Multi-Perspective: Research Progress of Probiotics on Waste Gas Treatment and Conversion

Abstract

The acceleration of industrialization and urbanization have led to the increasingly serious problem of waste gas pollution. Pollutants such as sulfur dioxide (SO₂), nitrogen oxides (NOx), volatile organic compounds (VOCs), ammonia (NH₃), formaldehyde (HCHO), and hydrogen sulfide (H₂S) emitted from industrial production, transportation, and agricultural activities have posed a major threat to the ecological environment and public health. Although traditional physical and chemical treatment methods can partially reduce the concentration of pollutants, they face three core bottlenecks of high cost, high energy consumption, and secondary pollution, and it is urgent to develop sustainable alternative technologies. In this context, probiotic waste gas treatment technology has become an emerging research hotspot due to its environmental friendliness, low energy consumption characteristics, and resource conversion potential. Based on the databases of PubMed, Web of Science Core Collection, Scopus, Embase, and Cochrane Library, this paper systematically searched the literature published from 2014 to 2024 according to the predetermined inclusion and exclusion criteria (such as research topic relevance, experimental data integrity, language in English, etc.). A total of 71 high-quality studies were selected from more than 600 studies for review. By integrating three perspectives (basic theory perspective, environmental application perspective, and waste gas treatment facility perspective), the metabolic mechanism, functional strain characteristics, engineering application status, and cost-effectiveness of probiotics in waste gas bioconversion were systematically analyzed. The main conclusions include the following: probiotics achieve efficient degradation and recycling of waste gas pollutants through specific enzyme catalysis, and compound flora and intelligent regulation can significantly improve the stability and adaptability of the system. This technology has shown good environmental and economic benefits in multi-industry waste gas treatment, but it still faces challenges such as complex waste gas adaptability and long-term operational stability. This review aims to provide useful theoretical support for the optimization and large-scale application of probiotic waste gas treatment technology, promote the transformation of waste gas treatment from ‘end treatment’ to ‘green transformation’, and ultimately serve the realization of sustainable development goals.

1. Introduction

With the acceleration of industrialization and urbanization, waste gas pollution has become a core issue that threatens the ecological environment and public health [1]. Industrial production, transportation, agricultural activities, and other processes emit a large amount of waste gas, including sulfur dioxide (SO₂), nitrogen oxides (NOx), volatile organic compounds (VOCs), ammonia (NH₃), formaldehyde (HCHO), hydrogen sulfide (H₂S), methane (CH₄) and other pollutants [2]. According to statistics, in 2024, the global fossil fuel industry’s methane emissions exceeded 120 million tons, which is the absolute ‘main source’ of methane emissions. Abandoned oil and gas wells and coal mines also emit about 8 million tons of methane, and the amount of methane leakage from oil and gas facilities detected by satellites in 2024 hit a record high. Industrial coating, textile dyeing and finishing, furniture manufacturing, packaging and printing, chemical fiber, and other industries are important sources of VOCs. Heavy industrial processes such as iron and steel, metallurgy, and cement produce a large amount of NO, SO₂, and carbon monoxide (CO). These pollutants will not only lead to the deterioration of air quality, cause environmental disasters such as haze and acid rain, but also cause serious damage to the human respiratory system and cardiovascular system; increase the incidence of respiratory diseases, cardiovascular diseases, cancer, and other diseases; and threaten human life and health [3]. Long-term exposure to high concentrations of particulate matter such as PM {2.5} will significantly increase the risk of lung cancer and other diseases [4]; SO₂ and NOx are the main precursors of acid rain. Acid rain can destroy soil and water ecosystems, affecting crop growth and aquatic organisms’ survival [5]. Toxic and harmful gases in the air are mainly divided into two categories. According to their different properties, they can be divided into irritating gases and asphyxiating gases [6]. Irritating gases are gases that irritate the eyes, respiratory mucosa, and skin, most commonly ammonia and formaldehyde [7]; asphyxiating gases refer to toxic gases that may cause hypoxia in the body, including hydrogen sulfide and phosphine, most of which are corrosive. Inhalation of certain high concentrations of harmful toxic gases can lead to poisoning, often accompanied by multi-functional disorders, which greatly threaten human health.

Traditional waste gas treatment methods can usually be divided into three categories: 1. Separation technology: Physical separation of pollutants from the airflow (such as adsorption, absorption). However, the adsorbent in the adsorption method needs to be replaced and regenerated regularly, and the cost is high; the combustion method consumes a lot of energy and may produce new pollutants [8]. 2. Destruction technology: Convert pollutants into harmless substances (such as incineration and photolysis) by chemical or biological methods. Although it can reduce the concentration of pollutants in the exhaust gas to a certain extent, there are problems such as high cost, high energy consumption, and easy to produce secondary pollution [9]. 3. Recycling technology: Capture and convert pollutants into recyclable resources. The specific treatment methods are as follows in Table 1.

Table 1. Introduction of traditional processing technology.

Technical Name	Core Technology Principle	Advantages	Disadvantages
Adsorption method	By using the huge surface area of porous solid materials, the pollutants in the gas are captured and fixed on the surface of their pores. When the adsorbent is saturated, it needs to be replaced or regenerated.	The technology is mature, the equipment is simple, and the operation is flexible. The treatment efficiency is high, and the removal effect of low-concentration pollutants is remarkable. Valuable solvents can be recycled (by steam or thermal regeneration).	The adsorbent capacity is limited, which needs to be replaced or regenerated regularly, and the operation cost is high. It is not suitable for high concentration, high humidity, or high temperature waste gas (humidity will reduce the adsorption capacity). The waste adsorbent may become a secondary pollutant, which needs to be properly treated. It is sensitive to particulate matter, and the exhaust gas needs to be pre-dusted.
Absorption method	The exhaust gas is in full contact with the absorption liquid, and the pollutants are transferred from the gas phase to the liquid phase by using the difference in solubility or chemical reaction activity of the pollutants in the absorption liquid, so as to achieve the purpose of purification.	The processing capacity is large, and the application range is wide. The removal efficiency of soluble pollutants is high, and the technology is mature. Specific pollutants can be treated by selecting different absorbents.	Secondary treatment is required to produce wastewater or waste absorption liquid, which increases the subsequent cost and complexity. There may also be equipment corrosion problems. The treatment effect of insoluble or insoluble VOCs is poor. High energy consumption (pump and fan power consumption).
Thermal combustion and Catalytic combustion	The exhaust gas is heated to a high temperature (usually 760–850 °C), so that the pollutant reacts with oxygen within a sufficient residence time and is completely oxidized and decomposed into CO2 and H2O. Under the action of catalysts (such as platinum, palladium, and other precious metals), the pollutants are oxidized at lower temperatures (usually 300–450 °C).	High destruction efficiency (>99%) and thorough disposal. It can handle complex and mixed VOCs airflow. Thermal combustion can recover heat energy (through heat exchangers) and reduce operating costs.	1. High initial investment and operating costs (fuel costs), especially thermal combustion. 2. Catalyst has a risk of poisoning (damaged by halogen, phosphorus, sulfur, heavy metals, and other substances), blockage, and wear, and the replacement cost is expensive. 3. Possibly produce secondary pollutants (such as NOx, especially at high temperatures). 4. Not suitable for the treatment of halogen-containing VOCs (which produce dioxins and acid gases).

In this context, the biological method of using compound probiotics to treat waste gas has gradually become a research hotspot because of its advantages of environmental friendliness, low cost, low energy consumption, and strong sustainability. Probiotics are a class of active microorganisms that are beneficial to the host. In waste gas treatment, they can transform pollutants in the exhaust gas into harmless or low-hazardous substances through their own metabolic activities [10]. Probiotics for waste gas treatment mainly include three categories of microorganisms. See (Table 2).

Table 2. Waste gas treatment probiotics.

Type	Characteristics	Specific Strains and Functions
Bacteria	By using the huge surface area of porous solid materials, the pollutants in the gas are captured and fixed on the surface of their pores. When the adsorbent is saturated, it needs to be replaced or regenerated.	Pseudomonas	Degradation of benzene, toluene, xylene, and other aromatic compounds
		Bacillus genus	Uses organic matter as carbon source and energy source, can form spores, and has strong environmental tolerance (such as resistance to drying, resistance to temperature change).
		Acinetobacter genus	Good at degrading alkane compounds
		Thiobacillus	Hydrogen sulfide (H2S) is oxidized to sulfuric acid (H2SO3) or sulfate [11]
		Nitrosomonas	Oxidation of ammonia (NH3) to nitrite (NO2−)
		Nitrobacter genus	Nitrite (NO2−) is oxidized to nitrate (NO3−)
Fungi	1. The mycelium network greatly increases the contact area with the gas, and the absorption efficiency of hydrophobic VOCs (such as olefins, benzene series) is much higher than that of bacteria [12]. 2. It can grow in a low pH environment, which is very suitable for dealing with the system that produces acidic substances in the process of degrading H2S, etc., and can avoid the acidification and collapse of the system. 3. The requirement for moisture is lower than that of bacteria, which reduces the energy consumption and cost of humidification.	Aspergillus niger	Degradation of toluene and other benzene series, stable in low pH biofilter, is a common industrial fermentation strain
		Aspergillus wentii	Degradation of sulfur-containing malodorous gases such as ethanethiol Separated from the biological filter for treating ethanethiol, which is suitable for malodorous gas treatment
		Trichoderma asperellum	Degradation of nitrogen oxides (NOx), which has a certain nitrification in the denitrification tower, can oxidize nitrite (NO2−) to nitrate (NO3−)
Actinobacteria	1. Secrete antibiotics, inhibit the growth of harmful bacteria in the system, and maintain the health of the microbial community. 2. Produce extracellular enzymes to help decompose some complex organic matter that is difficult to degrade. 3 Produce soil odor; the metabolism of the ‘soil’ itself can cover up or neutralize some stench.

This biological treatment method can not only effectively reduce the emission of exhaust pollutants and achieve non-toxic conversion [13], but also avoid the secondary pollution caused by traditional methods, which is of great significance for improving air quality and protecting the ecological environment.

The performance of a biological waste gas treatment system is affected by the interaction of multiple physical, chemical, and biological variables when a large-scale waste gas treatment is carried out in a factory. Optimization design in production is the process of balancing these parameters (as shown in Table 3). From the perspective of sustainable development, probiotic treatment of waste gas conforms to the concept of green development, helps to promote coordinated development of the economy and environment, and provides a new technical way to achieve sustainable development goals.

Table 3. Optimize the design in production.

Variable Category	Specific Parameters	Influence and Explanation
Operating parameters	Empty bed residence time (EBRT)	The average time that the exhaust gas stays in the packed bed. It is one of the most critical design parameters. High concentration and refractory waste gas requires longer EBRT (usually 30–100 s), otherwise it can be shortened (as low as 10–20 s).
	Temperature	Directly affects the microbial metabolic rate. The optimum temperature of mesophilic microorganisms is 25–40 °C. High temperature (>45 °C) will make the enzyme denature; too low (<10 °C) and it will greatly reduce activity.
	pH value	Different microorganisms have their optimum pH range. Bacteria are usually 6.5–8.5, and fungi are 4–7. The treatment of sulfur-containing waste gas will produce sulfuric acid, and the treatment of nitrogen-containing waste gas will produce nitric acid. It is necessary to add buffers (such as limestone, NaOH) or nutrient solution to stabilize the pH.
	Humidity	The filler humidity should be maintained at 40–60% (weight ratio). Too low humidity will lead to microbial inactivation and the biofilm drying; too high and it will block the pores of the filler, form an anaerobic zone, and increase the pressure drop.
Waste gas characteristics	Pollutant concentration and load	If the concentration is too low, the microorganism will be ‘starved to death’; too high of a concentration may lead to matrix inhibition (microbial poisoning) or too fast acid production, resulting in system pH collapse. Excessive load fluctuation will impact the stability of the system.
	Biodegradability of pollutants	Alkanes, alcohols, and phenols are easy to degrade. Aromatic hydrocarbons (benzene, xylene) and halogenated hydrocarbons (trichloroethylene) are difficult to degrade, requiring specific strains or longer acclimation time.
	Complexity of exhaust gas composition	When multiple pollutants coexist, there is a synergistic promotion or competitive inhibition effect. Compound microbial agents are needed to cope.
Microorganism-related	Nutrient supply (N, P, K)	It is necessary to provide balanced nutrition (such as BOD:N:P ≈ 100:5:1) for microorganisms to grow and maintain activity. Insufficient nutrition will limit growth, and excessive nutrition may cause bacteria to over reproduce and block the system.
	Oxygen (O2)	The aerobic process requires sufficient oxygen (usually O2 > 5%). For high concentration organic waste gas, it may be necessary to supplement the air.

This article includes but is not limited to the following databases: PubMed, Web of Science Core Collection, Scopus, Embase, and Cochrane Library. The basic period is more than 600 articles retrieved in the past ten years. All studies published within this time frame were included in the consideration, and non-English literature, meeting abstracts, research lacking full-text, and topic-independent research were clearly excluded. In total, 71 studies that met the criteria were included in this article from the remaining literature. The data extraction process is independently extracted by several authors and synthesized through discussion. This paper systematically reviews the application of probiotics in waste gas treatment and conversion from the perspectives of basic theoretical principles, environmental applications, and waste gas treatment facilities, aiming to provide theoretical support for technical optimization and large-scale applications and achieve green conversion.

2. Mechanism of Waste Gas Conversion Driven by Microbial Metabolism

As a ‘natural engineer’ of the earth’s material cycle, microorganisms can convert toxic and harmful gases into harmless substances through a series of enzyme-catalyzed reactions [14]. The waste gas treatment function of probiotics (generally refers to the functional microbial community with efficient degradation ability to environmental pollutants) is essentially a biological purification process achieved through energy metabolism and material transformation [15]. The core lies in the synergy of the flora and the precise regulation of the metabolic pathway. The conversion of waste gas by probiotics is based on the biotransformation process of pollutants driven by microbial metabolism, that is, microorganisms absorb pollutants in waste gas as a carbon source or energy or electron donor and convert them into harmless or low-toxic products through redox reactions under the catalysis of the intracellular enzyme system [16]. This process depends on the functional division of the compound probiotics: different strains form metabolic complementation for specific pollutants. For example, sulfur-oxidizing bacteria are specifically responsible for the oxidation of sulfur-containing compounds, while Pseudomonas and Bacillus dominate the degradation of VOCs [17]. Fungi can assist in the decomposition of complex organic compounds in a hypoxic environment. This ‘specific metabolism + synergistic metabolism’ model has greatly improved the treatment efficiency of mixed waste gas.

2.1. Biotransformation of Typical Waste Gases

The conversion of different types of waste gases by probiotics follows a specific metabolic pathway, and its core is to achieve ‘non-toxicity’ and ‘mineralization’ of pollutants through enzyme catalysis and energy transfer [18]. The catalytic effect and background of key enzymes in the biological treatment of waste gas are as follows. See Table 4.

Table 4. The catalytic effect and background of key enzymes in biological treatment of waste gas.

Enzyme Classification and Name	Target Pollutants	Catalytic Reaction (Simplified)	Functional Background and Importance	Representative Microorganisms
Methane Monooxygenase, MMO	CH4	CH4 + O2 + NADH → CH3OH + H2O + NAD+	It is the starting enzyme and key enzyme of methane metabolism, which can activate the inert C-H bond at room temperature and pressure, and oxidize methane to methanol. This is the first step in methane as a carbon and energy source.	Methylococcus Methylosinus
Toluene/Benzene Dioxygenase, TDO/BDO	Benzene, Toluene, Xylene	C6H6 + O2 + NADH → C6H6O2 + NAD+	Starting the degradation process of the aromatic ring, catalyzing the addition of oxygen molecules to the benzene ring, and opening its stable conjugated structure is the rate-limiting step for the degradation of refractory VOCs such as BTEX [19]. Under aerobic conditions, microorganisms hydroxylate the benzene ring through monooxygenase and dioxygenase, gradually decompose it into intermediate products such as catechol, and finally completely mineralize into CO2 and H2O through the tricarboxylic acid cycle (TCA) [20]; for complex VOCs (such as polycyclic aromatic hydrocarbons) that are difficult to metabolize directly, the flora can use a simple carbon source (such as glucose) as an energy source through a co-metabolism mechanism, while secreting enzymes to decompose the target pollutant [21].	Pseudomonas putida Ralstonia pickettii
Ammonia Monooxygenase, AMO	NH3	NH3 + O2 + 2H+ + 2e− → NH2OH + H2O	Oxidation of ammonia to hydroxylamine is the first step of nitrification and the starting point for the conversion of inorganic nitrogen pollutants. This enzyme is not specific to the substrate and may oxidize other substances.	Nitrosomonas
Nitrite Oxidoreductase, NOR	NO2−	NO2− + H2O → NO3− + 2H+ + 2e−	Nitrite is responsible for the oxidation of nitrite to nitrate, which is the second step of nitrification, converting toxic nitrite to less toxic nitrate.	Nitrobacter
Sulfite/Sulfate Reductase	SO42−/SO32−	SO42− + ATP + 8e− → SO32− → S2−	Under anaerobic conditions, it participates in the sulfate reduction process and finally reduces sulfate/sulfite to hydrogen sulfide (H2S) [22]. Note: This process is odorous and usually needs to be avoided.	Desulfovibrio [23]
Phosphotriesterase	organic phosphorus compound	(RO)3P=O + H2O → (RO)2P=O + ROH	The P-O-C or P-F bond in organophosphorus compounds is specifically hydrolyzed. It is a key enzyme for the degradation of phosphorus-containing poisons and can be used to treat the exhaust gas of phosphorus-containing pesticides.	Pseudomonas diminuta Flavobacterium spp.
Laccase	Phenols, Aromatic amines	Phenol + O2 → Quinone + H2O	A copper-containing polyphenol oxidase that uses molecular oxygen to oxidize a variety of phenolic and aromatic amine contaminants to produce water and unstable quinone intermediates, which are further polymerized or degraded.	Trametes versicolor Aspergillus
Peroxidase	A variety of refractory VOCs	Substrate + H2O2 → Oxidation product + 2H2O	Relying on hydrogen peroxide (H2O2) as a co-substrate, it can oxidize and decompose refractory pollutants with complex structures such as polycyclic aromatic hydrocarbons (PAHs) and chlorinated aromatic hydrocarbons and has a strong oxidizing ability.	Phanerochaete chrysosporium
GSH-dependent Formaldehyde Dehydrogenase, FDH Formate Dehydrogenase, FDH	Formaldehyde	HCHO + GSH + NAD+ → S-(Hydroxymethyl)GSH → S-Formyl GSH + NADH + H+ HCOOH + NAD+ → CO2 + NADH + H+	This is the core promoter of formaldehyde degradation in eukaryotes (such as fungi) and some bacteria. Firstly, formaldehyde is spontaneously combined with intracellular antioxidant glutathione (GSH) to generate hydroxymethyl glutathione, which is then oxidized by the enzyme. Subsequently, it is converted into formic acid by intracellular formaldehyde dehydrogenase, which is further decomposed into CO2 and H2O by formate dehydrogenase to achieve complete non-toxicity [24]. The intracellular metabolic rate of high-efficiency formaldehyde-degrading bacteria can reach 0.5–2.0 mmol/(g·h), which is significantly better than the saturation capacity limit of physical adsorption materials [25].	Candida spp. Pseudomonas putida

A variety of waste gases can react with organic acids to form onium salts [26]. The onium ion was first formed by the lone pair electrons of the nuclear atoms of the mononuclear hydrides of nitrogen, oxygen, and halogen as the cation formed by the protonation of the Bronsted base, which was called the XXium salt [27]. In the microecological environment of probiotics and organic acids, these toxic and harmful gases can obtain a proton and ionize and then form onium salts with organic acids. These onium salts are precipitated in different states (such as flocculent) in the sludge and are decomposed by subsequent redox [28]. The chemical equation is shown in Table 5.

Table 5. Reaction equation of organic acid and toxic and harmful gas.

Toxic and Harmful Gas	Reaction Chemical Equation
Hydrogen sulfide (SH2)	R1-COOH → R1-COO− + H+
	SH2 + H+ → SH3+ (Thiosulfonium)
	R1-COO− + SH3+ → R1-COOSH3 (Sulfonium salt)
Ammonia (NH3)	R2-COOH → R2-COO− + H+
	NH3 + H+ → NH4+ (Nitrinium)
	R2-COO− + NH4+ → R2-COONH4 (Ammonium salt)
Phosphine (PH3)	R3-COOH → R3-COO− + H+
	PH3 + H+ → PH4+ (Phosphonium)
	R3-COO− + PH4+ → R3-COOPH4 (Phosphonium salt)
Formaldehyde (HCHO)	R4-COOH → R4-COO− + H+
	HCHO + H+ → HCHOH+ (Carbonium)
	R4-COO− + HCHOH+ → R4-COOHCHOH (Carbonium salt)
Methane (CH4)	R5-COOH → R5-COO− + H+
	CH4 + H+ → CH5+ (Carbonium)
	R5-COO− + CH5+ → R5-COOCH5 (Carbonium salt)

2.2. Key Influencing Factors of Metabolic Regulation

The metabolic activity and waste gas conversion efficiency of probiotics are highly dependent on environmental conditions and the nutrient supply. The core regulatory factors include physical and chemical environmental parameters and nutrient substrate synergy. See Table 6.

Table 6. The catalytic effect and background of key enzymes in the biological treatment of waste gas.

Influencing Factors	Influence on the Principle of Microbial Metabolism	Best Range and Examples	Consequences of Improper Control
pH value	1. Enzyme activity: Most enzymes can only maintain their three-dimensional structure and active center in a specific pH range. Deviation from the optimum pH will denature and inactivate it. 2. Membrane permeability: Affecting cell membrane charge and permeability, thereby affecting nutrient absorption and waste discharge. 3. Substrate availability: Affecting the ionization state of certain pollutants (such as H2S, NH3), thereby affecting the ease of their use by microorganisms [29].	Bacteria: near neutral (6.5–8.0) Fungi: Acidic (4.0–7.0) [30] Nitrifying bacteria: 7.5–8.5 Sulfur-oxidizing bacteria: 1.0–3.0 (extreme acidophilic)	Peracid/peralkali: Enzyme activity decreased sharply; cell membrane damage; the microbial community structure is unbalanced and the function collapses. For example, the treatment of sulfur-containing waste gas to produce sulfuric acid; if not buffered, the system pH plummets, inhibiting the vast majority of microorganisms.
Temperature	1. Reaction rate: For every 10 °C increase in temperature, the enzymatic reaction rate increases by about 1–2 times (Q10 law). 2. Enzyme and membrane stability: Too high temperature will cause enzyme denaturation, membrane lipid excessive flow, and disintegration; if the temperature is too low, enzyme activity will be extremely low, membrane fluidity will be poor, and metabolism will be stagnant [31]. 3. Gas phase mass transfer: It affects the solubility and diffusion rate of pollutants in gas–liquid biofilm.	Mesophilic microorganisms: 25–40 °C Thermophilic microorganisms: 50–60 °C (suitable for high temperature waste gas) Ambient temperature: usually refers to 15–30 °C	Too high: Rapid microbial inactivation, system collapse [32]. Too low: Slow metabolism, low processing efficiency, difficult to start. Excessive fluctuation: Impacts the microbial community and selects strains with poor adaptability.
Oxygen concentration	Microorganisms need to breathe to produce energy (ATP) when degrading pollutants (as a carbon source and energy). This process requires a final electron acceptor to receive electrons produced in biochemical reactions. Oxygen (O2) is the final electron acceptor with the highest energy production efficiency.	At high concentrations, aerobic respiration is performed to degrade most VOCs (benzene, toluene, phenol, etc.) [33] and inorganic substances (H2S, NH3). Common flora such as Pseudomonas and Bacillus.	The oxygen concentration is generally required to be no less than 5–10% to ensure the smooth progress of aerobic metabolism. For high concentration organic waste gas, it is necessary to supplement air or oxygen to prevent a decrease in treatment efficiency and the generation of odorous by-products due to hypoxia.
Carbon nitrogen phosphorus ratio (C:N:P)	1. Cell synthesis: Microbial synthesis of their own cytoplasm needs to follow a certain nutritional ratio. Nutrient imbalance limits microbial growth. 2. Enzyme synthesis: C, N, and P are the basic elements of key metabolites such as synthetase, ATP, and NADH [34].	Classical ratio: BOD:N:P = 100:5:1 (When treating inorganic waste gas such as H2S, an additional carbon source is needed) [35].	C high and N/P low: Malnutrition, poor growth of zoogloea, treatment efficiency decreased. High N/P and low C: Overgrowth of microorganisms may lead to sludge bulking or excessive biofilm thickness, resulting in blockage.
cofactor	1. Enzyme function core: Many enzymes (especially the key enzymes for degradation of special pollutants) require cofactors to perform catalytic functions. 2. Energy metabolism: Coenzymes (such as NAD+, FAD) are involved in electron transfer and energy (ATP) production.	Trace elements: Fe, Mo, Mg, Co, Ni, Cu, etc. (are part of many coenzymes) [36] Vitamins: Such as B vitamins, as coenzyme precursors [37].	Deficiency: Key metabolic pathways are disrupted, and pollutants cannot be degraded even in the presence of bacteria. This is a common but easily overlooked reason for poor system performance.
Substrate inhibition	High concentration substrates (pollutants) may cause the following: 1. Toxic effects: Direct destruction of cell membranes, enzyme saturation inactivation, or interference with central metabolism. 2. Osmotic stress: High concentrations of organic matter lead to a decrease in water activity and cause cell dehydration.	It varies with microorganisms and pollutants. For example, VOC concentrations >5–10 g/m3 may inhibit many bacteria.	Performance: When the inlet concentration suddenly increases (impact load), the treatment efficiency does not rise but falls, or even drops, to zero. Microorganisms need a long recovery time.
Product inhibition	Accumulation of metabolites may cause the following: 1. Feedback inhibition: Terminal products inhibit the activity of key enzymes at the front end of the metabolic pathway. 2. Change the pH of the environment: Such as degradation of chlorine, sulfur, and nitrogen waste gas to produce HCl, H2SO2, and HNO3, resulting in system acidification.	For example, the accumulation of H2S produced by sulfate-reducing bacteria inhibits its own activity.	Performance: The initial effect of the system is good, but with the operation, the efficiency continues to decline. It is often accompanied by dramatic changes in pH.

2.3. Types of Probiotics Converting Harmful Gases

In the process of industrialization and urbanization, harmful gases such as sulfur dioxide (SO₂), ammonia (NH₂), and volatile organic compounds (VOCs) continue to be emitted, causing environmental disasters, such as haze and acid rain, and seriously threatening public health. The traditional physical and chemical treatment technology is limited by high energy consumption, secondary pollution, and cost bottlenecks, and it is urgent to develop sustainable solutions. As a natural ‘biocatalyst’, probiotics are becoming an emerging force for waste gas purification due to their efficient metabolic diversity and environmental compatibility. The following table summarizes the types of probiotics that convert harmful gases; see (Table 7).

Table 7. Types of probiotics converting harmful gases.

Name of Bacteria	Convertible Gas Name	Transformation Mechanism	Biotransformation Products	Reaction Conditions	Ref
Acidithiobacillus thiooxidans	H2S	Under aerobic conditions, H2S is gradually oxidized by sulfur oxidase system, and the electron transfer chain drives energy metabolism to produce sulfuric acid or elemental sulfur.	S0, SO42−	The optimum temperature is 28–30 °C; the optimum pH is 1.0–3.5; chemoautotrophic bacteria; aerobic metabolism.	[38]
Vibrio natriegens	BTEX, PAHs	Synthetic biology modified chassis bacteria, integrated 5 degradation gene clusters, expressed benzene ring lyase, oxygenase, etc., and directly mineralized aromatic hydrocarbons into small molecules.	CO2, H2O	The optimum temperature is 30–37 °C; the optimum pH is 7.0–8.5; aerobic metabolism.	[39]
Bacillus amyloliquefaciens T-5	VOCs	It secretes antibacterial VOCs (such as aldehydes and ketones), destroys the virulence genes of R. solanacearum, and weakens its biofilm formation ability.	Non-toxic small molecule metabolites	The optimum temperature is 30–37 °C; the optimum pH is 6.5–7.5; the optimum C/N ratio is 20:1; aerobic metabolism.	[40]
Pseudomonas	VOCs	Mineralization of n-butyl acetate and other pollutants with VOCs as a carbon source.	O2, H2O	The optimum temperature is 20–30 °C; the optimum pH is 6.5–7.5; the optimum C/N ratio is 5:1; aerobic metabolism.	[41]
Alcaligenes faecalis UA	H2SO4	Secretes acid-resistant urease, decomposes urea to produce alkali and neutralize acidic wastewater, and precipitates heavy metal ions.	Carbonate, heavy metal precipitates	The optimum temperature is 30–37 °C; the optimum pH is 7.0–8.0; aerobic metabolism.	[42]
Bacillus subtilis yb-1	NH3, H2S	Reduce pH to reduce odor production.	Short-chain fatty acid	The optimum temperature is 30–37 °C; the optimum pH is 6.5–7.5; the optimum C/N ratio is 25:1; aerobic metabolism.	[43]
Acinetobacter pittii	H2S	H2S can be converted to SO42−.	SO42−	The optimum temperature was 30–35 °C; the optimum pH was 6.5–7.5; the optimum C/N ratio was 5:1; and strict aerobic metabolism was carried out.	[44]

3. Genetic Characteristics of Probiotics Converting Harmful Gases

The genetic characteristics of microorganisms determine their core metabolic capacity, environmental adaptability, and engineering potential, which is the theoretical basis for evaluating and optimizing their ability to treat harmful gases. By analyzing the composition, structure, and regulatory sequence of the gene cluster, its metabolic pathway can be completely restored, and the rate-limiting steps of the degradation process, intermediate products, and induction conditions of gene expression (such as which pollutants trigger gene expression) can be understood. This provides a theoretical basis for optimizing operating conditions. The following are the genetic characteristics associated with the waste gas treatment capacity of seven microorganisms in Section 2.3. See Table 8.

Table 8. Genetic characteristics associated with waste gas treatment capabilities of 7 microorganisms.

Microbial Name	Key Genetic Characteristics
Acidithiobacillus thiooxidans	1. Chemoautotrophic-related gene cluster: There is a complete Calvin–Benson–Bassham cycle gene for fixing CO2 as a carbon source. 2. Sulfur oxidation gene cluster: There is the sox gene cluster (soxXYZABCD), doxDA, etc., encoding the key enzyme system that oxidizes reducing sulfide (H2S, S0) to sulfuric acid (H2SO4). 3. Extreme acid resistance mechanism: The genome contains a strong proton pump system, cell membrane, and enzyme adapted to an extremely low pH environment, allowing it to survive in high acidity.
Vibrio natriegens	1. Natural high-speed metabolic gene background: It has a very high number of ribosomal RNA operons, supporting its ultra-fast protein synthesis and growth rate. 2. Integration of exogenous degradation pathways: Through plasmid or genome integration, the degradation gene clusters of specific VOCs (such as benzene series) (such as tod or tom operons from Pseudomonas) were introduced. 3. Artificial optimization elements: The introduced genes are usually controlled by strong promoters to maximize expression; it may also contain resistance markers and stability elements.
Bacillus amyloliquefaciens T-5	1. Antibacterial substance synthesis gene cluster: It has a non-ribosomal synthase (NRPS) gene cluster that encodes the synthesis of antimicrobial lipopeptides (such as surfactin, iturin, and fengycin). 2. Substrate degradation related genes: Genes encoding extracellular enzymes (such as proteases, amylases, and possibly specific oxygenases) that degrade complex organic matter. 3. Spore-forming gene: It has a complete spo gene cluster, which enables it to form spores with strong resistance.
Pseudomonas spp.	1. A large library of degradation plasmids: Usually carrying a variety of mobile genetic elements (such as plasmids, transposons), which carry a variety of degradation operons (such as tod for toluene, alk for alkanes, nah for naphthalene). The regulatory network is complex: There are complex regulatory genes (such as xyIR, todS/todT) that can sense the presence of substrates and fine-tune the expression of degradation genes. 3. Environmental adaptation genes: It has genes encoding a variety of efflux pumps and membrane proteins to help it resist organic solvents and heavy metal stress.
Alcaligenes faecalis UA	1. Denitrification-related genes: The amoCAB gene cluster, which is used to oxidize ammonia (NH3) to hydroxylamine, and the nitrite oxidoreductase (nxr) gene, which was used to oxidize nitrite (NO2−) to nitrate (NO3−). 2. Aerobic/facultative anaerobic metabolism: Some strains have nitrate reductase (nar) gene, which can be denitrified under anoxic conditions.
Bacillus subtilis yb-1	1. Spore-forming genes: Having a complete spo gene cluster is the basis of its stress resistance. 2. Antibacterial peptide synthesis genes: Usually have genes encoding a variety of antimicrobial peptides (such as subtilin). 3. Substrate degradation genes: There are abundant genes encoding extracellular hydrolases.
Acinetobacter pittii	1. Alkane hydroxylase gene: alkB gene encodes a key enzyme that hydroxylates the end of alkane, which is the first step in the degradation of long-chain alkane. 2. Biofilm formation related genes: It has genes encoding pilus, extracellular polysaccharides, etc., so that it can strongly adhere to form biofilms. 3. Drug resistance gene island: The mobile drug resistance gene island is often integrated into the genome, which is the source of its hospital infection ability but has little effect on waste gas treatment.

4. Environmental Application Scenarios—Multi-Domain Waste Gas Treatment Solutions

Probiotic technology has realized the engineering application of waste gas treatment in many scenarios such as industry, agriculture, and urban management by virtue of its characteristics of ‘targeted degradation + environmental friendliness’, and has formed customized solutions for different pollution sources. It includes the stable emission of VOCs and sulfur-containing waste gas in industrial waste gas treatment, the control of odorous gases such as NH₃ and H₂S in animal husbandry and waste treatment in agricultural activities, and the efficient treatment of urban waste gas.

4.1. Industrial Waste Gas Pollution Control

VOCs and sulfur-containing waste gas emitted from industrial production (such as chemical, printing, pharmaceutical) have complex components and large concentration fluctuations. Probiotic technology has achieved stable and standard emissions through bioreactors [45]. For example, the application of a biofilter, biotrickling filter, and biological washing tower in chemical industry parks, food processing and fermentation, and livestock and poultry breeding have formed a mature technology system [46]. Specific industries and microbial treatment technologies are shown in the table below. See Table 9.

Table 9. Specific industries and microbial treatment technologies.

Industry Category	Main Exhaust Gas Components	Evaluated Microbial Treatment Technologies	Core Functional Microorganisms	Technical Advantages and Precautions
Petrochemical industry	Benzene, toluene, xylene (BTEX), and other volatile organic compounds (VOCs).	Biotrickling filter, biofilter	Pseudomonas, Bacillus	It has good removal effect on a variety of VOCs; attention should be paid to the possible impact of fluctuations in exhaust gas concentration.
Printing, spraying	VOCs such as ketones (such as acetone), esters, benzene series, etc.	Biotrickling filter, biological scrubber	Nocardia, Mycobacterium	It is suitable for low-concentration and high-air-volume exhaust gas; the treatment effect of VOCs with poor water solubility may be limited, and composite microbial agents or pretreatment can be used.
Food processing, fermentation	Ethanol, organic acids (such as acetic acid), aldehydes, odorous gases	Biofilter	Yeast (such as Candida), lactic acid bacteria, compound microbial agents	The operation cost is low; however, the exhaust gas may contain fungicides or high concentrations of salt, which requires screening of tolerant strains and control of environmental conditions (such as pH, humidity).
Pharmaceutical industry	Solvent VOCs (such as dichloromethane), sulfur-containing or nitrogen-containing organic matter, fermentation tail gas	Biotrickling filter (domesticated strains for specific compounds), composite biological treatment (combined with other technologies)	Specific domesticated degrading bacteria (such as some Pseudomonas) and fungi (such as Aspergillus)	Exhaust gas may contain biological inhibitory substances or antibiotics, which are highly toxic to microorganisms and require long-term domestication or physicochemical pretreatment.
Leather, papermaking	Hydrogen sulfide (H2S), mercaptan, total reduced sulfur (TRS), formaldehyde	Biotrickling filter, biofilter	Sulfur-oxidizing bacteria (such as Thiobacillus), formaldehyde-degrading bacteria (such as Hyphomicrobium)	The waste gas contains odorous substances with strong hydrophobicity (such as thiols), and the mass transfer efficiency is the key. Segmented treatment or surfactant addition can be used to promote microbial absorption.

Among them, a large coal chemical enterprise uses ceramsite-activated carbon composite filler biofilter to treat waste gas containing benzene, toluene, and xylene (inlet concentration of 200–800 mg/m³). By inoculating Pseudomonas and Bacillus complex bacteria, the removal rate of VOCs is stable at more than 90% under the condition of an empty bed residence time of 30–60 s [47], and the outlet concentration meets the limit requirements of petrochemical industry pollutant emission standards. The low-concentration VOCs (100–300 mg/m³) in the printing industry can be treated by a biotrickling filter [48], using ethanol as a co-metabolism carbon source, so that the degradation rate of ethyl acetate, butanone, and other solvents can reach 95%, and the operating cost is only 1/3 of the activated carbon adsorption method [49].

For the H₂S-containing waste gas (concentration of 500–5000 mg/m³) in the chemical and refining industries, the probiotic activated-carbon washing tower achieves efficient desulfurization through gas countercurrent contact [50]. The experimental equipment is shown in Figure 1. Thiobacillus thiooxidans and Acidithiobacillus ferrooxidans (concentration of ≥10⁸ CFU/mL) were inoculated into the spray liquid in the tower, and activated carbon was added. Using the adsorption of activated carbon, the probiotics and organic acids contained in the microecological liquid entered the micropores of activated carbon and adhered to the inner surface of activated carbon. The valve was opened, and hydrogen sulfide was introduced into a multiple U-tube containing activated-carbon and probiotic liquid for adsorption. After H₂S was absorbed in the liquid phase, it was oxidized by the flora to elemental sulfur (recovery rate > 85%). After treatment, the concentration of H₂S in the exhaust gas can be reduced to <10 mg/m³.

Figure 1. High concentration hydrogen sulfide waste gas adsorption experimental equipment diagram.

4.2. Agricultural Source Pollution Control

Animal husbandry and waste treatment in agricultural activities are the main sources of odorous gases such as NH₃ and H₂S (Table 10). Probiotic technology can achieve pollution control through ‘source control + process treatment’.

Table 10. Waste gas treatment in agricultural activities.

Industry Category	Main Exhaust Gas Components	Evaluated Microbial Treatment Technologies	Core Functional Microorganisms	Technical Advantages and Precautions
Livestock and poultry breeding, composting	Malodorous gases such as ammonia (NH3), hydrogen sulfide (H2S), volatile organic acids, phenols, etc.	Biological filter (commonly used organic filler)	Nitrifying bacteria, sulfur-oxidizing bacteria, actinomycetes	The investment operation cost is relatively low; the composition of exhaust gas is complex, and the concentration fluctuates greatly. It has high requirements for the compound degradation ability of microorganisms, and it is necessary to ensure sufficient residence time.

The regulation of intestinal flora by adding probiotics to feed can reduce NH₃ emissions from the source [51]. Studies have shown that adding 0.1–0.5% of Lactobacillus plantarum (concentration 10⁹ CFU/g) to pig feed can inhibit urease activity by adjusting intestinal pH value (reducing 0.5–1.0 units) and reduce NH₃ produced by protein decomposition [52]. The experimental data show that the NH₃ concentration in the piglet house can be reduced by 30–40%, and the H₂S emission in the feces can be reduced by more than 25%. In addition, the farm manure treatment process uses a composite microbial agent (Bacillus + yeast) spray, which can immediately decompose the escaped odorous substances, and the treatment efficiency is more than 80% [53].

In the composting process of straw and livestock manure, anaerobic bacteria are prone to produce odorous gases such as H₂S and methyl mercaptan, which can be significantly inhibited by inoculation of compound microbial agents (such as Desulfovibrio + actinomycetes) [54]. When ‘straw–chicken manure’ compost was used in a vegetable planting base, the release of H₂S in the compost was reduced by 60–70% compared with the control group when 1.0% compound microbial agent (concentration of 10⁸ CFU/g) was added, and the composting time was shortened by 5–7 days [55]. The effective nitrogen content in organic fertilizer increased by 15–20%, which achieved the dual benefits of ‘deodorization–upgrading’.

4.3. Urban Waste Treatment and Formaldehyde Adsorption in New Houses

The composition of waste gas in municipal waste transfer stations and landfills is complex (including H₂S, methyl mercaptan, CH₄, VOCs, etc.) [56]. Probiotic technology can achieve efficient treatment through flexible application (Table 11). The main toxic and harmful gas in the new room is formaldehyde [57].

Table 11. Waste gas treatment in urban garbage.

Industry Category	Main Exhaust Gas Components	Evaluated Microbial Treatment Technologies	Core Functional Microorganisms	Technical Advantages and Precautions
Sewage treatment, waste treatment	Malodorous gases such as hydrogen sulfide (H2S), ammonia (NH3), and thiols.	Biological filter (commonly used organic filler)	Acidithiobacillus thiooxidans, Nitrifying bacteria/denitrifying bacteria	The removal efficiency of inorganic odors such as H2S and NH3 is high (often ≥98%). Acidic substances may be produced during the treatment process, and attention should be paid to pH control and filler maintenance.

For the short-term concentrated emission of high concentration odor gas (H₂S concentration can reach 100–500 mg/m³) from the transfer station [58], the environmental control probiotic spray system was used for treatment. The system combines a compound microbial agent (sulfur-oxidizing bacteria + yeast) with a surfactant, and fully contacts with the exhaust gas through high-pressure atomization (droplet diameter 5–20 μm). The decomposition rate of H₂S, methyl mercaptan, and other substances under the action of the flora is >95% within 10–30 min [59].

The formaldehyde adsorption facility in the new room is to place an activated carbon pad in the new room that adsorbs a microecological probiotic liquid with 100 ppm phosphonium ions (Figure 2) [60]. A large fan is placed on the side of the mat to allow the indoor air containing formaldehyde to be filtered through the mat, and the indoor walls and floors are sprayed with a microecological probiotic liquid diluted 10 times with 100 ppm phosphonium ions. It has a good adsorption effect.

Figure 2. New room formaldehyde adsorption experimental equipment diagram.

5. Engineering Design and Optimization

The large-scale application of probiotic waste gas treatment technology needs to realize the dynamic balance of ‘efficiency–cost–stability’ through three core paths: bioreactor design innovation, flora immobilization technology upgrading, and precise regulation of process parameters. At the level of bioreactor design, it is necessary to develop a multi-stage coupling structure (such as a ‘biotrickling filter–activated-carbon adsorption’ combined system), and use the microporous structure of ceramsite or modified plastic filler to enhance the gas–liquid mass transfer efficiency, so that the unit reactor volume treatment capacity is greatly improved. The immobilization technology of bacteria is the key guarantee. The loss rate of bacteria is reduced, and the service life is greatly prolonged by sodium alginate–PVA double carrier embedding method or magnetic nanoparticles loading functional bacteria (such as Thiobacillus thiooxidans). Process parameter optimization focuses on operation threshold control. At present, the design and performance of microbial waste gas treatment engineering are summarized into the following five parts through the types of pollutants (Table 12).

Table 12. Design and performance of microbial waste gas treatment engineering.

Pollutant Type	Process Conditions (Temperature, pH, Residence Time, etc.)	Reactor/Technology Type	Removal Rate	Core Microorganisms (If Specified)
VOCs mixed gas (alcohol, phenol, aldehyde, ketone, etc.)	pH: 6.5–7.5 Temperature: 25–28 °C DO: ≥0.3 mg/L Contact time: 15–20 s	Two-stage biological contact purification tower (including specific filler)	H2S: close 100% (160 → 0.1 mg/m3) NH3: ~73% (6 → 1.6 mg/m3) VOCs: ~98% (1200 → 22)	Pseudomonas, Bacillus, Arthrobacter, and other dominant flora
H2S, NH3, mercaptan and other odorous gases	pH: 6–7 Temperature: 26–34 °C	Biofilter, biotrickling filter, and a variety of combined processes (such as biotrickling filter–chemical absorption, –activated-carbon adsorption, –photocatalysis)	Odorous substances: >95% TVOCs: >90%	Composite functional flora screened for representative pollutants
The stench of stale landfill waste (H2S, NH3, CH4, VOCs)	Comprehensive application of a technology system (direct injection of pile body, fog gun, drone spraying, biological filter)	MPI environmental control probiotic integrated control technology system (not a single reactor)	CH4: 92.7% H2S: 100% Odor concentration: 62.85–88.24% NH3: 89.37% (7.9 → 0.84 mg/m3)	MPI environmental control probiotics (compound microbial agent)
Antibiotic production tail gas (Complex VOCs, odor, dust, high temperature, and high humidity)	A large-scale integrated system with an investment of more than 1.6 billion yuan (87 washing towers, 13 molecular sieve units, 11 combustion units, etc.)	Integrated process of ‘negative pressure collection + pretreatment + molecular sieve adsorption concentration + high temperature oxidation combustion’ technology	VOCs and stench: ≥95%	Domesticated microbial communities
Organic waste gas	pH: 6–7 Temperature: 30 ± 4 °C (26–34 °C)	Airlift packed bioreactor	Effectively degraded	Domesticated microbial communities

5.1. Mainstream Bioreactor Technology and Characteristics

A bioreactor is the core place for probiotics to come in contact with waste gas and complete metabolic transformation. Its structural design directly affects mass transfer efficiency and microbial activity [61]. The current mainstream technologies include a biofilter, biotrickling filter, and membrane bioreactor. The biofilter is composed of a filler layer, an air distribution system, and a spray system. The filler (such as ceramsite, activated-carbon, bark, etc.) provides an attachment carrier for the flora [62]. The porous structure of ceramsite/activated-carbon (porosity 50–70%) can enhance the gas residence time and bacterial colonization area. The biofilter of a printing workshop uses ‘ceramsite + activated-carbon’ composite filler (volume ratio 1:1), which is 15–20% higher than the VOCs removal rate of a single filler. During the operation, the waste gas enters from the bottom, comes into contact with the biofilm on the surface of the filler, and is degraded. It is suitable for the treatment of low and medium concentration waste gas (VOCs < 1000 mg/m³), and the empty bed residence time is usually 30–120 s [63]. The core difference between the biotrickling filter and the biofilter is the addition of a liquid circulation system. The nutrient solution (including carbon source, nitrogen source, and trace element) flows from top to bottom through the spray device to form a liquid film on the surface of the filler. When the exhaust gas is the countercurrent contact, the pollutant is first dissolved in the liquid film and then degraded by the flora [64]. The design optimizes mass transfer efficiency, especially for the waste gas with high water solubility (such as H₂S and NH₃). When the biotrickling filter of a chemical enterprise treats the waste gas containing H₂S, the mass transfer coefficient is increased by 30% by adjusting the liquid circulation (2–5 L/m²·h), and the treatment load reaches 50–100 g/(m³·h), which is significantly higher than that of the biofilter [65]. Membrane bioreactor (MBR) combines ultrafiltration membrane module with biological reaction tank [66]. The interception of membrane can maintain the concentration of bacteria in the reactor at 10⁹–10¹⁰ CFU/mL (5–10 times that of traditional reactor), which greatly improves the degradation efficiency [67]. An electronic factory uses MBR to treat formaldehyde-containing waste gas (inlet concentration of 50–200 mg/m³). Under the condition of hydraulic retention time (HRT) of 1–2 h, the formaldehyde removal rate is stable at more than 95%, and the membrane module can intercept more than 99% of the functional bacteria, avoiding the loss of flora, and the operation cycle is extended to 3–6 months [68].

5.2. Immobilization Technology of Bacteria

Free bacteria are easy to lose with the fluid in the reactor and are sensitive to environmental fluctuations. Bacteria immobilization technology significantly improves the stability of microorganisms by limiting them in a specific space. Encapsulation immobilization uses natural polymer materials (such as sodium alginate) or synthetic materials (such as polyvinyl alcohol, PVA) to embed the flora in gel microspheres to form a ‘microecosystem’ [69]. For example, the desulfurization bacteria were embedded in 4% sodium alginate +2% CaCl₂, the diameter of the microspheres was 2–3 mm, and the porosity was >60%. It can not only allow the diffusion of pollutants and nutrients but also protect bacteria from external shocks. In the treatment of H₂S-containing waste gas, the half-life of the embedded bacteria (the time when the activity decreased to the initial 50%) was 2–3 times longer than that of the free bacteria. PVA gel is more suitable for high flow rate reactors due to its high mechanical strength (compressive strength > 0.5 MPa) [70]. The new carrier enhances the treatment efficiency through the synergistic effect of ‘adsorption–degradation’. The surface of modified biochar (such as corncob biochar oxidized by nitric acid) is rich in functional groups such as carboxyl and hydroxyl groups, and the adsorption capacity of VOCs can reach 0.1–0.5 g/g. At the same time, its porous structure provides attachment sites for the flora [71], forming a virtuous cycle of ‘adsorption enrichment–biodegradation’. A study showed that the removal rate of benzene series of modified biochar carrier was 40–50% higher than that of ordinary carrier [72]. Nanofiber carriers (such as polylactic acid nanofibers prepared by electrospinning) can load more bacteria due to their large specific surface area (>100 m²/g), and the fiber gap is conducive to gas mass transfer, which is excellent in low-concentration waste gas treatment [73].

5.3. Multivariable Discussion of Process Design and Scale

The design and amplification of microbial waste gas treatment systems is far from simple proportional amplification but requires systematic optimization and balance of multiple interrelated variables. For example, the core engine of technology: microbial strains. Screening and domestication of specific functional bacteria for target pollutants is the basis for efficient treatment. In practice, it is more inclined to use complex bacteria to degrade complex mixtures and enhance system stability by using the synergistic effect between microorganisms. The engineered strains need to tolerate pressure such as concentration shock, pH change, toxic substances (such as intermediate metabolites), and drying. When the microbial strains are prepared, immobilization is also an important step. Immobilizing microorganisms to form biofilms on the surface of the filler is the key to maintaining high biomass, preventing the loss of bacteria, and ensuring the treatment effect.

Secondly, the reactor design and well-designed operating parameters are used to create the best microenvironment. It usually includes two parts: the selection of reactor type and the coupling of process parameters. Common reactor types and their characteristics are shown in the following figures. The most suitable type should be selected according to the characteristics of exhaust gas: (Figure 3).

Figure 3. Selection of bioreactors.

The coupling of process parameters is mainly divided into three aspects: Empty bed residence time (EBRT) determines the contact time between exhaust gas and microorganisms, which is the core design parameter. The pH, temperature, and humidity must be maintained within the optimal range of microbial activity. Finally, appropriate nutrients can provide the necessary nitrogen, phosphorus, potassium, and trace elements for microbial growth.

By optimizing the inoculation ratio, microbial concentration, and intelligent regulation, the metabolic potential of probiotics can be maximized. For example, when the inoculation ratio (the mass ratio of functional flora to carrier) is controlled at 1.0–2.0, the flora can quickly form a biofilm without clogging the carrier pores [74]; the concentration of bacterial agent should be ≥10⁷ CFU/mL. If the concentration is too low, the start-up cycle will be prolonged, and if the concentration is too high, the activity may be decreased due to nutritional competition. The debugging data of a biofilter showed that when the concentration of microbial agent increased from 10⁶ CFU/mL to 10⁸ CFU/mL, the VOCs treatment efficiency increased from 60% to 90%, but when it continued to increase to 10⁹ CFU/mL, the efficiency increase was less than 5%, and the cost and benefit needed to be balanced [75]. Through on-line monitoring of pH, temperature, DO, and other parameters, combined with automatic feedback device to adjust nutrient solution supply, stable operation is achieved [76]. For example, an industrial waste gas treatment station uses a pH sensor (accuracy ± 0.1) to link with an automatic dosing pump. When the pH of the reactor deviates from the optimal range (±0.5), the acid/alkali regulator is automatically added [77]; the temperature sensor cooperates with the heating/cooling device to control the reactor temperature in the optimal range of ±1 °C, so that the fluctuation range of treatment efficiency is controlled within 5%, which greatly reduces the cost of manual operation and maintenance [78].

6. Whole Life Cycle Cost Disassembly

The cost of probiotics to treat waste gas needs to cover ‘initial investment + operating cost + maintenance cost’ and is significantly affected by the treatment scale and pollutant complexity (taking the 10,000 m³/h waste gas treatment system as an example). The initial investment cost control scheme is shown in Table 13. Table 14 shows the operation and maintenance plan. Table 15 shows the factors affecting the cost.

Table 13. Initial investment cost.

Cost Item	Specific Content	Cost Range (Ten Thousand Yuan)	Proportion
Reactor equipment	Biofilter/trickling filter main body, air distribution/spray system	80–150	50–60%
Carrier material	Volcanic rock/activate-carbon/modified straw	20–50	15–20%
Monitoring and control system	pH meter, temperature sensor, fan, frequency converter	15–30	10–15%
Fungi culture and domestication equipment	Laboratory shake flask, seed tank (on demand)	5–15	5–10%
Installation and debugging	Pipeline connection, system test run	10–20	5–10%

Table 14. Operation and maintenance costs.

Cost Item	Calculation Basis	Cost Range (Yuan/m3 Waste Gas)	Proportion
Fungi and nutrition supplement	Monthly supplementation (100–200 yuan/kg agent), nutrients.	0.1–0.3	10–15%
Energy consumption	Fan (0.5–1.0 kW·h/m3), spray pump (0.2–0.5 kW·h/m3), heating (on demand)	0.3–0.8	40–50%
Manual maintenance	1–2 people/class (monitoring parameters, cleaning carrier)	0.1–0.2	10–15%
Carrier replacement	Replaced every 1–3 years (based on 1/3 of carrier cost)	0.05–0.15	5–10%
Wastewater/waste treatment	Regular discharge of spray liquid (to be treated to meet the standard)	0.1–0.3	15–20%

Table 15. Cost-influencing factors.

Factors	Description and Optimization Direction
Exhaust gas complexity	The cost of single pollutant (such as toluene) is low (0.5–1.0 CNY/m3). Mixed pollutants (such as VOCs + H2S + NH3) require a variety of bacteria, and the cost increases by 20–30%.
Processing scale	When the scale is greater than 50,000 m3/h, the unit cost can be reduced by 15–25% (scale effect).
Carrier reusability	Degradable carriers such as modified straw need to be replaced frequently, while ceramic carriers can be reused for 3–5 years, with lower long-term costs.
Parameter control accuracy	Automatic control systems (such as AI regulating pH and temperature) can reduce energy consumption and bacterial waste and reduce operating costs by 10–15%.

7. Conclusions

In this study, the research progress of probiotic technology in the field of waste gas treatment and conversion was systematically reviewed by integrating three perspectives: basic theory, environmental application, and engineering facilities. The core conclusions are as follows:

The purification of waste gas by probiotics is essentially an enzyme-catalyzed process driven by microbial metabolism. Key functional enzymes (such as methane monooxygenase, toluene dioxygenase, ammonia monooxygenase, glutathione-dependent formaldehyde dehydrogenase, etc.) are the core engines of pollutant biotransformation, and their activities are regulated by temperature, pH, oxygen concentration, nutrient balance, and the characteristics of pollutants. Understanding and optimizing these variables is the basis for improving degradation efficiency. Different microbial strains show specific degradation of specific pollutants due to their unique genetic characteristics and metabolic pathways. Bacterium (such as Pseudomonas, Bacillus) is the main degradation of VOCs; fungi have advantages in the treatment of hydrophobic VOCs by virtue of their mycelium structure. Chemoautotrophic bacteria (such as Thiobacillus thiooxidans) are the key to the treatment of inorganic odor gases (H₂S, NH₃). The construction of a functional complementary composite flora is an effective strategy for the treatment of multi-component mixed waste gas.

Probiotic technology has shown good application potential in many fields such as industry (petrochemical industry, printing and spraying), agriculture (livestock and poultry breeding, composting), and urban management (garbage treatment, new house formaldehyde). It can effectively treat VOCs, H₂S, NH₃, and other pollutants and has the advantages of environmental friendliness and relatively low operating costs. In the process of engineering, the successful large-scale application of technology depends on the rational design of the bioreactor (biofilter, biotrickling filter, membrane bioreactor), the optimization of microbial immobilization technology (such as embedding method, new carrier), and intelligent process control (pH, temperature, nutrient addition). However, the initial investment cost, adaptability to complex exhaust gas components, and long-term operational stability are still current challenges.

In summary, probiotic waste gas treatment technology represents a promising ‘green conversion’ scheme. Future research should focus on strengthening the performance of the strain by means of synthetic biology, dealing with complex scenarios through multi-technology coupling processes, and guiding its sustainable large-scale application through comprehensive life cycle assessment, so as to provide strong technical support for the realization of the ‘double carbon’ goal and environmental pollution control.

8. Future Prospect

At present, there are two technical bottlenecks. Insufficient environmental adaptability and heavy metal inhibition: Hg²⁺ and Pb²⁺ in industrial waste gas bind to the enzyme activity center of the flora (such as sulfhydryl group), resulting in inactivation (when the concentration of Hg²⁺ reaches 10 mg/m³, the activity of desulfurization bacteria decreases by >70%). 2. The influence of extreme conditions: Low temperature (90%) plugs the pores of the carrier, and the treatment efficiency fluctuates by 30–50%. The cost control problem is that the production cost of microbial inoculants is high (about 2000–5000 yuan per ton of liquid microbial inoculants), accounting for 30–50% of the operation and maintenance cost of large-scale projects.

It is necessary to promote life cycle assessment (LCA) and technical and economic analysis (TEA), which are the key links of technology from laboratory to large-scale engineering application, and also the weaknesses of current research. Future work must include systematic LCA and TEA research: LCA can systematically quantify the environmental impact of ‘cradle to grave’ (including the whole process of microbial agent production, carrier manufacturing, reactor construction, operation energy consumption, waste treatment, etc.), including carbon emissions, water consumption, ecotoxicity, and other indicators, and compare them with traditional physical and chemical technologies to objectively evaluate environmental sustainability advantages. TEA needs to accurately calculate investment and operating costs on a larger scale (such as 100,000 m³/h) to reveal the scale effect. The potential of new low-cost carriers (such as agricultural waste modification), in situ regeneration technology of microbial agents, and energy and resource recovery (such as biomass and sulfur) to reduce unit treatment costs is evaluated to provide a clear economic basis for investors and policy makers.

Probiotic waste gas treatment technology takes efficient microbial mineralization as the core advantage and realizes multi-scenario application through basic theoretical innovation and engineering optimization. In the future, it is necessary to break through the bottleneck of environmental adaptability and cost by relying on synthetic biology (strengthening bacterial tolerance), technology coupling (photocatalysis/electrochemical assistance), and a circular economy model (pollutant → resource conversion). Interdisciplinary collaboration and policy support will promote this technology to become a key support for ‘reducing pollution and carbon’ and help achieve the goal of ‘double carbon’. Although probiotic waste gas treatment technology has made significant progress, it still faces bottlenecks in complex environmental adaptability and cost control. In the future, it is necessary to promote technological breakthroughs through multidisciplinary integration.