substances. Engineering practice demonstrates that combined processes centered on ozone catalytic oxidation can achieve COD removal rates exceeding 91%, with some projects reporting removal efficiencies of 95.8% for COD, 99.2% for ammonia nitrogen, and 92.5% for total nitrogen. The addition of catalysts substantially improves ozone oxidation efficiency, reducing the required reaction time from 160 minutes to 120 minutes, while operational costs drop from RMB 61.49/m³ to RMB 46.12/m³. This approach offers a technically feasible and economically sound engineering solution for the advanced treatment of landfill leachate.

1. Industry Challenges in Landfill Leachate Treatment

Treating landfill leachate has long been a persistent challenge in solid waste management. The key pollution indicators are daunting: COD concentrations can reach tens of thousands of mg/L, and ammonia nitrogen often exceeds 1,000 mg/L—orders of magnitude higher than typical municipal wastewater. As landfills age, the biodegradability of leachate continues to decline, with BOD₅/COD ratios frequently falling below 0.1, and the proportion of recalcitrant organics increasing. Leachate contains numerous toxic substances, including aromatics, halogenated compounds, and heavy metals, posing serious risks to soil, groundwater, and ecosystems.

Conventional treatment technologies face inherent limitations. Membrane processes (nanofiltration/reverse osmosis) can achieve stable effluent quality but suffer from severe membrane fouling, high replacement costs, and the production of concentrate volumes equal to 15–30% of the original feed. This concentrate has COD levels in the thousands of mg/L, high salinity, and is practically non-biodegradable. The Fenton oxidation method relies heavily on chemical reagents and generates difficult-to-dispose iron sludge. Biological processes (e.g., MBR) are constrained by poor biodegradability and often fail to meet discharge standards consistently. Against this backdrop, advanced oxidation technologies that combine high degradation efficiency with stable operation have become a key focus for the industry.

2. Water Quality Characteristics and Treatment Difficulties of Landfill Leachate

The difficulty in treating landfill leachate stems from its unique water quality characteristics.

High organic matter concentration. COD levels typically range from 500 to 5,000 mg/L, and in some raw leachates can reach tens of thousands of mg/L. These organics are dominated by large, recalcitrant molecules such as humic and fulvic acids, with humic substances accounting for over 50% of dissolved organic carbon.

High ammonia nitrogen. Ammonia nitrogen frequently exceeds 1,000 mg/L, leading to a severely imbalanced carbon-to‑nitrogen ratio. High ammonia concentrations exert toxic inhibition on biological treatment systems, greatly undermining the stability of conventional biochemical processes.

Extremely poor biodegradability. The BOD₅/COD ratio of aged leachate is often below 0.1. Conventional biological treatment is nearly ineffective—microorganisms cannot readily utilize these macromolecular organics as carbon sources.

Complex composition and high variability. Leachate contains a variety of toxic and hazardous substances, including aromatics, halogenated compounds, and heavy metals. Both water quality and flow rate fluctuate significantly with seasonal changes and landfill age, introducing substantial uncertainty into process design and operational management.

These characteristics give rise to three core difficulties:

• Removal of refractory organics — humic macromolecules are resistant to biodegradation;
• Disposal of membrane concentrate — membrane-based processes produce large volumes of highly concentrated waste;
• Stringent discharge limits — the new GB 16889‑2024 standard imposes tighter requirements on COD, ammonia nitrogen, and total nitrogen.

3. Principles and Advantages of Ozone Catalytic Oxidation Technology

Ozone catalytic oxidation technology is designed precisely to address these challenges.

Technical principle. Under the action of a catalyst, ozone (O₃) undergoes a chain reaction to generate hydroxyl radicals (·OH), which have an oxidation potential of 2.8 V. ·OH possesses extremely strong oxidizing power and can non‑selectively attack organic molecules, effectively breaking benzene rings, conjugated double bonds, and other refractory structures, thereby decomposing large humic macromolecules into smaller fragments. Fourier‑transform infrared spectroscopy shows that ozone can completely eliminate certain functional groups in humic substances, such as Ar‑O structures.

Compared with ozonation alone, the introduction of a catalyst brings a qualitative improvement. Active sites on the catalyst surface effectively promote ozone decomposition to generate ·OH, significantly accelerating the reaction rate. Research data indicate that with catalyst addition, the time required for ozonation of biochemically treated leachate effluent to reach COD discharge standards can be reduced from 160 minutes to 120 minutes, with operating costs decreasing from RMB 61.49/m³ to RMB 46.12/m³. The average COD removal rate with a catalyst is more than 104% higher than that achieved by ozone alone.

Key advantages.

• No secondary pollution — no sludge or concentrate is produced; organics are converted mainly to CO₂ and H₂O;
• Stable operation — heterogeneous catalysts offer irreplaceable advantages over homogeneous catalysts in catalytic ozonation of wastewater;
• Enhanced biodegradability — recalcitrant macromolecules are “fragmented” into smaller, biodegradable compounds, raising the BOD₅/COD ratio from 0.008 to 0.26.

4. Typical Process Routes and Engineering Design Schemes

Depending on influent quality and treatment objectives, several mature process combinations incorporating ozone catalytic oxidation are available for leachate treatment.

Route 1: Pretreatment + MBR biological treatment + ozone catalytic oxidation. This route is suitable for small‑scale transfer stations where leachate production is low but pollutant concentrations are high. In the pretreatment unit (coagulation + screw‑press solid‑liquid separation), average removal rates for COD and SS are 29.81% and 41.54%, respectively. The MBR biological unit achieves removal rates of 97.99% for COD, 97.88% for NH₃‑N, and 93.52% for TN, serving as the core pollutant‑removal step. Ozone catalytic oxidation then acts as a polishing step to ensure final effluent compliance.

Route 2: VFL biological treatment + ozone catalytic oxidation. Pilot studies on mature landfill leachate show that under a COD/TN ratio of 2.3–3.0, the VFL biological system provides average removals of 72.7% for COD, 99.6% for NH₃‑N, and 97.9% for TN. Subsequent ozone catalytic oxidation at an O₃/COD ratio of 2 reduces effluent COD to an average of 80.4 mg/L, corresponding to a removal rate of 91.1%. The combined process consistently meets discharge standards.

Route 3: Iron‑carbon micro‑electrolysis coupled with ozone catalytic oxidation. For leachate characterized by “high COD, high ammonia nitrogen, low biodegradability, and high toxicity,” the process sequence is “pre‑aeration ammonia stripping + iron‑carbon micro‑electrolysis – ozone catalytic coupling + two‑stage AO + advanced ozone oxidation.” Optimal parameters include: pre‑aeration intensity 1.2 m³/(m²·h), iron‑carbon packing ratio 60%, reaction pH 3–4, ozone dosage 80 mg/L (catalytic stage) and 40 mg/L (polishing stage), and a two‑stage AO hydraulic retention time of 18 hours (6 h anoxic + 12 h aerobic).

Route 4: Coagulation‑sedimentation + ozone catalytic oxidation + A/O biological treatment. Coagulation‑sedimentation is placed first to remove metal ions and suspended solids. Ozone catalytic oxidation is the core step, breaking down humic macromolecules, and the subsequent A/O stage further removes residual organics. This scheme replaces the conventional nanofiltration membrane process, thereby eliminating concentrate production.

5. Full‑Scale Engineering Cases and Operational Performance

Case 1: A 300‑tpd leachate treatment upgrade project. The plant adopted the “pre‑aeration ammonia stripping + iron‑carbon micro‑electrolysis – ozone catalytic coupling + two‑stage AO + advanced ozone oxidation” process. Operating results demonstrated COD, ammonia nitrogen, and total nitrogen removal rates of 95.8%, 99.2%, and 92.5%, respectively, with effluent consistently meeting the Table 4 discharge standards of GB 16889‑2024. The integrated treatment cost per ton was 23.6% lower than that of the conventional “biological + membrane separation” process.

Case 2: A small municipal solid waste transfer station. The “pretreatment + MBR biological + ozone catalytic oxidation” combination was employed. During the monitoring period, all units worked synergistically to achieve stable compliance. Ozone catalytic oxidation, serving as the final polishing step, effectively addressed the challenge of treating leachate with small flow but high pollutant strength.

Case 3: Advanced treatment of MBR effluent at a landfill site. Using a supported metal‑oxide catalyst at an ozone dosage of 40 mg/L and an HRT of 45 minutes, the COD removal rate reached 92.1%.

Case 4: Treatment of reverse osmosis concentrate from a landfill. Under conditions of pH 8.0, temperature 30°C, ozone feed rate approximately 5 g/h, and reaction time 90 minutes, the removal efficiencies for COD, color, and humic substances in the concentrate were 67.6%, 98.0%, and 86.1%, respectively. The BOD₅/COD ratio increased from 0.008 to 0.26, demonstrating a significant improvement in biodegradability.

6. Key Considerations for Catalyst Selection

The catalyst is central to ozone catalytic oxidation, and its selection directly affects treatment efficiency and operating costs.

Active components. Metal‑oxide catalysts are widely used. MnO₂‑based catalysts show good performance in treating refractory organics via ozonation. Mixed oxides of manganese and cerium exhibit excellent catalytic activity. Copper‑based catalysts demonstrate good stability during repeated use.

Support and specific surface area. The specific surface area of the catalyst directly influences the number of active sites and the contact efficiency with pollutants. Heterogeneous catalysts offer advantages over homogeneous ones in wastewater treatment. Common support materials include activated carbon and alumina.

Engineering selection factors.

• Poisoning resistance — heavy metals and chloride ions in leachate can deactivate the catalyst. Deactivation often results from deposition of inorganic salts on the catalyst surface, hindering contact with ozone;
• Mechanical strength — the catalyst must withstand bed pressure and hydraulic scouring;
• Service life — long‑term stability must be evaluated, with periodic assessment of activity decay;
• Economic viability — catalyst addition significantly improves ozone utilization efficiency.

7. Engineering Design and Operational Maintenance Considerations

Design essentials.

• System positioning — ozone catalytic oxidation is typically installed as a polishing step after biological treatment. It is advisable to place the ozone system downstream of the MBR to avoid clogging of the ozone contactor micro‑bubble diffusers by suspended solids.
• Ozone dosage — generally in the range of 30–120 mg/L, adjusted dynamically based on water quality. Studies show that a dosage of 90 mg/L and a reaction time of 90 minutes yield favorable performance.
• Hydraulic retention time (HRT) — recommended 30–120 minutes.
• pH control — a slightly alkaline pH around 8.0 favors ·OH generation.
• Catalyst packing — the bed height and space velocity should be properly matched; fixed‑bed or fluidized‑bed configurations can be adopted.

Operations and maintenance.

• Online monitoring — install online monitoring systems to adjust ozone dosage in real time.
• Off‑gas treatment — incorporate an off‑gas destruction unit to ensure safe discharge.
• Periodic evaluation — regularly measure effluent COD, ammonia nitrogen, and total nitrogen; monitor catalyst activity and replenish or regenerate as needed.
• System integration — employ automated controls to coordinate with upstream processes for whole‑system synergy.

8. Summary and Outlook

Ozone catalytic oxidation technology leverages the synergistic action of catalysts and ozone to generate highly reactive hydroxyl radicals that decompose refractory humic macromolecules in landfill leachate into biodegradable small molecules. This approach fundamentally addresses the secondary pollution problem of membrane concentrate—producing neither sludge nor concentrate, and converting organics essentially to CO₂ and H₂O.

Full‑scale projects have already demonstrated both technical and economic feasibility. A 300‑tpd upgrade project achieved COD, ammonia nitrogen, and total nitrogen removals of 95.8%, 99.2%, and 92.5%, respectively, with a 23.6% reduction in overall treatment cost. The VFL‑ozone catalytic oxidation combination yielded a COD removal rate of 91.1%. With catalyst addition, reaction time dropped from 160 to 120 minutes, and operational cost decreased from RMB 61.49/m³ to RMB 46.12/m³. These figures confirm that ozone catalytic oxidation has become one of the most promising and practically deployable technologies for advanced leachate treatment.

Looking ahead, three major trends will shape the development of ozone catalytic oxidation:

• Continuous improvement in catalyst performance — development of more active, longer‑lasting, and more poisoning‑resistant catalytic systems;
• Deeper process integration — closer synergy with biological and membrane units;
• Accelerated adoption of intelligent O&M — real‑time online regulation will make system operation more precise and efficient.

With the implementation of the new GB 16889‑2024 standard and increasingly stringent environmental regulations, ozone catalytic oxidation will continue to play an ever‑more important role in landfill leachate treatment.

author: Gloria
date:2026/8/10