厌氧消化作为一种高效的废物处理方法，因其能耗低、占地面积小、处理效率高等优点而被广泛关注。然而，厌氧菌对毒物敏感，废水和废渣中的抑制物质往往是导致厌氧反应不稳定和失败的主要原因。本文将重点探讨厌氧反应中无机物质的毒性与抑制，以期为厌氧工艺的优化提供理论依据。
一、无机抑制性物质的分类
在厌氧生物反应器中，无机抑制性物质是一类能够对厌氧微生物生长代谢及生物反应过程产生不良影响的重要化学物质。这类物质主要包括氧气、氨氮、硫化物及硫酸盐、无机盐类（如钠离子、钙离子、镁离子）、重金属等。
氧气是厌氧消化过程中的首要抑制因素，即使在微小的浓度下也会对厌氧微生物的活性产生显著的抑制作用。这是因为厌氧微生物在缺氧条件下才能有效进行产甲烷反应，而氧气的存在会干扰这一过程，损害产甲烷菌的生理机能，从而降低整体厌氧消化速率和效率。
氨氮是另一个关键的抑制因子，当废水中氨氮浓度过高时，它可以通过多种途径抑制厌氧微生物的活性。一方面，高浓度的氨氮可以与微生物体内的蛋白质结合，形成不易分解的化合物，导致微生物营养不足；另一方面，氨氮还会影响微生物的能量代谢过程，降低其生物活性。
硫化物和硫酸盐也是重要的无机抑制性物质。硫化物主要来源于硫酸盐还原菌在厌氧环境下的还原反应产物，过量的硫化物会与产甲烷菌竞争氢气和电子供体，从而对产甲烷过程产生抑制作用。硫酸盐则可能在某些条件下形成硫酸根离子，进一步影响厌氧消化过程中的pH值和氧化还原电位，不利于厌氧微生物的正常生长与代谢。
此外，高浓度的无机盐类如钠离子、钙离子、镁离子等也会对厌氧微生物产生抑制作用。这些离子的存在可能会改变反应器内的渗透压平衡，导致微生物细胞内外的水分失衡，进而影响微生物的生理机能。
重金属元素同样对厌氧微生物具有显著的抑制作用。例如，铜、锌、镉、铅等重金属离子在废水中超标时，能够与微生物体内的酶蛋白结合，破坏其结构功能，或者通过干扰电子传递链等途径阻碍微生物的正常能量代谢过程。
二、无机物质的毒性研究
氨氮
氨氮是废水中常见的无机氮化合物，其主要存在形式为铵离子（NH⁺）和游离氨（NH₃，FA）。其中，游离氨具有良好的膜渗透性，能够通过细胞膜进入微生物细胞内，是抑制作用产生的主要原因。研究表明，氨氮对厌氧微生物的抑制作用主要表现在于微生物的能量代谢过程。在能量代谢过程中，氨氮能够与ATP（腺苷三磷酸）结合，形成氨酰磷酸（AMP），从而导致ATP含量下降。当氨氮浓度在200mg/L以下时，其对厌氧降解过程具有促进作用，因为氮也是厌氧微生物所需的重要营养物。然而，当氨氮浓度升高时，会显著抑制产甲烷菌（MPB）的活性，导致甲烷产量下降。这种抑制作用可能会对整个厌氧生物处理过程产生负面影响，影响废水的处理效果。
亚硝氮和硝氮
亚硝氮（NO₂⁻-N）和硝氮（NO₃⁻-N）是另一种常见的无机氮化合物。这两种物质在厌氧微生物处理废水的过程中，表现出显著的抑制作用。亚硝氮对厌氧颗粒污泥的比产甲烷活性（SMA）具有明显的抑制作用，且这种抑制作用呈现出浓度依赖性，即亚硝氮浓度越高，SMA受到的抑制程度越大。研究表明，亚硝氮的半抑制浓度（EC₅₀）约为12mgN·L⁻¹，这意味着当亚硝氮浓度超过这个阈值时，其对SMA的抑制作用将变得显著。类似的，硝氮对SMA的抑制作用与亚硝氮相似，其EC₅₀约为30mgN·L⁻¹。高浓度的亚硝氮和硝氮不仅影响厌氧微生物的能量代谢过程，还破坏微生物细胞膜结构。这包括改变细胞膜的通透性，阻碍物质运输和能量转换等关键生理功能。
重金属
重金属离子对厌氧微生物的抑制作用也非常明显。例如，可溶性的低浓度铜盐、锌盐和镍盐等重金属盐类，在水中具有相当大的毒性。这些重金属离子能够与厌氧微生物的酶系统结合，干扰其正常的生理代谢过程，从而对微生物产生抑制作用。然而，厌氧微生物对重金属离子有一定的适应性。在废水的厌氧处理过程中，产生的S²⁻和CO₃²⁻等阴离子会与金属离子发生沉淀反应，形成难溶性的硫化物和碳酸盐沉淀物。这些沉淀物会附着在微生物细胞表面或聚集在细胞周围的水环境中，降低金属离子的浓度。此外，硫酸盐还原所生成的硫化物也可以有效地降低重金属离子的毒性。硫化物与重金属离子反应生成稳定的硫化物沉淀物，进一步减少了水中重金属离子的浓度。这些因素共同作用，使得在废水的厌氧处理过程中，即使存在重金属离子污染的情况，厌氧微生物也能在一定程度上保持其生理活性和处理效能。
无机盐类
无机盐类的抑制性只在浓度非常高时才会显现。例如，高浓度的Na⁺对未经驯化的厌氧菌有抑制作用。这是因为高浓度的无机盐离子会导致细胞内外的离子平衡失调，影响细胞的正常生理功能。然而，经过驯化的厌氧菌能够适应这种高浓度的离子环境。当Na⁺浓度高于约5000mg/L时，即具有明显的阳离子毒性。然而，经过驯化后，当废水中Na⁺浓度高达15000mg/L时，厌氧反应器去除污染物的速率仍可相当于低钠对照系统的50%。这表明驯化后的厌氧菌具有一定的耐受性。
硫化物及硫酸盐
硫酸盐在厌氧处理过程中会被硫酸盐还原菌（SRB）还原成硫化物，这些硫化物主要含有H₂S、HS⁻、S²⁻。其中H₂S的毒害作用最大。H₂S呈电中性，能穿过带负电的菌体细胞膜，破坏其蛋白质。此外，硫化物几乎对所有的厌氧细菌都有直接或可逆的毒害作用。少量的硫酸盐（或硫化物）有益于厌氧消化过程，但当废水中硫酸盐含量过高时（如超过一定阈值），会对厌氧生物处理产生严重的抑制作用甚至导致处理系统的崩溃。因此在实际操作中需要严格控制废水中的硫酸盐含量以确保厌氧生物处理的顺利进行。
三、无机物质的抑制机制
无机物质对厌氧微生物的抑制作用是一个复杂且多途径的过程，主要包括以下几个方面：
破坏微生物细胞膜结构
高浓度的无机物质如亚硝氮、硝氮和重金属离子会对微生物细胞膜产生强烈的毒害作用。这些无机物质能够穿透细胞膜，与膜上的蛋白质、脂质等成分发生相互作用，导致细胞膜结构发生改变，通透性增加。细胞膜是微生物细胞的重要屏障，一旦其完整性遭到破坏，微生物的代谢活动将受到严重影响，包括物质运输、能量转换、信息传递等过程。
影响微生物能量代谢过程
氨氮、亚硝氮和硝氮等无机氮化合物在厌氧微生物的代谢过程中起着重要作用，但高浓度时也会对微生物的能量代谢产生抑制作用。这些无机氮化合物能够与微生物体内的酶活性位点结合，干扰酶的催化反应，从而影响微生物的能量转换过程。例如，硝氮可以竞争性地抑制电子传递链上的酶活性，导致ATP含量下降。ATP是微生物生长和繁殖的重要能量来源，其含量下降将直接影响微生物的生长速度和繁殖能力。
抑制辅酶活性
辅酶F₄₂₀和辅酶M是厌氧微生物代谢过程中的重要辅酶，它们参与了许多重要的生化反应。然而，一些无机物质如重金属离子等能够与这些辅酶结合，抑制其活性。这种抑制作用会导致微生物的代谢效率下降，活性降低，从而影响整个厌氧微生物群落的代谢过程。
改变微生物群落结构
无机物质的抑制作用还会导致厌氧微生物群落结构的变化。在无机物质的抑制作用下，一些对无机物质具有抗毒性和适应能力的细菌菌属会成为优势菌属，而一些敏感菌属则会逐渐减少或消失。这种群落结构的改变可能会影响整个厌氧发酵过程的稳定性和效率。
四、结论与展望
厌氧反应中无机物质的毒性与抑制是影响厌氧工艺稳定运行的重要因素。本文综述了厌氧反应中常见无机物质的毒性及其抑制机制，为厌氧工艺的优化提供了理论依据。未来的研究应进一步深入探讨无机物质对厌氧微生物的抑制机理，以及如何通过驯化、调节pH值、添加抑制剂等措施来减轻或消除无机物质的抑制作用，提高厌氧工艺的稳定性和处理效率。
英文版（English version）：
Anaerobic digestion, as an efficient waste treatment method, has garnered widespread attention due to its low energy consumption, small footprint, and high treatment efficiency. However, anaerobic bacteria are sensitive to toxins, and inhibitory substances in wastewater and sludge are often the main reasons for the instability and failure of anaerobic reactions. This article will focus on discussing the toxicity and inhibition of inorganic substances in anaerobic reactions, aiming to provide a theoretical basis for the optimization of anaerobic processes.
I. Classification of Inorganic Inhibitory Substances
In anaerobic bioreactors, inorganic inhibitory substances are a class of important chemical substances that can adversely affect the growth, metabolism, and biological reaction processes of anaerobic microorganisms. These substances mainly include oxygen, ammonia nitrogen, sulfides and sulfates, inorganic salts (such as sodium ions, calcium ions, magnesium ions), heavy metals, etc.
Oxygen is the primary inhibitory factor in the anaerobic digestion process, and even small concentrations can significantly inhibit the activity of anaerobic microorganisms. This is because anaerobic microorganisms can only effectively carry out methanogenic reactions under anaerobic conditions, and the presence of oxygen interferes with this process, damaging the physiological functions of methanogens, thereby reducing the overall rate and efficiency of anaerobic digestion.
Ammonia nitrogen is another key inhibitory factor. When the concentration of ammonia nitrogen in wastewater is too high, it can inhibit the activity of anaerobic microorganisms through multiple pathways. On the one hand, high concentrations of ammonia nitrogen can combine with proteins in microbial cells to form compounds that are difficult to decompose, leading to nutritional deficiencies in microorganisms; on the other hand, ammonia nitrogen can also affect the energy metabolism process of microorganisms, reducing their biological activity.
Sulfides and sulfates are also important inorganic inhibitory substances. Sulfides mainly come from the reduction products of sulfate-reducing bacteria under anaerobic conditions, and excessive sulfides can compete with methanogens for hydrogen and electron donors, thereby inhibiting the methanogenic process. Sulfates may form sulfate ions under certain conditions, further affecting the pH value and redox potential in the anaerobic digestion process, which is not conducive to the normal growth and metabolism of anaerobic microorganisms.
In addition, high concentrations of inorganic salts such as sodium ions, calcium ions, and magnesium ions can also inhibit anaerobic microorganisms. The presence of these ions may change the osmotic balance inside the reactor, leading to an imbalance of water inside and outside the microbial cells, thereby affecting the physiological functions of the microorganisms.
Heavy metal elements also have a significant inhibitory effect on anaerobic microorganisms. For example, when copper, zinc, cadmium, lead, and other heavy metal ions exceed the standard in wastewater, they can bind with enzyme proteins in microbial cells, destroying their structural functions, or hinder the normal energy metabolism process of microorganisms by interfering with electron transport chains.
II. Toxicity Study of Inorganic Substances
Ammonia Nitrogen
Ammonia nitrogen is a common inorganic nitrogen compound in wastewater, mainly existing in the form of ammonium ions (NH₄⁺) and free ammonia (NH₃, FA). Among them, free ammonia has good membrane permeability and can enter microbial cells through the cell membrane, which is the main reason for the inhibitory effect. Studies have shown that the inhibitory effect of ammonia nitrogen on anaerobic microorganisms is mainly manifested in the energy metabolism process of microorganisms. In the energy metabolism process, ammonia nitrogen can combine with ATP (adenosine triphosphate) to form amide phosphate (AMP), resulting in a decrease in ATP content. When the concentration of ammonia nitrogen is below 200mg/L, it has a promoting effect on the anaerobic degradation process, because nitrogen is also an important nutrient required by anaerobic microorganisms. However, when the concentration of ammonia nitrogen increases, it will significantly inhibit the activity of methanogenic bacteria (MPB), resulting in a decrease in methane production. This inhibitory effect may have a negative impact on the entire anaerobic biological treatment process, affecting the treatment effect of wastewater.
Nitrite and Nitrate
Nitrite (NO₂⁻-N) and nitrate (NO₃⁻-N) are another common inorganic nitrogen compounds. These two substances show significant inhibitory effects in the process of anaerobic microbial treatment of wastewater. Nitrite has a clear inhibitory effect on the specific methanogenic activity (SMA) of anaerobic granular sludge, and this inhibitory effect shows a concentration-dependent manner, that is, the higher the concentration of nitrite, the greater the degree of inhibition of SMA. Studies have shown that the half-inhibitory concentration (EC₅₀) of nitrite is about 12mgN·L⁻¹, which means that when the concentration of nitrite exceeds this threshold, its inhibitory effect on SMA will become significant. Similarly, the inhibitory effect of nitrate on SMA is similar to that of nitrite, and its EC₅₀ is about 30mgN·L⁻¹. High concentrations of nitrite and nitrate not only affect the energy metabolism process of anaerobic microorganisms but also destroy the cell membrane structure of microorganisms. This includes changing the permeability of the cell membrane, hindering material transport and energy conversion, and other key physiological functions.
Heavy Metals
The inhibitory effect of heavy metal ions on anaerobic microorganisms is also very obvious. For example, soluble low-concentration copper salts, zinc salts, and nickel salts have considerable toxicity in water. These heavy metal ions can bind with the enzyme system of anaerobic microorganisms, interfering with their normal physiological metabolism, thereby inhibiting the microorganisms. However, anaerobic microorganisms have a certain adaptability to heavy metal ions. In the process of anaerobic treatment of wastewater, the anions such as S²⁻ and CO₃²⁻ produced will react with metal ions to form insoluble sulfide and carbonate precipitates. These precipitates will adhere to the surface of microbial cells or gather around the microbial cells in the aqueous environment, reducing the concentration of metal ions. In addition, sulfides generated by sulfate reduction can also effectively reduce the toxicity of heavy metal ions. Sulfides react with heavy metal ions to form stable sulfide precipitates, further reducing the concentration of heavy metal ions in the water. These factors work together to ensure that even if there is heavy metal ion pollution in the wastewater, anaerobic microorganisms can maintain their physiological activity and treatment efficiency to a certain extent.
Inorganic Salts
The inhibitory effect of inorganic salts is only apparent when the concentration is very high. For example, high concentrations of Na⁺ have an inhibitory effect on unacclimated anaerobic bacteria. This is because high concentrations of inorganic salt ions can cause an imbalance in ion balance inside and outside the cell, affecting the normal physiological function of the cell. However, acclimated anaerobic bacteria can adapt to this high concentration of ion environment. When the concentration of Na⁺ is higher than about 5000mg/L, it has obvious cationic toxicity. However, after acclimation, when the concentration of Na⁺ in wastewater is as high as 15000mg/L, the rate at which the anaerobic reactor removes pollutants can still be equivalent to 50% of the low-sodium control system. This indicates that acclimated anaerobic bacteria have a certain tolerance.
Sulfides and Sulfates
Sulfates will be reduced to sulfides by sulfate-reducing bacteria (SRB) during the anaerobic treatment process, and these sulfides mainly contain H₂S, HS⁻, S²⁻. Among them, H₂S is the most toxic. H₂S is electrically neutral and can pass through the negatively charged microbial cell membrane, destroying its proteins. In addition, sulfides have a direct or reversible toxic effect on almost all anaerobic bacteria. A small amount of sulfate (or sulfide) is beneficial to the anaerobic digestion process, but when the content of sulfate in wastewater is too high (such as exceeding a certain threshold), it will have a serious inhibitory effect on anaerobic biological treatment and even cause the collapse of the treatment system. Therefore, it is necessary to strictly control the content of sulfates in wastewater in actual operation to ensure the smooth progress of anaerobic biological treatment.
III. Inhibition Mechanism of Inorganic Substances
The inhibitory effect of inorganic substances on anaerobic microorganisms is a complex and multi-pathway process, mainly including the following aspects:
Destruction of Microbial Cell Membrane Structure
High concentrations of inorganic substances such as nitrite, nitrate, and heavy metal ions will have a strong toxic effect on the microbial cell membrane. These inorganic substances can penetrate the cell membrane and interact with proteins, lipids, and other components on the membrane, causing changes in the cell membrane structure and increased permeability. The cell membrane is an important barrier for microbial cells, and once its integrity is damaged, the metabolic activities of microorganisms will be severely affected, including material transport, energy conversion, information transmission, and other processes.
Influence on Microbial Energy Metabolism Process
Ammonia nitrogen, nitrite, and nitrate, and other inorganic nitrogen compounds play an important role in the metabolism process of anaerobic microorganisms, but high concentrations can also inhibit the energy metabolism of microorganisms. These inorganic nitrogen compounds can combine with the active sites of enzymes in microbial bodies, interfering with the catalytic reaction of enzymes, thereby affecting the energy conversion process of microorganisms. For example, nitrate can competitively inhibit the enzyme activity on the electron transport chain, resulting in a decrease in ATP content. ATP is an important source of energy for microbial growth and reproduction, and its content decline will directly affect the growth rate and reproductive capacity of microorganisms.
Inhibition of Coenzyme Activity
Coenzyme F₄₂₀ and coenzyme M are important coenzymes in the metabolic process of anaerobic microorganisms, and they participate in many important biochemical reactions. However, some inorganic substances such as heavy metal ions can combine with these coenzymes to inhibit their activity. This inhibitory effect will reduce the metabolic efficiency of microorganisms, reduce their activity, and thus affect the metabolic process of the entire anaerobic microbial community.
Change of Microbial Community Structure
The inhibitory effect of inorganic substances will also lead to changes in the structure of the anaerobic microbial community. Under the inhibitory effect of inorganic substances, some bacterial genera that are resistant to inorganic substances and have adaptability will become dominant bacterial genera, while some sensitive bacterial genera will gradually decrease or disappear. This change in community structure may affect the stability and efficiency of the entire anaerobic fermentation process.
IV. Conclusion and Outlook
The toxicity and inhibition of inorganic substances in anaerobic reactions are important factors affecting the stable operation of anaerobic processes. This article reviews the toxicity and inhibition mechanisms of common inorganic substances in anaerobic reactions, providing a theoretical basis for the optimization of anaerobic processes. Future research should further explore the inhibition mechanism of inorganic substances on anaerobic microorganisms, as well as how to reduce or eliminate the inhibitory effect of inorganic substances through acclimation, pH regulation, addition of inhibitors, and other measures to improve the stability and treatment efficiency of anaerobic processes.
