Research progress on ammonia suppression in anaerobic digestion process?

With the continuous deepening of theoretical research on anaerobic digestion, the research, development and application of anaerobic digestion processes have achieved rapid development. However, low treatment efficiency and poor operational stability are common problems in anaerobic digestion. , one of the main reasons is ammonia inhibition caused by ammonia accumulation. The article briefly describes the mechanism of ammonia inhibition during anaerobic digestion and the main influencing factors of ammonia inhibition, introduces the current research status of microbial changes during ammonia inhibition, summarizes methods to eliminate and alleviate ammonia inhibition, and proposes anaerobic digestion Key research directions for ammonia inhibition.

As a sustainable biological treatment technology that can obtain energy, anaerobic digestion has been widely used in practical engineering. In the anaerobic digestion process, ammonia suppression is considered an important factor affecting its overall performance. Although nitrogen is an essential element for microbial metabolism in anaerobic digestion systems, excessive ammonia nitrogen in anaerobic digestion systems often inhibits the normal life activities of microorganisms, especially methanogens.

It focuses on the latest mechanism research on anaerobic digestion ammonia inhibition at home and abroad, and elaborates on its main influencing factors and elimination measures, including microbial domestication, pH value adjustment, temperature control and C/N ratio adjustment. etc., to provide certain reference and reference for future research on the engineering application of anaerobic digestion technology.

As a sustainable biological treatment technology that can obtain energy, anaerobic digestion can generate biomass energy (mainly methane CH 4 with a content of 60 to 70) during the digestion process. It is a very Promising green energy that can mitigate climate change.

Anaerobic digestion has the characteristics of stably degrading organic pollutants and generating renewable energy during the digestion process. It has been widely used in environmental protection and resource utilization. During the anaerobic digestion process, nitrogen-containing substances in the fermentation materials, such as proteins, urea and nucleic acids, are eventually converted into ammonia nitrogen (including free ammonia NH3 and NH4) under the action of microorganisms.

Although ammonia nitrogen is a necessary nutrient for anaerobic microorganisms in the anaerobic digestion process, and a certain concentration of ammonia nitrogen can provide a good buffering effect for the digestion process, too high ammonia nitrogen concentration in the system will inhibit microorganisms. function, which is considered to be the main factor leading to the collapse of the anaerobic digestion reaction system. In recent years, a large number of studies have been carried out at home and abroad on ammonia suppression during anaerobic digestion. This article focuses on summarizing the formation mechanism, influencing factors and countermeasures of ammonia suppression during anaerobic digestion, in order to provide certain guarantees for the stable operation of the anaerobic digestion reaction system. reference.

1 Research on the mechanism of ammonia inhibition in anaerobic digestion

There are many studies on the formation mechanism of ammonia inhibition during anaerobic digestion, but these studies are not complete enough and there is no unified understanding. .

The anaerobic digestion process usually includes five steps: material dissolution, hydrolysis, acidification, acetification and methanogenesis. Therefore, the stable operation of the anaerobic digestion system mainly depends on hydrolytic fermentation bacteria, acidogenic bacteria and methanogenesis. The synergistic effect of bacteria and other microorganisms under normal physiological activities, among which methanogens have the worst tolerance to ammonia concentration in the system.

Ammonia suppression is usually manifested as a decrease in biogas production and the accumulation of volatile fatty acids VFA in a stably operating anaerobic digestion system. Many researchers have put forward conjectures on the mechanism of ammonia inhibition formation, such as: changes in intracellular pH of methanogens, an increase in energy required to maintain normal cell life activities, and inhibition of specific enzymatic reactions.

In anaerobic digestion aqueous solutions, NH 4 ions and free ammonia (NH3) are the two main forms of ammonia present. Free ammonia is considered to be the main factor causing system inhibition due to its good permeability.

Some scholars have speculated through pure strain culture experiments that the impact of ammonia inhibition on methanogens in the system is mainly reflected in the following two aspects:

1) NH 4 ions may directly inhibit 2) Hydrophobic free ammonia molecules may enter bacterial cells through passive diffusion, causing proton imbalance or intracellular potassium deficiency.

Free ammonia NH3 enters microbial cells through passive diffusion, combines with extracellular proton H and is converted into NH 4, thereby causing intracellular pH changes.

In order to maintain intracellular proton balance, cells consume energy for active transport through potassium pumps on the cell membrane, moving intracellular potassium ions out of the cell to maintain intracellular pH, thus increasing the demand for cell maintenance energy. And restrict some specific enzymatic reactions.

There are many studies on the ammonia inhibition threshold (see Table 1). Hejnfelt and Angelidaki found that ammonia may occur in the anaerobic digestion system when the total ammonia nitrogen concentration is 1 500 to 7 000 mg/L. inhibition phenomenon. The difference in ammonia inhibition thresholds of different anaerobic digestion systems is mainly affected by the characteristics of the digestion matrix and inoculum materials, digestion temperature, pH in the system and acclimation time.

Table 1 Ammonia inhibition thresholds in different anaerobic digestion systems

Currently, research on the ammonia inhibition mechanism in the anaerobic digestion process mainly focuses on the population structure and diversity of methanogens in the system. Changes in properties with increasing ammonia concentration. Some scholars have shown that during the anaerobic digestion process, hydrogenotrophic methanogens have stronger tolerance to ammonia concentration in the system than acetic acid-type methanogens.

Gao et al. designed a food waste anaerobic digestion reactor test and found that as the ammonia concentration in the system continued to increase, the relative activity of coenzyme F 420 (CoF 420) in the reactor in the final stage of the test It is 2. 4 times that of the initial stage, and the increase rate reaches 114.

Because the biochemical effect of CoF 420 is a low-potential electron transfer carrier, it specifically reduces bicarbonate to hydrogen, which is then used by hydrogenotrophic methanogens to synthesize methane, thus proving that as the reactor As the ammonia concentration increases, hydrogenotrophic methanogens gradually dominate, which promotes the activity of specific CoF 420.

Niu et al. used 16S rRNA molecular biology technology to study the changes in the methanogen community with different ammonia nitrogen concentrations in the system during the high-temperature anaerobic digestion of chicken manure by ammonia nitrogen.

The results show that the hydrogenotrophic Methanothermobacter has developed from 9.3 in the initial stage to 95 in the ammonia inhibition stage, which greatly exceeds the acetotrophic Methanosarcina. , playing a dominant role in the methanogenesis process of high ammonia nitrogen systems. Demirel and Scherer's study also reached the same conclusion.

2 Influencing factors and elimination measures of ammonia suppression in anaerobic digestion

Ammonia suppression often has a serious impact on the anaerobic digestion system, and even eventually leads to system collapse. Therefore, many studies have investigated the influencing factors of ammonia inhibition and measures to delay or eliminate it. It mainly includes microbial domestication, pH value adjustment, temperature control, C/N ratio adjustment, etc.

2. 1 Microbial domestication

High concentration of ammonia nitrogen in the system can directly inhibit microbial activity, which is often the primary reason for the failure of anaerobic digestion reactors. Studies have shown that anaerobic digestion reactors that treat wastewater with low ammonia nitrogen concentrations often have better adaptability to high ammonia nitrogen concentrations in the system.

By gradually increasing the ammonia nitrogen level in the system and domesticating the microorganisms in the anaerobic digestion system, the tolerance of the methanogenic microbial population to ammonia nitrogen can be improved, which is an effective and economical way.

Abouelenien et al. finally achieved a gas production of 31 mL/g (VS) under high ammonia nitrogen concentration by acclimating the microorganisms in the system for 254 days. The research results of Sung and Liu showed that although the overall activity of methanogens in the system decreased under high ammonia nitrogen acclimation, their tolerance to ammonia nitrogen and pH changes was enhanced.

Gao et al. gradually tamed ammonia tolerance in situ, and finally realized that the anaerobic digestion reactor can still operate normally and effectively when the total ammonia nitrogen in the system reaches 4 275 mg/L.

Although many studies have confirmed that domestication can enhance the adaptability of microorganisms in the system to different ammonia nitrogen concentrations, but whether it is due to the transformation of the metabolic pathways of the existing microorganisms in the system, or whether it is domestication that can adapt to different ammonia nitrogen concentrations. The new microbial population with high ammonia nitrogen concentration has not reached absolute awareness.

Gao et al.’s study showed that within the range of the total ammonia nitrogen concentration in the anaerobic digestion system from 2 341 mg/L (stage 1) to 4 293 mg/L (stage 5), Methanogens ( Methanobacteriaceae) phase

The abundance increased from 36.16 in stage 1 to 69.73 in stage 5, with a growth rate of 92.84. The relative abundances of Methanosaetaceae and Methanosarcinaceae dropped from 31.01 and 26.81 in stage 1 to 8.13 and 2.77 respectively. Under high ammonia nitrogen concentrations, hydrogenotrophic methanogens gradually dominate, and the acetate-utilizing methanogenic pathway becomes the rate-limiting step in the anaerobic digestion methanogenic process.

Research shows that when the ammonia concentration in the anaerobic digestion system exceeds 3 g/L, the acetic acid-utilizing methanogens in the system begin to be inhibited, and the activity of mutually trophic acetic acid-oxidizing bacteria (SAOB) gradually increases. It can degrade acetic acid into hydrogen and carbon dioxide, and then synthesize methane through hydrogenotrophic methanogens.

Karakashev et al.’s research also showed that as the ammonia concentration in the system increases, the activity of syntrophic acetic acid-oxidizing bacteria (SAOB) gradually exceeds that of acetic acid-utilizing methanogens, and they have an advantage in the process of consuming acetic acid.

2. 2 pH value adjustment

The pH value of the anaerobic digestion system is not only closely related to the normal life activities of microorganisms, but also closely related to the concentration of free ammonia in the system. It can be seen from formula (1) that as the pH in the system increases, more NH 4 in the system will be converted into free ammonia molecules (NH3).

The methanogenic bacteria in the system are most sensitive to ammonia concentration, and their activity is first affected, which then leads to the accumulation of VFA. When it accumulates to a certain extent, the pH in the system decreases, ultimately leading to a decrease in the gas production of the system. , or even crash. Effective control of pH in the system is critical to slowing down the effects of ammonia inhibition.

2. 3 Temperature control

Temperature is considered to be a significant factor affecting the ammonia inhibition threshold of the system. It is closely related to the growth rate of microorganisms and the concentration of free ammonia in the anaerobic digestion system (see formula (1)) Close contact. It is generally believed that as the temperature increases, the metabolic rate of microorganisms accelerates, which will also lead to a corresponding increase in the concentration of free ammonia in the system.

In addition, many scholars have found that for high-nitrogen-containing organic waste, high-temperature (50 to 60 ℃) anaerobic digestion is more susceptible to ammonia inhibition than medium-temperature (30 to 40 ℃) anaerobic digestion. Causing system instability.

Hejnfelt and Angelidaki conducted anaerobic digestion of slaughterhouse waste. The results showed that the total nitrogen concentration of the final system in the high-temperature anaerobic reactor (55 ℃) reached 7 000 mg/L (the free ammonia concentration reached 999 mg /L), while the medium-temperature anaerobic reactor (37 °C) still operates stably when the free ammonia concentration reaches 400 mg/L.

Gallert and Winter designed two sets of domestic waste anaerobic digestion reactors with reaction temperatures of 37 and 55°C respectively. When the two sets of reactors reached 50°C inhibition, the corresponding free ammonia concentrations were respectively 220,690 mg/L. However, Masse et al. studied low-temperature (10°C) anaerobic digestion of pig manure and found that low-temperature anaerobic digestion was more tolerant to high ammonia nitrogen concentrations in the system than higher-temperature and medium-temperature anaerobic digestion. It is speculated that the possible reason is that under low-temperature conditions, the system The ratio of free ammonia concentration to total ammonia nitrogen is low.

2. 4 C/N value adjustment

For anaerobic digestion systems, if the C/N ratio of fermentation raw materials is too high, it will cause insufficient nitrogen sources in the system, thereby making the carbon sources unable to On the contrary, if the C/N ratio is too low, it will easily lead to the accumulation of ammonia in the system and inhibit the anaerobic digestion microorganisms. Selecting reaction substrates with different characteristics for combined anaerobic digestion is a cost-effective and easy-to-operate measure to reduce the risk of ammonia inhibition in the system.

Adjusting the C/N ratio of fermentation raw materials can effectively control the total ammonia nitrogen concentration level in the system. However, this process is relatively slow, and it can only play a corresponding role when the methanogenic microorganisms in the system are in an active state. If the methanogenic microorganisms in the anaerobic digestion system have been severely suppressed and the C/N ratio of the fermentation feedstock is adjusted at this time, the system will be difficult to recover.

De Vries et al.’s study showed that compared with the separate anaerobic digestion of corn green feed, its combined anaerobic digestion with beetroot and wheat yeast increased the production of bioenergy glycerol by 568%. Hejnfelt and Angelidaki found that adding 5 pork by-products to pig manure for combined mesophilic anaerobic digestion increased methane production by 40% compared with anaerobic digestion of pig manure alone.

Molinuevo-Salces et al. put a certain amount of vegetable processing waste into pig manure for anaerobic digestion. The results showed that the VS degradation rate in the system was improved.

Shanmugam and Horan conducted anaerobic digestion of tannery wastewater. The experimental design was to feed wastewater with a C/N ratio of 3. 2 to 30. The results showed that the anaerobic digestion system had a system pH of 6. 5 , when the C/N ratio is 15, the cumulative gas production reaches the maximum, and the system free ammonia concentration (817 mg/L) is low at this time. Compared with the original wastewater without adjusting the C/N ratio, the release of free ammonia is reduced by 80%.

2. 5 Other influencing factors and measures

Many scholars have found that initial biomass, feeding frequency, addition of membrane contactor components, microbial immobilization, trace element addition and dilution, etc. All have an impact on ammonia suppression in the anaerobic digestion system. By taking certain measures, the impact of ammonia suppression can be mitigated.

Markou et al. studied the anaerobic digestion of Spirulina and designed the initial biomass to be 125, 250 and 500 mg/L respectively. The results showed that the lower the initial biomass of the anaerobic digestion system, the more susceptible it is to ammonia inhibition. . It is speculated that the higher the biomass, the faster the rate of assimilation of ammonia nitrogen in the system.

Tice et al. conducted a study on microbial fuel cells, and the results showed that under the condition that the substrate was 2 g/L sodium acetate, the high feeding frequency (feeding once every 2 days) group had the highest total energy consumption in the system. When the ammonia nitrogen concentration reaches 4 000 mg/L, a high power density (1. 1 ~ 1. 9 W/m 2 ) can still be obtained.

The low feeding frequency (feeding once every 6 days) group achieved a lower power density when the total ammonia nitrogen concentration of the system reached 2 500 mg/L.

By increasing the feeding frequency, the tolerance of electrogenic microorganisms to ammonia inhibition can be enhanced. It was also pointed out that the power density curve is more effective in predicting ammonia inhibition in microbial fuel cells than continuous current monitoring.

Lauterbock et al. added a hollow fiber membrane contactor component to the anaerobic digestion system of slaughterhouse waste to continuously remove ammonia nitrogen in the system. The results showed that its free ammonia concentration was 1 compared to that of the control reactor. 000 ~ 1 200 mg/L, reduced by approximately 70.

While the system is more stable, it also achieves higher methane production and treatment efficiency. Adding inert load materials (clay, activated carbon, zeolite, etc.) to the anaerobic digestion reactor for microbial immobilization can effectively slow down ammonia inhibition and make the digestion process more stable.

Sasaki et al. added carbon fiber textiles (CFT) to a high-temperature anaerobic digestion reactor to immobilize methanogens. The results showed that when the total ammonia nitrogen in the system reached 3 000 mg/L, the reactor adding CFT Still running stable. And through 16S rRNA gene analysis, a large number of methanogens are attached to CFT. The control reactor without CFT had a lower methane yield when the total ammonia nitrogen in the system reached 1 500 mg/L.

Banks et al. added trace elements selenium and cobalt to the anaerobic digestion system and optimized their dosage. They found that under optimal conditions, the total ammonia nitrogen concentration of the system could be controlled to 5 000 mg/L and Stable operation, while the total ammonia nitrogen concentration in the control group reached 6 100 mg/L.

Kayhanian points out that dilution can directly reduce the ammonia load of the anaerobic digestion system, thereby moderating the effects of ammonia suppression. However, this method is not economical because the reactor volume will be wasted after dilution and the treatment efficiency will be reduced.

In addition, some physical and chemical methods, such as chemical precipitation, stripping, etc., can be used to remove ammonia nitrogen in the anaerobic digestion reactor and slow down ammonia suppression. Research has found that the two-phase anaerobic digestion system has better tolerance to ammonia nitrogen in the system than the traditional single-phase anaerobic digestion system, and can also improve the treatment efficiency of the system.

Therefore, the use of a two-phase anaerobic digestion system to treat high-nitrogen waste can be considered to reduce the impact of excessive ammonia nitrogen on reaction efficiency during the digestion process.

3 Conclusion

At present, a lot of research has been carried out at home and abroad on the suppression of ammonia in anaerobic digestion, but the existing research is more focused on the effect of ammonia suppression on methanogenic microbial communities. changing patterns, and measures to alleviate ammonia inhibition are still at the laboratory level.

Although the anaerobic digestion system where ammonia suppression occurs can still be restored by taking measures, the process requires a certain amount of time to recover, seriously affecting the continuity and efficiency of the digestion process, especially for actual projects. Unavoidable.

Therefore, future research directions should focus more on the interaction between methanogens and other anaerobic microorganisms during the ammonia suppression process and the prediction and simulation of ammonia suppression in the system. In the early stages of ammonia suppression, The system takes appropriate measures to avoid the adverse effects of ammonia suppression on the system.

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