In the activated sludge system of wastewater treatment, microorganisms serve as the core "executors" for pollutant purification. Depending on their energy acquisition methods and carbon source utilization forms, these microorganisms can be broadly classified into two categories: autotrophic and heterotrophic. These two types exhibit fundamental differences in metabolic mechanisms, functional roles, and environmental adaptability, collectively forming the ecological structure of activated sludge. However, their pathways of action and core values differ significantly. A deep understanding of these differences is crucial for optimizing wastewater treatment processes and enhancing purification efficiency.
1. Core Differences: The Fundamental Distinction Between Energy Sources and Carbon Source Utilization
The source of energy and the source of carbon are fundamental indicators distinguishing autotrophic and heterotrophic microorganisms. These two key factors directly determine their metabolic direction and survival dependence, as well as the underlying logic for their distinct roles in the activated sludge system.
(1) Autotrophic microorganisms: "self-sufficient" inorganic matter transformers
The core characteristic of autotrophic microorganisms is their ability to independently synthesize organic compounds from inorganic carbon, using inorganic substances as "energy fuel" without relying on external organic matter, functioning as the "producers" in ecosystems.
In terms of energy acquisition, these microorganisms obtain energy by oxidizing inorganic substances. For instance, nitrifying bacteria derive energy by oxidizing ammonia nitrogen (NH₄⁺→NO₂⁻→NO₃⁻), while nitrite-oxidizing bacteria oxidize nitrite (NO₂⁻→NO₃⁻). Sulfur-oxidizing bacteria, on the other hand, generate energy by oxidizing sulfides (e.g., H₂S→S→SO₄²⁻). Regarding carbon source utilization, they rely solely on carbon dioxide (CO₂) or carbonates (such as HCO₃⁻) as their sole carbon source, converting inorganic carbon into organic carbon through photosynthesis or chemosynthesis to construct their cells and carry out metabolic activities. This "self-sufficient" characteristic enables them to survive without depending on organic pollutants in wastewater.
(2) Heterotrophic Microorganisms: "Externally Dependent" Organic Matter Degraders
Heterotrophic microorganisms are the exact opposite of autotrophic ones. They cannot utilize inorganic substances for energy or autonomously synthesize organic carbon, relying instead on pre-existing organic matter from the external environment as both "energy source" and "carbon source." This makes them functionally equivalent to "consumers" and "decomposers" in an ecosystem.
In terms of energy acquisition, these microorganisms obtain energy by decomposing organic pollutants in wastewater (such as carbohydrates, proteins, fats, etc., quantified by COD, i.e., chemical oxygen demand). For instance, aerobic heterotrophic bacteria break down glucose into CO₂ and H₂O while releasing energy for their own metabolism. Regarding carbon source utilization, they directly absorb organic carbon from wastewater (such as COD components and small organic molecules) without the need for autonomous synthesis. Their metabolic activities entirely depend on the concentration and types of organic pollutants in the wastewater.
II. Functional Roles: Different Roles in the Activated Sludge Purification System
Based on differences in energy and carbon source utilization, autotrophic and heterotrophic microorganisms in activated sludge systems perform distinctly different purification functions. The former focuses on inorganic matter transformation, while the latter focuses on organic matter degradation, working synergistically to ensure effective wastewater purification.
(1) Autotrophic microorganisms: Focused on "nitrogen and sulfur removal," treating inorganic pollutants
Autotrophic microorganisms play a central role in activated sludge by facilitating the transformation and removal of inorganic substances, with nitrosomonas (including Nitrosomonas and Nitrobacter) being the most representative. These bacteria are key players in wastewater nitrogen removal processes. Under aerobic conditions, Nitrosomonas first oxidizes ammonia nitrogen (NH₄⁺) in wastewater into nitrite (NO₂⁻), which is then further oxidized into nitrate (NO₃⁻) by Nitrobacter. This process, known as the "nitrification reaction," is the core step in biological nitrogen removal. Without autotrophic nitrifying bacteria, the ammonia nitrogen in wastewater cannot be converted into nitrate, which can subsequently be removed through denitrification, ultimately leading to excessive ammonia nitrogen levels in the effluent.
In addition, a few autotrophic sulfur-oxidizing bacteria can oxidize sulfides in wastewater, converting them into harmless sulfates and preventing the toxic inhibition of sulfides on microorganisms, thereby ensuring the stable operation of the activated sludge system. However, it should be noted that autotrophic microorganisms have an extremely slow metabolic rate (with a typical generation cycle of 10-30 hours) and are sensitive to environmental conditions (such as temperature, dissolved oxygen, and pH). Consequently, their proportion in the activated sludge system is usually low (approximately 5%-10%).
(2) Heterotrophic microorganisms: Core "COD degradation," constructing sludge flocs
Heterotrophic microorganisms are the "main force" of activated sludge, accounting for over 90% of its population. Their primary functions are concentrated in two major aspects: organic matter degradation and sludge floc formation, which directly determine the COD removal efficiency in wastewater and the settling performance of activated sludge.
In the degradation of organic matter, aerobic heterotrophic bacteria break down macromolecular organic compounds (such as starch, lipids, and proteins) in wastewater into smaller organic molecules through aerobic respiration. These smaller molecules are further decomposed into inorganic products like CO₂ and H₂O, thereby reducing the COD value of the wastewater. This is the core objective of treating domestic sewage and industrial organic wastewater. For instance, in urban wastewater treatment plants, heterotrophic bacteria can reduce the influent COD from 300-500 mg/L to below 50 mg/L, meeting discharge standards.
In the formation of sludge flocs, certain heterotrophic microorganisms (such as actinomycetes and fungi) secrete viscous substances like polysaccharides and proteins, which aggregate dispersed microbial cells into structurally stable flocs (i.e., activated sludge flocs). These flocs not only encapsulate pollutants and enhance degradation efficiency but also settle rapidly in sedimentation tanks, achieving sludge-water separation and preventing microbial loss with the effluent. If the activity of heterotrophic bacteria is insufficient or their floc-forming capability is weak, it can lead to excessive suspended solids (SS) in the effluent, and in severe cases, cause "sludge bulking," destabilizing the system.
3、 Environmental adaptability: Different requirements for process conditions
The metabolic characteristics of autotrophic and heterotrophic microorganisms differ, resulting in different requirements for the environmental conditions of the activated sludge system, such as dissolved oxygen, temperature, and nutrient ratio. Optimizing these conditions is the key to ensuring the collaborative work of the two types of microorganisms.
（1） Autotrophic microorganisms: highly sensitive to environmental conditions
The metabolic activity of autotrophic microorganisms (especially nitrifying bacteria) requires strict environmental conditions, and even small parameter fluctuations can affect their activity:
-Dissolved oxygen (DO): Adequate dissolved oxygen is required for nitrification reaction, and DO needs to be maintained at 2mg/L. If DO is below 1mg/L, the activity of nitrifying bacteria will be significantly inhibited, and the efficiency of ammonia nitrogen oxidation will sharply decrease;
-Temperature: The optimal temperature is 20-30 ℃. When the temperature is below 10 ℃, the metabolic rate of nitrifying bacteria will decrease by more than 50%. In winter, sewage treatment plants often encounter the problem of insufficient ammonia nitrogen removal rate;
-PH value: The suitable range is 7.5-8.5. If the pH is below 6.5 or above 9.0, nitrifying bacteria will stop metabolism due to enzyme activity inhibition;
-Nutrient ratio: does not require a large amount of organic carbon, but is sensitive to organic carbon - if the COD in the sewage is too high, heterotrophic bacteria will compete with autotrophic bacteria for dissolved oxygen and space, inhibiting the growth of nitrifying bacteria.
（2） Heterotrophic microorganisms: highly tolerant to environmental conditions
Compared to autotrophic microorganisms, heterotrophic microorganisms have stronger environmental adaptability and a wider tolerance range for process parameters:
-Dissolved oxygen (DO): Aerobic heterotrophic bacteria require DO to be maintained at 1-2mg/L to meet their metabolic needs, while some facultative heterotrophic bacteria (such as denitrifying bacteria) can still degrade organic matter through anaerobic respiration under anaerobic conditions;
-Temperature: The optimal temperature is 15-35 ℃, but it can still maintain a certain level of activity within the range of 5-40 ℃, and its tolerance to low temperatures is much better than that of autotrophic bacteria;
-PH value: The suitable range is 6.0-9.0, and some heterotrophic bacteria (such as fungi) can still survive under acidic conditions at pH 5.0 or alkaline conditions at pH 10.0;
-Nutrient ratio: Adequate organic carbon is required and sensitive to carbon to nitrogen ratio (C/N) - usually requiring a C/N ratio of 5-10:1. If the carbon source is insufficient, heterotrophic bacteria will experience a decrease in activity and COD removal rate due to "starvation".
4、 Collaboration and Competition: Microbial Relationships in Activated Sludge Systems
In the activated sludge system, autotrophic and heterotrophic microorganisms do not exist independently, but have a dual relationship of "synergy" and "competition", and the balance between the two directly affects the effectiveness of sewage treatment.
（1） Collaborative relationship: complementary functions, jointly completing purification
The synergy between the two is mainly reflected in the "denitrification process": autotrophic nitrifying bacteria convert ammonia nitrogen into nitrate (nitrification process), while heterotrophic denitrifying bacteria, under anaerobic conditions, use organic carbon in wastewater as an electron donor to reduce nitrate to nitrogen (N ₂) and release it into the air (denitrification process) - without autotrophic bacteria, denitrifying bacteria have no "substrate" to use; If heterotrophic bacteria are lacking, the nitrate produced by nitrifying bacteria cannot be removed, and ultimately the total nitrogen cannot meet the standard. In addition, heterotrophic bacteria can reduce the organic load in wastewater after degrading COD, creating a suitable living environment for autotrophic bacteria sensitive to organic carbon and indirectly promoting their activity.
（2） Competitive relationship: resource competition, affecting system balance
The competition between the two mainly focuses on "dissolved oxygen" and "living space": when the COD concentration in sewage is too high, heterotrophic bacteria will rapidly reproduce due to "sufficient food", consume a large amount of dissolved oxygen, and the activity of autotrophic bacteria will be inhibited due to "hypoxia", resulting in the phenomenon of "good COD removal effect but poor ammonia nitrogen removal effect"; On the contrary, if the COD concentration in wastewater is too low (such as industrial wastewater), the activity of heterotrophic bacteria is insufficient, and stable sludge flocs cannot be formed. Autotrophic bacteria will also be lost due to "carrier deficiency", affecting nitrification efficiency. Therefore, in practical processes, it is necessary to balance the competitive relationship between the two by adjusting parameters such as inlet water load and reflux ratio. For example, when treating high COD wastewater, "segmented inlet water" can be used to reduce local organic load and ensure the dissolved oxygen demand of nitrifying bacteria.
5、 Summary: Core Differences and Technological Significance between Two Types of Microorganisms
The difference between autotrophic and heterotrophic microorganisms in activated sludge is essentially the difference in "energy sources and carbon source utilization methods", which extends to a series of differences in functional positioning, environmental adaptability, and microbial relationships between the two (as shown in Table 1).
Understanding these differences has important guiding significance for optimizing sewage treatment processes: for example, when treating high ammonia nitrogen and low COD sewage (such as aquaculture wastewater), it is necessary to focus on ensuring the survival conditions of autotrophic bacteria (increasing DO, controlling temperature), and appropriately adding carbon sources to meet the denitrification needs of heterotrophic bacteria; When treating high COD and low ammonia nitrogen wastewater (such as food wastewater), it is necessary to control the organic load, avoid excessive growth of heterotrophic bacteria and inhibit autotrophic bacteria, and ensure that COD and ammonia nitrogen meet the standards simultaneously. In short, the stable operation of an activated sludge system is essentially a "dynamic balance" between autotrophic and heterotrophic microorganisms. Only by accurately matching the needs of both can the maximum efficiency of sewage treatment be achieved.
