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For the treatment of four types of pollutants including hydrogen sulfide (H₂S), ammonia (NH₃), VOCs and odor, the treatment process shall be determined according to pollutant properties, concentration, air volume and emission standards. Some technologies can realize the coordinated removal of multiple pollutants. The mainstream treatment technologies and combined schemes for each pollutant are listed as follows:
1. Hydrogen Sulfide (H₂S) Treatment Technology
H₂S is an acidic, reductive and toxic gas with a typical rotten-egg odor. The core treatment principle is sulfur removal by oxidation or pollutant fixation through acid-base neutralization.
1.1 Alkali Solution Absorption Method
Principle: Alkaline solutions such as NaOH and Na₂CO₃ are used for spray absorption to generate sodium sulfide or sodium hydrosulfide. This method is suitable for low-concentration H₂S waste gas.
Advantages: Simple equipment, low investment and convenient operation; compatible with ammonia absorption process.
Disadvantages: The waste liquid requires secondary treatment and may cause secondary pollution; limited removal efficiency for high-concentration H₂S.
1.2 Dry Oxidation Method (Iron Oxide Method / Activated Carbon Adsorption Oxidation)
Iron Oxide Method: Iron oxide desulfurizer reacts with H₂S to generate iron sulfide, and the adsorbent is regenerated by air ventilation after saturation.
Activated Carbon Adsorption Oxidation Method: Activated carbon adsorbs H₂S and oxidizes it into elemental sulfur via surface catalytic reaction.
Advantages: No secondary waste liquid, high treatment efficiency (over 99%), applicable to low-concentration and small-air-volume waste gas.
Disadvantages: Desulfurizer requires regular replacement or regeneration; the operating cost increases with air volume.
1.3 Biological Method (Biological Filter / Biological Trickling Filter)
Principle: Microorganisms (such as sulfur-oxidizing bacteria) convert H₂S into elemental sulfur or sulfate.
Advantages: Low operating cost and no secondary pollution; suitable for coordinated treatment of medium and low concentration malodorous waste gas.
Disadvantages: Large floor space and high sensitivity to temperature and humidity.
1.4 Catalytic Oxidation Method (Claus Process / Selective Catalytic Oxidation)
Claus Process: High-concentration H₂S (>15%) is combusted to generate SO₂, and then elemental sulfur is produced by the reaction between residual H₂S and SO₂ under catalysis.
Selective Catalytic Oxidation: H₂S is directly oxidized into elemental sulfur under low-temperature catalyst.
Advantages: Realize sulfur resource recovery for high-concentration H₂S with high removal efficiency.
Disadvantages: High investment and complex process; applied to high-concentration H₂S waste gas in coal chemical and petroleum refining industries.
2. Ammonia (NH₃) Treatment Technology
NH₃ is an alkaline and water-soluble gas with pungent odor. The core treatment methods are acid-base neutralization and catalytic decomposition.
2.1 Acid Solution Absorption Method
Principle: Acidic solutions such as sulfuric acid and hydrochloric acid are used for spray absorption to generate ammonium salts including ammonium sulfate and ammonium chloride.
Advantages: Simple equipment with high efficiency (over 95%); ammonium salts can be recycled as fertilizer resources.
Disadvantages: Strict pH control is required to prevent ammonia escape; multi-stage absorption is necessary for high-concentration ammonia waste gas.
2.2 Biological Method
Principle: Nitrifying bacteria convert ammonia nitrogen into nitrate or nitrite.
Advantages: Low operating cost and no secondary pollution; capable of coordinated treatment of H₂S and odor.
Disadvantages: Strict limit on inlet concentration (NH₃ < 500mg/m³) with long startup cycle.
2.3 Catalytic Decomposition Method
Principle: NH₃ is decomposed into N₂ and H₂O under high temperature (300~400℃) and catalytic condition.
Advantages: No secondary pollutants; suitable for high-temperature and low-concentration ammonia waste gas.
Disadvantages: High investment and high energy consumption; dust and sulfur impurities need to be removed by pretreatment.
2.4 Adsorption Method
Principle: Adsorbents such as acidic activated carbon and zeolite are adopted to adsorb NH₃.
Advantages: Simple operation and small floor space; suitable for low-concentration and small-air-volume waste gas.
Disadvantages: Adsorbents require regular regeneration or replacement, leading to high operating cost.
3. VOCs Treatment Technology
VOCs contain complex components including hydrocarbons, alcohols and esters. The treatment technology is selected according to concentration, boiling point and flammability, which is divided into recovery method and destruction method.
3.1 Recovery Method (For medium and high concentration VOCs with recovery value)
3.1.1 Activated Carbon Adsorption
Principle: The porous structure of activated carbon adsorbs VOCs, and steam desorption is applied for regeneration after adsorption saturation.
Advantages: Low investment and simple operation; suitable for low-concentration and large-air-volume VOCs.
Disadvantages: Not applicable to high-boiling and easily polymerized VOCs; activated carbon needs periodic replacement.
3.1.2 Zeolite Rotor Concentration + Incineration
Principle: The zeolite rotor adsorbs and concentrates low-concentration VOCs, and the concentrated gas is sent to incinerator for degradation.
Advantages: High removal efficiency (>95%) and low energy consumption; suitable for large-air-volume and low-concentration VOCs.
Disadvantages: Relatively high investment; dust and moisture need to be eliminated by pretreatment.
3.1.3 Condensation Recovery Method
Principle: VOCs are converted from gaseous state to liquid state for recovery through cooling and pressurization.
Advantages: Realize resource recovery; suitable for high-concentration and high-boiling VOCs (such as gasoline and organic solvents).
Disadvantages: High energy consumption and low efficiency for low-concentration VOCs.
3.1.4 Membrane Separation Method
Principle: Selective permeable membrane separates VOCs from air, and the enriched pollutants are recycled or destroyed.
Advantages: Low energy consumption and no secondary pollution; suitable for medium and high concentration VOCs.
Disadvantages: The membrane is prone to blockage with high investment; pretreatment is mandatory.
3.2 Destruction Method (For low-concentration VOCs without recovery value)
3.2.1 Catalytic Oxidation (CO)
Principle: Under catalytic condition, VOCs are oxidized and decomposed into CO₂ and H₂O at low temperature (200~400℃).
Advantages: Low energy consumption and high treatment efficiency; suitable for medium and low concentration VOCs.
Disadvantages: Catalyst is easy to be poisoned (sulfur and chlorine impurities require pretreatment); high investment.
3.2.2 Thermal Oxidation (TO)
Principle: VOCs are directly combusted and decomposed under high temperature (600~800℃).
Advantages: Wide application range without catalyst; suitable for high-concentration VOCs with complex components.
Disadvantages: High energy consumption; waste heat recovery system is recommended.
3.2.3 Photocatalytic Oxidation (UV Photolysis)
Principle: Ultraviolet light excites catalysts such as TiO₂ to generate hydroxyl radicals for oxidative degradation of VOCs.
Advantages: Simple equipment and low operating cost; suitable for low-concentration malodorous VOCs.
Disadvantages: Treatment efficiency is greatly affected by humidity and dust; intermediate by-products are easily generated.
3.2.4 Plasma Method
Principle: High-voltage plasma generates high-energy electrons to crack molecular chains of VOCs.
Advantages: Small floor space; applicable for coordinated treatment of low-concentration VOCs and odor.
Disadvantages: High energy consumption, limited efficiency for high-concentration VOCs, and ozone by-product generation.
4. Odor Treatment Technology
Odor is a kind of mixed pollutant containing H₂S, NH₃, VOCs, mercaptan and other components. Coordinated treatment and broad-spectrum technologies are preferred.
4.1 Biological Method (Mainstream Technology)
Process Type: Biological filter, biological trickling filter, biological scrubber.
Principle: Microorganisms degrade mixed odor pollutants into harmless substances.
Advantages: Low operating cost and no secondary pollution; suitable for medium and low concentration mixed odor (municipal sewage, garbage disposal, chemical waste gas).
Disadvantages: Large floor space and high sensitivity to temperature and pH value.
4.2 Advanced Oxidation Processes (AOPs)
Principle: Hydroxyl radicals are generated by ozone oxidation, Fenton oxidation and other methods to degrade odor molecules.
Advantages: High treatment efficiency and fast reaction rate; suitable for high-concentration and refractory odor.
Disadvantages: High energy consumption; ozone is prepared on-site and tail gas ozone removal is required.
4.3 Combined Adsorption-Oxidation Method
Principle: Activated carbon or zeolite adsorbs odor, combined with catalytic oxidation or photo-oxidation for advanced treatment.
Advantages: High efficiency and wide applicability; suitable for low-concentration multi-component odor.
Disadvantages: Relatively high operating cost; adsorbents require regular maintenance.
4.4 Chemical Scrubbing Method
Principle: Mixed scrubbing liquid containing acid, alkali and oxidant (such as sodium hypochlorite) is used for neutralization and oxidation of odor pollutants.
Advantages: Fast treatment speed and compact equipment; suitable for high-concentration water-soluble odor.
Disadvantages: Various scrubbing liquids are required; waste liquid needs secondary treatment.
5. Multi-Pollutant Cooperative Treatment Scheme
In practical engineering, most waste gas is mixed waste gas containing H₂S+NH₃+VOCs+odor. The recommended combined processes are as follows:
5.1 Chemical Scrubber + Biological Filter
Process Flow: Waste gas → Pretreatment (Dedusting & Cooling) → Acid-base Scrubber (H₂S & NH₃ removal) → Biological Filter (VOCs and residual odor degradation) → Discharge.
Application: Medium and low concentration mixed waste gas (pharmaceutical, chemical and breeding industry).
5.2 Zeolite Rotor Concentration + Catalytic Combustion + Alkali Scrubber
Process Flow: Waste gas → Pretreatment → Zeolite Rotor (VOCs concentration) → Catalytic Combustion (VOCs destruction) → Alkali Scrubber (remove by-product SO₂) → Discharge.
Application: Large-air-volume low-concentration VOCs waste gas containing a small amount of H₂S (coating and printing industry).
5.3 Plasma + Photocatalytic Oxidation + Activated Carbon Adsorption
Process Flow: Waste gas → Plasma (macromolecular VOCs and odor cracking) → Photocatalytic Oxidation (advanced degradation) → Activated Carbon Adsorption (terminal purification) → Discharge.
Application: Low-concentration refractory mixed odor (garbage transfer station and sewage treatment plant).
