AC/MnO₂-TiO₂ Catalyst Application and Ecological Safety Assessment

Abstract

Using municipal wastewater treatment plant effluent as makeup water for urban landscape water bodies is an effective way to alleviate urban water scarcity, but residual refractory organic pollutants pose challenges to water quality improvement and ecological safety. This study adopts an activated carbon supported catalyst AC/MnO₂-TiO₂ for heterogeneous catalytic ozonation deep treatment of urban landscape water. The effects of ozone flow rate, reaction time and catalyst dosage on permanganate index (CODMn) removal were investigated, and process parameters were optimized via response surface methodology. Organic degradation mechanisms and ecotoxicity changes were analyzed through GPC, GC-MS and ECOSAR methods. Under the optimal conditions of 18.15 mg/min ozone flow rate, 57.19 min reaction time and 48.14 g/L catalyst dosage, the CODMn removal rate reaches 80.84%. After treatment, the proportion of macromolecular organics (1–100 ku) decreases from 43.9% to 21.2%, while small molecular organics (≤1 ku) increase from 42.9% to 74.5%. Typical pollutants including o-xylene, dodecamethylpentasiloxane and octamethylcyclotetrasiloxane are effectively degraded. The acute toxicity to fish, daphnia and green algae is completely eliminated, and chronic toxicity is reduced to non-toxic or slightly toxic levels. This technology provides a reliable technical solution for water quality improvement and ecological risk control of urban landscape water bodies.

1. Technical Background and Challenges

1.1 Demand for Landscape Water Quality Improvement

Against the background of global water scarcity, reclaimed water from municipal wastewater treatment plants has become a common makeup source for urban landscape water bodies, effectively reducing the consumption of freshwater resources. However, secondary effluent from wastewater treatment plants still contains residual refractory organic matter, trace pollutants and disinfection by-products. Long-term accumulation of these pollutants will weaken the self-purification capacity of landscape water bodies, leading to problems such as chromaticity deterioration, odor, eutrophication and blackening, and even pose long-term ecological risks through food chain enrichment.

Conventional landscape water treatment technologies such as aeration and biological filtration have limited removal efficiency for refractory macromolecular organics and trace toxic substances, making it difficult to meet the requirements of high-quality landscape water environments. Efficient deep treatment technologies are urgently needed to improve water quality, reduce ecological risks and enhance the self-purification capacity of water bodies.

1.2 Advantages of Catalytic Ozonation Technology

As a typical advanced oxidation process, heterogeneous catalytic ozonation promotes the decomposition of ozone through solid catalysts to generate highly reactive hydroxyl radicals (·OH), achieving efficient degradation of refractory organic pollutants. Compared with single ozonation, catalytic ozonation has faster reaction rate, wider applicability to pollutant types, and higher ozone utilization efficiency.

For landscape water treatment, catalytic ozonation has additional unique advantages:

• Effectively removes chromaticity and odor-causing substances, improving the sensory quality of landscape water
• Increases dissolved oxygen level in water bodies, which is conducive to the growth of indigenous microorganisms and enhances water self-purification capacity
• Significantly reduces the ecotoxicity of water bodies and improves ecological safety
• No excess sludge production, with simple operation and management

2. Catalyst Characterization and Performance Basis

2.1 AC/MnO₂-TiO₂ Catalyst Properties

The AC/MnO₂-TiO₂ catalyst uses granular activated carbon as the carrier, loaded with MnO₂ and TiO₂ as active components, with a particle size of 1–3 mm and a bulk density of 690 g/L.

SEM characterization shows that the catalyst surface has a rough, multi-layered structure with dense irregular particles, providing abundant active sites for catalytic reaction. Nitrogen adsorption-desorption tests confirm a specific surface area of 306.784 m²/g, a pore volume of 0.276 cm³/g and an average pore diameter of 3.755 nm. The mesopore-dominated pore structure is suitable for mass transfer and diffusion in liquid-phase reactions, providing favorable conditions for ozone, pollutants and active sites to fully contact.

2.2 Adsorption Contribution Verification

To clarify whether the pollutant removal effect is dominated by catalytic oxidation or adsorption, dynamic adsorption tests were carried out. The results show that after adsorption saturation (20 h of circulation), the CODMn removal rate by adsorption alone stabilizes at about 24.47%. Since all catalytic ozonation experiments are conducted after the catalyst reaches adsorption saturation, the contribution of physical adsorption to CODMn removal within the reaction time can be ignored, and the pollutant removal effect is dominated by catalytic oxidation.

3. Process Parameter Optimization

3.1 Single-Factor Influence Analysis

3.1.1 Catalyst Dosage

With an ozone flow rate of 20 mg/min and reaction time of 45 min, the CODMn removal rate increases from 48.29% to 86.00% when the catalyst dosage rises from 20 g/L to 60 g/L. The increase in catalyst dosage provides more active sites, promotes ozone decomposition into hydroxyl radicals, and accelerates the degradation of organic matter.

However, when the dosage exceeds 60 g/L, the removal rate shows a downward trend, as excessive catalyst leads to non-productive decomposition of ozone and free radical scavenging effect. Considering removal efficiency and economic cost, the optimal dosage range is 40–80 g/L. At a dosage of 40 g/L, the effluent CODMn reaches 4.66 mg/L, meeting the Class III standard of China’s Environmental Quality Standards for Surface Water (GB 3838-2002).

3.1.2 Reaction Time

With 40 g/L catalyst dosage and 20 mg/min ozone flow rate, the CODMn removal rate rises rapidly to 52.26% within the first 10 minutes of reaction, as high-concentration pollutants are rapidly degraded by ozone and hydroxyl radicals. After that, the degradation rate slows down gradually: the removal rate reaches 68.70% at 45 min, and only increases by 2.91 percentage points when extended to 60 min. This is because the remaining organic matter is mostly refractory small molecular intermediates, which are difficult to be further oxidized. The recommended optimal reaction time is 45 min for engineering design.

3.1.3 Ozone Flow Rate

With 40 g/L catalyst dosage and 45 min reaction time, the CODMn removal rate increases significantly when ozone flow rate rises from 5 mg/min to 20 mg/min, reaching a maximum of 72.65% at 20 mg/min. Higher ozone concentration provides more oxidizing species and accelerates pollutant degradation.

When the ozone flow rate exceeds 20 mg/min, the removal rate decreases instead. Excessive ozone leads to overflow without full mass transfer, reducing ozone utilization efficiency; meanwhile, a large number of intermediate products generated by rapid oxidation may increase the complexity of water quality. The optimal ozone flow rate is determined to be 20 mg/min under the test conditions.

3.2 Response Surface Methodology Optimization

3.2.1 Model Construction and Validation

Based on single-factor test results, a three-factor three-level Box-Behnken experiment was designed with ozone flow rate, reaction time and catalyst dosage as independent variables, and CODMn removal rate as the response value. A quadratic polynomial regression model was established, with analysis of variance confirming that the model is highly significant (P < 0.0001), with a determination coefficient R² of 0.9976 and adjusted R² of 0.9945. The difference between adjusted R² and predicted R² is less than 0.2, indicating that the model has high fitting accuracy and can reliably predict the treatment effect.

Among the three factors, ozone flow rate has the most significant influence on removal efficiency, followed by reaction time, while the influence of catalyst dosage is relatively small within the test range. The interaction between ozone flow rate and reaction time is the most significant.

3.2.2 Optimal Process Parameters

Through model solving, the optimal process parameters for maximum CODMn removal are obtained: ozone flow rate of 18.15 mg/min, reaction time of 57.19 min, and catalyst dosage of 48.14 g/L. Under these conditions, the model predicts a CODMn removal rate of 82.18%.

Parallel verification tests show that the actual average CODMn removal rate is 80.48%, with a relative error of only 2.1% from the predicted value, confirming the reliability and accuracy of the model for engineering design guidance.

4. Organic Degradation Mechanism and Ecotoxicity Assessment

4.1 Molecular Weight Distribution Evolution

Gel permeation chromatography (GPC) tests show that catalytic ozonation significantly changes the molecular weight distribution of organic matter in landscape water:

• Macromolecular organics with molecular weight of 1–100 ku (mainly humic acid, fulvic acid and other aromatic substances) decrease from 43.9% to 21.2% after treatment
• Small molecular organics with molecular weight ≤1 ku (mainly carbonate, amino acids and other simple organics) increase from 42.9% to 74.5%

Compared with single ozonation, catalytic ozonation further strengthens the breakdown of macromolecular refractory organics into small molecular biodegradable substances. These small molecular products are more easily metabolized by indigenous microorganisms in landscape water bodies, which helps to enhance the natural self-purification capacity of the water ecosystem.

4.2 Trace Organic Compound Transformation

GC-MS analysis confirms that typical trace organic pollutants in raw water including o-xylene, dodecamethylpentasiloxane, 4,5,6,7-tetramethoxyflavone, 3,5-dimethylbenzyl chloride, octamethylcyclotetrasiloxane and diethyldimethyllead are effectively degraded after catalytic ozonation treatment, with their characteristic peak intensities significantly reduced.

Meanwhile, new small molecular intermediate products are detected in the effluent, including 2,4-pyrimidinedione, 2-methylthio-6-methylpyrazine and 2-amino-5-methylbenzoic acid, indicating that toxic macromolecular pollutants are converted into low-toxicity or easily degradable small molecular substances through oxidation.

4.3 Ecotoxicity Risk Reduction

ECOSAR toxicity assessment shows that raw water organic pollutants have acute and chronic toxic effects on fish, daphnia and green algae, among which 4,5,6,7-tetramethoxyflavone has the highest toxicity with lg LC₅₀ (fish) of -4.34, belonging to the highly toxic grade.

After catalytic ozonation treatment:

• The acute toxicity (LC₅₀ and EC₅₀) of all detected organics to the three aquatic organisms is above the non-toxic threshold (lg k > 2), meaning acute toxicity is completely eliminated
• The chronic toxicity is significantly reduced, with most intermediate products at non-toxic or slightly toxic levels

This confirms that catalytic ozonation can effectively reduce the ecological risk of landscape water replenished by reclaimed water, providing safety guarantee for the aquatic ecosystem.

5. Engineering Application Analysis

5.1 Applicable Scenarios

This catalytic ozonation technology is suitable for the following landscape water treatment scenarios:

• Urban landscape rivers and lakes replenished by municipal wastewater treatment plant effluent
• Deep purification and water quality maintenance of urban black and odorous water bodies
• Circulating purification of park waters, residential landscape waters and commercial waterscapes
• Quality improvement of slightly polluted surface water bodies to meet higher water quality standards

5.2 Key Engineering Design Considerations

• Reactor configuration: Plug-flow or completely mixed catalytic oxidation tanks are recommended, with catalysts filled in fixed bed form to avoid loss and facilitate backwashing. The height-diameter ratio should be controlled within 3:1 to 5:1 to ensure sufficient ozone mass transfer time.
• Ozone distribution system: Titanium alloy microporous aerators are adopted to improve ozone mass transfer efficiency and reduce tail gas escape. The aeration device should be arranged in sections to realize graded ozone dosing according to pollutant concentration gradient.
• Tail gas treatment: Equipped with activated carbon ozone destruction device to ensure that the ozone concentration in exhaust gas meets occupational safety and environmental protection standards.
• Bypass circulation design: Set up bypass circulation pipelines to flexibly adjust hydraulic retention time according to influent water quality fluctuations, ensuring stable effluent quality.
• Online monitoring system: Configure online monitoring of CODMn, ORP, dissolved ozone and ozone tail gas concentration to realize automatic linkage control of ozone dosing.

5.3 Operation and Maintenance Best Practices

• Perform regular backwashing on the catalyst layer to remove surface suspended solids and maintain exposure of active sites, usually once every 7–15 days according to water turbidity.
• Regularly evaluate catalyst activity, and carry out in-situ regeneration every 2–3 years to restore catalytic performance, extending the total service life of the catalyst to more than 5 years.
• Adjust ozone dosage and reaction time according to seasonal water quality changes: appropriately increase ozone dosage or extend HRT in low temperature seasons or when influent pollutant load rises.
• Establish ozone leakage early warning mechanism in the operation area, and configure ventilation and protective equipment to ensure the safety of operation and maintenance personnel.

6. SYNERAQUA Technical Perspective

At SYNERAQUA, we recognize that catalytic ozonation is a highly efficient and sustainable solution for landscape water quality improvement and micro-polluted water remediation, perfectly aligning with the growing demand for ecological urban water environments.

Our skid-mounted modular catalytic ozonation systems integrate optimized catalyst filling structures, high-efficiency ozone mass transfer units and automatic control systems, enabling fast on-site deployment for both new construction and retrofitting projects. Combined with our smart water management platform, the system can automatically adjust ozone dosage and hydraulic retention time based on real-time influent water quality, improving ozone utilization efficiency by 15–20% and reducing operating energy consumption. We also provide customized supported catalyst development for different water quality characteristics, as well as full-lifecycle O&M services including catalyst regeneration and activity testing, to help clients achieve long-term stable landscape water quality and ecological safety.

7. Conclusion

The AC/MnO₂-TiO₂ catalytic ozonation process provides an efficient and reliable technical solution for quality improvement of urban landscape water replenished by reclaimed water, with the following core conclusions:

1. Ozone flow rate, reaction time and catalyst dosage all have significant effects on CODMn removal. Under the optimized conditions of 18.15 mg/min ozone flow rate, 57.19 min reaction time and 48.14 g/L catalyst dosage, the CODMn removal rate reaches 80.84%, and the effluent can meet Class III surface water quality standards.
2. Catalytic ozonation effectively breaks macromolecular refractory organic matter into small molecular biodegradable substances, significantly improving the biodegradability of water bodies and helping to enhance the self-purification capacity of landscape water ecosystems.
3. Typical toxic organic pollutants are effectively degraded after treatment. The acute toxicity to aquatic organisms such as fish, daphnia and green algae is completely eliminated, and chronic toxicity is reduced to non-toxic or slightly toxic levels, greatly reducing ecological risks.
4. The process has the advantages of fast reaction speed, no sludge production and simple operation, and has broad application prospects in urban landscape water treatment, black and odorous water remediation and micro-polluted water quality improvement.

FAQ

Q1: What is the core difference between catalytic ozonation and conventional single ozonation for landscape water treatment?
Catalytic ozonation uses solid catalysts to promote ozone decomposition into hydroxyl radicals with stronger oxidizing ability, which can degrade refractory organic pollutants that cannot be removed by single ozonation. It also has higher ozone utilization efficiency, better removal effect on chromaticity and toxicity, and lower operating cost per unit pollutant removal.

Q2: What is the service life of the AC/MnO₂-TiO₂ catalyst and can it be regenerated?
Under normal operation conditions, the catalyst has an effective service life of 2–3 years before obvious activity decline. It can be regenerated through in-situ thermal activation or chemical cleaning, and the total service life can be extended to more than 5 years after regeneration, which greatly reduces long-term operating costs.

Q3: Can catalytic ozonation alone make reclaimed water-replenished landscape water meet Class III surface water standards?
For most secondary effluent-replenished landscape water, catalytic ozonation can effectively reduce CODMn, chromaticity and toxicity to meet Class III standards. For water bodies with high nitrogen and phosphorus loads, it is recommended to combine with ecological treatment units such as constructed wetlands to achieve comprehensive standard compliance.

Q4: How to ensure the safety of ozone application in engineering projects?
Standard engineering configurations include a closed reactor design, an activated carbon ozone tail gas destruction device, and an ozone concentration online monitoring and leakage alarm system. The operation area is equipped with ventilation facilities and personal protective equipment, which can fully ensure the safety of operation and the surrounding environment in compliance with relevant national standards.