In the nitrogen cycle of wastewater treatment systems and natural aquatic environments, the conversion of organic nitrogen to ammonia nitrogen is one of the core processes. This process, known as ammonification, serves as a fundamental step in nitrogen transformation, directly influencing the efficiency of subsequent denitrification and other nitrogen removal reactions. It plays a critical role in controlling nitrogen pollution in water bodies. Organic nitrogen is widely present in domestic sewage, industrial wastewater, and natural water bodies, with its primary sources including nitrogen-containing organic compounds such as proteins, amino acids, urea, nucleic acids, and humic substances. These substances must be decomposed through microbial metabolic processes, ultimately converting into ammonia nitrogen (ce{NH3-N} or ce{NH^{+}_{4}-N}), which then participates in subsequent nitrogen migration and transformation.
1. The Core Process of Organic Nitrogen Conversion to Ammonia Nitrogen—Ammonification
Ammonification refers to the biochemical reaction in which nitrogen-containing groups in organic nitrogen compounds are gradually decomposed under the catalysis of microorganisms, ultimately releasing ammonia nitrogen. Depending on the types of microorganisms involved and the reaction conditions, ammonification can be classified into aerobic and anaerobic ammonification. Although their reaction pathways and dominant microorganisms differ, the final products primarily consist of ammonia nitrogen.
Ammonification under aerobic conditions
Aerobic ammonification is the process by which aerobic microorganisms oxidize and decompose organic nitrogen compounds in an oxygen-rich environment. It features fast reaction rates and high conversion efficiency, serving as the primary form of organic nitrogen transformation in the aerobic stage of wastewater treatment (such as the aeration tank in the activated sludge process).
Transformation pathways of proteinaceous organic nitrogen
Protein is one of the most common organic nitrogen pollutants in water bodies, and its conversion to ammonia nitrogen involves two key reactions. The first step is protein hydrolysis, catalyzed by proteases secreted by aerobic microorganisms, which breaks down large protein molecules into smaller polypeptides and amino acids. Proteases, including trypsin and pepsin, exhibit specificity in cleaving peptide bonds within protein molecules. The second step is amino acid deamination, the core process of ammonification, where amino acids, under the action of deaminase, lose their amino group (NH₂) through oxidative deamination, reductive deamination, or hydrolytic deamination, converting it into ammonia nitrogen.
Taking oxidative deamination as an example, its reaction can be represented as:
ce{R-CH(NH2)-COOH + O2 -> R-CO-COOH + NH3}
The ammonia (ce{NH3}) generated by the reaction combines with hydrogen ions in water to form ammonium ions (ce{NH^{+}_{4}}). The ratio between the two depends on the pH of the water. When the pH is alkaline, ammonia (ce{NH3}) predominates; when the pH is acidic, ammonium ions (ce{NH^{+}_{4}}) dominate.
2. Transformation Pathways of Organic Nitrogen in Urea Compounds
Urea is a significant component of organic nitrogen in domestic sewage. Its ammonification process is catalyzed by urease, occurring under mild conditions and proceeding rapidly in an aerobic environment. Urease breaks the amide bond in the urea molecule, directly decomposing it into ammonium nitrogen and carbon dioxide. The reaction equation is as follows:
ce{CO(NH2)2 + H2O -> 2NH3 + CO2}
This reaction does not require an amino acid intermediate stage, exhibits extremely high conversion efficiency, and serves as one of the primary sources of ammonia nitrogen in domestic wastewater.
(2) Ammonification under anaerobic conditions
Anaerobic ammonification is the process by which anaerobic or facultative anaerobic microorganisms ferment and decompose organic nitrogen compounds in an oxygen-free environment, commonly occurring in the anaerobic stages of wastewater treatment (such as anaerobic digesters), sediments, and hypoxic water bodies. Compared to aerobic ammonification, anaerobic ammonification proceeds at a slower rate and is accompanied by the production of gases like methane and hydrogen sulfide.
The decomposition of organic nitrogen by anaerobic microorganisms also begins with the hydrolysis of macromolecular organic compounds, such as proteins, which are broken down into amino acids by anaerobic proteases. Subsequently, the amino acids release ammonia nitrogen through reductive deamination or fermentative deamination. Taking reductive deamination as an example, the reaction equation is:
ce{R-CH(NH2)-COOH + 2H -> R-CH2-COOH + NH3}
Additionally, in anaerobic environments, complex organic nitrogen compounds such as nucleic acids and humus can also be gradually decomposed by microorganisms, releasing ammonia nitrogen. However, the conversion process is more complex and involves the synergistic action of multiple enzymes.
II. Major Microbial Groups Involved in Ammonification
The essence of ammonification is the metabolic process of microorganisms, involving a diverse range of microbial species, including bacteria, fungi, actinomycetes, and more. Different microorganisms exhibit variations in their ability to decompose organic nitrogen and their adaptability to environmental conditions.
```(1) Bacterial Groups```
Bacteria are the dominant microorganisms in ammonification, primarily categorized into aerobic and anaerobic types. Aerobic ammonifying bacteria include genera such as Bacillus, Pseudomonas, and Proteus, which rapidly proliferate under aerobic conditions and exhibit high protease and deaminase activity, enabling efficient protein and amino acid decomposition. Anaerobic ammonifying bacteria are represented by genera like Clostridium and methanogens. Clostridium can decompose proteins under anaerobic conditions to produce ammonia nitrogen and organic acids, while methanogens utilize simple organic nitrogen compounds for further fermentation and participate in ammonification reactions.
(2) Fungal and actinomycete taxa
Fungi and actinomycetes also play an important role in organic nitrogen conversion, especially in the treatment of wastewater containing complex organic nitrogen, such as printing and dyeing wastewater and pharmaceutical wastewater. Fungi such as Aspergillus and Penicillium can secrete various extracellular enzymes to decompose bound organic nitrogen in recalcitrant organic compounds such as cellulose and lignin; Streptomyces, a genus of actinomycetes, has a strong ability to decompose humic organic nitrogen. The enzymes produced by their metabolism can break the stable structure of humic substances and release ammonia nitrogen.
3, Key factors affecting the conversion of organic nitrogen to ammonia nitrogen
The efficiency of ammonification is influenced by various environmental factors and substrate characteristics. In sewage treatment systems, regulating these factors can effectively improve the conversion rate of organic nitrogen to ammonia nitrogen, creating favorable conditions for subsequent nitrification and denitrification.
(1) Temperature
Temperature is the core factor affecting microbial enzyme activity, directly determining the rate of ammonification reaction. The suitable growth temperature for ammonifying microorganisms is 20 ℃ -35 ℃. Within this temperature range, enzyme activity is high, and the ammonification reaction rate accelerates with increasing temperature; When the temperature is below 10 ℃, the metabolic rate of microorganisms significantly decreases, enzyme activity is inhibited, and the efficiency of ammonification decreases significantly; When the temperature exceeds 40 ℃, enzyme proteins in microbial cells will undergo denaturation, leading to the stagnation of ammonification reaction. In actual sewage treatment, it is often necessary to extend the hydraulic retention time or increase the sludge concentration under low temperature conditions in winter to compensate for the decrease in ammonification efficiency.
(2) PH value
The pH value indirectly affects ammonification by influencing the growth environment and enzyme activity of microorganisms. The suitable pH range for aerobic ammonifying microorganisms is 6.5-8.0, during which the protease and deaminase activities of the microorganisms are highest; When the pH value is below 5.5 or above 9.0, the spatial structure of the enzyme will be disrupted, microbial growth will be inhibited, and ammonification reaction will be hindered. Anaerobic ammonifying microorganisms have a relatively wide adaptability range to pH values, with a suitable pH range of 6.0-7.5. A slightly acidic environment is more conducive to the fermentation metabolism of anaerobic ammonifying bacteria. In addition, pH value can also affect the form of ammonia nitrogen, which in turn affects the substrate supply for subsequent nitrification reactions.
(3) Dissolved oxygen (DO)
Dissolved oxygen is a key condition for distinguishing aerobic ammonification from anaerobic ammonification. In an aerobic environment, the dissolved oxygen concentration needs to be maintained at 2mg/L-4mg/L to meet the respiratory needs of aerobic ammonification microorganisms. At this time, aerobic ammonification dominates and the conversion efficiency is high; When the dissolved oxygen concentration is below 0.5mg/L, the activity of aerobic microorganisms is inhibited, and anaerobic ammonifying microorganisms become the dominant microbial group, resulting in a slower ammonification reaction rate. In processes such as A ²/O and oxidation ditch in sewage treatment, the synergistic process of organic nitrogen ammonification, nitrification, and denitrification can be achieved by controlling the dissolved oxygen concentration in different areas.
(4) Types and concentrations of organic nitrogen substrates
The type and concentration of organic nitrogen matrix directly affect the rate and degree of ammonification. Small molecule organic nitrogen compounds (such as amino acids and urea) can be directly absorbed and utilized by microorganisms, with a fast rate of ammonification conversion; Large molecule organic nitrogen compounds (such as proteins and nucleic acids) need to undergo hydrolysis reactions to decompose into small molecule substances, with a longer conversion period. In addition, when the concentration of organic nitrogen is too high, it can cause an imbalance in the osmotic pressure of microbial cells, inhibiting microbial growth; When the concentration is too low, it cannot provide sufficient nutrition for microorganisms, and the efficiency of ammonification reaction is low. In practical engineering, for high concentration organic nitrogen wastewater, pretreatment processes (such as hydrolysis acidification) are often used to decompose large molecular organic nitrogen into small molecular substances, thereby improving the efficiency of subsequent ammonification treatment.
(5) Microbial community structure
The diversity and abundance of microbial communities are the core biological factors affecting ammonification. When the variety of ammonifying microorganisms in the system is abundant and the number of dominant bacterial groups is sufficient, the efficiency of organic nitrogen decomposition and transformation is higher; On the contrary, if the microbial community structure is single or there are inhibitory substances (such as heavy metals, toxic organic compounds) that cause the death of dominant microbial communities, ammonification will be severely affected. During the start-up phase of the sewage treatment system, efficient ammonifying microbial communities can be quickly established by adding ammonifying agents or inoculating mature sludge, shortening the system commissioning cycle.
4, The Environmental and Engineering Significance of Organic Nitrogen Conversion to Ammonia Nitrogen
The conversion of organic nitrogen to ammonia nitrogen is a key link in the nitrogen cycle and has significant importance in both natural environments and wastewater treatment projects.
In natural water bodies, ammonia nitrogen produced by ammonification can provide nitrogen sources for phytoplankton, algae, etc., promoting the material cycle of aquatic ecosystems; However, excessive ammonia nitrogen can lead to eutrophication of water bodies, causing environmental problems such as algal blooms and red tides. In sewage treatment engineering, ammonification is a prerequisite step for biological denitrification. Only by efficiently converting organic nitrogen into ammonia nitrogen can sufficient substrates be provided for subsequent nitrification reactions (ammonia nitrogen converted to nitrate nitrogen) and denitrification reactions (nitrate nitrogen converted to nitrogen), achieving complete removal of nitrogen. In addition, in the anaerobic digestion process, the ammonia nitrogen produced by ammonification can neutralize the organic acids produced during the digestion process, maintain the stability of the system pH value, and ensure the smooth progress of anaerobic digestion.
V. Conclusion
The conversion of organic nitrogen to ammonia nitrogen is a complex microbial mediated process, influenced by multiple factors such as temperature, pH value, dissolved oxygen, and substrate properties. A deep understanding of the mechanism and influencing factors of ammonification has important theoretical and practical significance for optimizing sewage treatment processes and improving biological nitrogen removal efficiency. With the continuous improvement of water environment governance requirements, it is necessary to further study the metabolic regulation mechanism of ammonifying microorganisms, develop efficient ammonifying bacterial agents and process optimization strategies in the future, and provide stronger technical support for solving the problem of nitrogen pollution in water bodies.
