Precautions for the application of ozone decomposition catalysts in the cover of exhaust gas in sewage treatment plants

Wastewater treatment plant wastewater treatment plant structures such as equalization tanks, aeration tanks, and sludge thickening tanks collect exhaust gases with near-saturated humidity (relative humidity often exceeding 95%), coexistence of hydrogen sulfide (5–50 ppm) and ammonia (5–30 ppm), and the presence of submicron-sized aerosols and oil mist. Under these conditions, ozone decomposition catalysts face a triple failure mechanism: competitive adsorption of water vapor, synergistic poisoning by sulfur and ammonia, and physical blockage. Without targeted design, the lifespan of the catalyst can be shortened to 20%–30% of that in normal industrial exhaust gas environments. Accelerated aging data from laboratories show that under conditions of 90% relative humidity and 20 ppm H₂S + 20 ppm NH₃, the ozone conversion rate of manganese-based catalysts drops from 99% to 27% after 300 hours of operation. Therefore, a combination of measures, including pretreatment dehumidification, selection of sulfur and ammonia-resistant formulations, and operational monitoring and early warning systems, is necessary to ensure the long-term stable operation of the catalytic system.

I. Typical Characteristics of Covered Exhaust Gas from Wastewater Treatment Plants The composition of exhaust gas varies across different units in a wastewater treatment plant, but common characteristics are evident. The H₂S concentration in the exhaust gas from the equalization tank and anaerobic tank is typically 5–50 ppm, reaching over 100 ppm in some high-concentration conditions; the NH₃ concentration is generally 5–30 ppm. The H₂S concentration in the exhaust gas from the aeration tank is lower (<5 ppm), but it carries a large amount of liquid droplets and bioaerosols. The exhaust gas temperature is generally 15–35°C, with a relative humidity of 95%–100%. Even after cooling and effluent separation during pipeline transportation, the relative humidity often remains above 80% when entering the treatment unit. The gas also contains siloxanes, trace amounts of oil mist, and organic sulfides (such as methanethiol).

Compared to coal-fired flue gas or chemical tail gas, wastewater treatment plant exhaust gas has three key differences: First, it contains high concentrations of both reducing sulfur (H₂S) and NH₃ in close proportions; second, it has extremely high water vapor content and almost no drying section; and third, it has a high concentration of submicron-sized aerosols (10³–10⁵ particles/cm³) with adhesive properties. These characteristics determine that the failure mode is not simply sulfur poisoning, but rather a synergistic destruction caused by water vapor, sulfur-ammonia salts, and particulate matter.

II. Three Major Failure Mechanisms and Quantitative Data

1. Competitive Adsorption of Water Vapor and Water Film Barrier under High Humidity Water molecules and ozone directly compete for active sites on the catalyst. Test data shows that under conditions of 50 ppm ozone inlet concentration, 10,000 h⁻¹ space velocity, and 25°C, when the relative humidity increases from <5% to 90%, the initial conversion rate decreases from 99.2% to 84.6%; after 200 hours of continuous operation, the conversion rate at 90% humidity further decreases to 68.2%, and the specific surface area decreases by 41%. High humidity not only causes reversible competitive adsorption but also triggers irreversible hydrothermal aging (growth of active component crystallites). In saturated humidity exhaust gas, a continuous water film easily forms on the catalyst surface, and ozone must first dissolve in the water film before diffusing to the active sites, increasing mass transfer resistance by an order of magnitude.

2. Synergistic Poisoning of Hydrogen Sulfide and Ammonia: Rapid Deposition of Ammonium Sulfate

A strong synergistic effect is observed when H₂S and NH₃ coexist. Parallel accelerated aging experiments (RH=85%, 25°C, ozone 50 ppm) showed that after 300 hours of operation with 20 ppm H₂S alone, the conversion rate was 52.7%; with 20 ppm NH₃ alone, it was 81.6%; while the conversion rate when both coexisted was only 26.8%, much faster than simple superposition. The mechanism is that H₂S is oxidized to SO₂/SO₃ on the catalyst surface, which then reacts with NH₃ to form ammonium sulfate or ammonium sulfite. These ammonium salts are solid at room temperature and rapidly deposit on the catalyst surface and within the pores, causing the specific surface area to decrease from 156 m²/g to 68 m²/g, and the pore volume to decrease by more than 60%. This salt deposit does not depend on the consumption of active components and its formation rate is much faster than that of sulfur poisoning alone.

3. Physical Blockage by Aerosols and Oil Mist The bioaerosol particles in the exhaust gas are mainly between 0.1 and 10 µm, with particles of 0.3–1 µm being most likely to enter the catalyst micropores and deposit. Oil mist originates from the evaporation of fan lubricating oil. These substances neither react nor decompose, yet form a sticky coating on the catalyst surface. Actual operational data shows that when only a primary filter is installed at the front end, the catalyst conversion rate at the inlet section is only 38% of the fresh sample after six months of operation, while the outlet section is 82%. After ultrasonic cleaning, the inlet section recovers to 67%, indicating that about half of the loss comes from washable physical deposits, and the other half from chemical poisoning.

III. Targeted Selection and Operational Recommendations

1. Pretreatment Configuration High-efficiency demisting/condensation dehumidification reduces relative humidity to below 70%; two-stage washing (acid washing for ammonia removal + alkaline washing for sulfur removal) controls H₂S to below 5 ppm and NH₃ to below 10 ppm; the terminal is equipped with an F7 or higher grade filter to intercept particles larger than 0.3 µm, with a filtration efficiency of no less than 85%.

2. Catalyst Selection Mn-Ce composite oxide or Mn-Ce-Sn ternary system is preferred. Cerium and tin, as anti-sulfur agents, can delay the sulfation rate (comparative tests show that the Mn-Ce catalyst maintains a conversion rate of 68% after 500 hours of operation at 20 ppm H₂S and RH=80%, while the single Mn catalyst only achieves 41%). A hydrophobic support (silanized SiO₂/TiO₂) can mitigate competitive adsorption of water vapor. Full analysis data of the exhaust gas must be provided to the supplier during the selection process.

3. Operation Monitoring and Early Warning Establish a monitoring system for outlet ozone concentration (online/daily), bed pressure differential (twice a week), and scrubber outlet H₂S/NH₃ (weekly). Early warning thresholds are: outlet ozone increase >0.5 ppm/day for 3 consecutive days, pressure differential increase >30% from the initial level, H₂S >10 ppm, and NH₃ >15 ppm. When multiple indicators trigger simultaneously, the system should be shut down for sampling and evaluation.

4. Post-poisoning treatment: On-site segmented sampling and activity testing should be conducted. If only the inlet end is deactivated (<40%), 30%–50% of the inlet section catalyst can be replaced; if both ends are <50%, the entire section needs to be replaced; if physical blockage is the primary cause, water washing regeneration should be attempted. Thermal regeneration (300–350°C nitrogen purging) has limited effectiveness in this scenario (conversion rate only recovers from 31% to 44%) and should not be used as a primary method.

IV. Economic Considerations Taking a project handling 10,000 m³/h of air volume as an example (catalyst 3 m³, unit price approximately 30,000 RMB/m³): Without pretreatment, the annual catalyst replacement cost is 120,000–180,000 RMB (lifespan 6–9 months); with only a demister + ordinary catalyst, the annual cost is 80,000–120,000 RMB (lifespan 12–18 months); with two-stage scrubbing + demister + anti-sulfur catalyst, the annual cost is 30,000–50,000 RMB (lifespan 24–36 months), and the pretreatment operation cost is 40,000–60,000 RMB/year, resulting in a total annual cost of 70,000–110,000 RMB, which is lower than the first two options. Sufficient pretreatment and the use of an anti-sulfur catalyst offer greater economic advantages in long-term operation.

The successful application of ozone decomposition catalysts in covered wastewater treatment plant exhaust gases hinges on the accurate identification and targeted design of three key characteristics: high humidity, coexistence of sulfur and ammonia, and aerosols. Through pretreatment dehumidification, anti-sulfur formulations, and monitoring and early warning systems, the stable operating cycle can be extended from several months to over two years.

author：Gloria
date:2026-05-07