Comprehensive Explanation of the Principles of Anaerobic Wastewater Treatment

In the process of sewage treatment, anaerobic biological treatment of wastewater is also known as anaerobic digestion or anaerobic fermentation in the early stages; It refers to the process in which organic matter is decomposed and produces CH4 and CO2 under anaerobic conditions by the combined action of multiple (anaerobic or facultative) microorganisms.

1、 Basic biological processes in anaerobic biological treatment
1. The research on anaerobic microbiology based on the three-stage theory shows that methanogenic bacteria are a very special class of archaea. In addition to their taxonomy and unique academic structure, their main characteristic is that methanogenic bacteria can only use some simple organic substances as substrates, mainly some simple one carbon substances such as formic acid, methanol, methylamines, and H2/CO2, and only acetic acid is present in the two carbon substances, And other fatty acids containing two or more carbons and alcohols other than methanol cannot be used. ① Hydrolysis and fermentation stage: ② Hydrogen producing acetic acid stage: Hydrogen producing acetic acid bacteria convert fatty acids such as propionic acid, butyric acid, and ethanol into acetic acid, H2/CO2; ③ Methane production stage: Methanogenic bacteria utilize acetic acid and H2, CO2 to produce CH4; It is generally believed that about 70% of CH4 is produced from the decomposition of acetic acid during anaerobic biological treatment, while the rest is produced from H2 and CO2.
2. Four stage theory: In fact, based on the above three stage theory, a type of bacteria - homotypic acetic acid producing bacteria - has been added, whose main function is to synthesize H2/CO2 produced by hydrogen producing acetic acid producing bacteria into acetic acid. However, research has shown that in reality, the amount of acetic acid synthesized from H2/CO2 is relatively small, accounting for only about 5% of the total amount of acetic acid in anaerobic systems. Overall, the "three-stage theory" and "four stage theory" are widely recognized as comprehensive and accurate descriptions of anaerobic biological treatment processes.
2、 The main microorganisms in anaerobic digestion process are mainly introduced, including fermentation bacteria (acid producing bacteria), hydrogen producing acetic acid producing bacteria, methane producing bacteria, etc.
1. Fermentation bacteria (acid producing bacteria): There are two main functions of fermentation acid producing bacteria: ① hydrolysis - hydrolysis of insoluble organic matter into soluble organic matter under the action of extracellular enzymes; ② Acidification - converting soluble macromolecular organic compounds into fatty acids, alcohols, etc; The main fermentation acid producing bacteria include Clostridium, Bacteroidetes, Vibrio butyricum, Bifidobacterium, etc; The hydrolysis process is slow and influenced by various factors (pH, SRT, organic matter types, etc.), sometimes becoming a rate limiting step in anaerobic reactions; The rate of acid production reaction is relatively fast; Most are anaerobic bacteria, but there are also a large number of facultative anaerobic bacteria; It can be divided by function: cellulose degrading bacteria, hemicellulose degrading bacteria, starch degrading bacteria, protein degrading bacteria, fat degrading bacteria, etc.
2. Hydrogen producing and acetic acid producing bacteria: The main function of hydrogen producing and acetic acid producing bacteria is to oxidize and decompose various advanced fatty acids and alcohols into acetic acid and H2; To provide a suitable substrate for methanogenic bacteria, they often coexist and interact with each other in anaerobic systems. The main reactions for hydrogen to acetic acid production include: Note that the above reactions can only proceed smoothly when the concentration of acetic acid is very low and the hydrogen partial pressure in the system is also very low. Therefore, the smooth progress of hydrogen to acetic acid production often requires the subsequent methane production reaction to timely consume its two main products, acetic acid and H2. The main hydrogen producing and acetic acid producing bacteria are mainly: Actinobacteria, Actinobacteria, Clostridium, and Bacillus; Most are strictly anaerobic or facultative anaerobic bacteria. 3. The main function of methanogenic bacteria is to convert the products of hydrogen producing acetic acid bacteria - acetic acid and H2/CO2- into CH4 and CO2, enabling the anaerobic digestion process to proceed smoothly; It can be mainly divided into two categories: acetic acid trophic and H2 trophic methanogenic bacteria, also known as acetic acid methanogenic bacteria and hydrogen methanogenic bacteria; Generally speaking, there are fewer types of acetic acidotrophic methanogens in nature, with only Methanosarcina and Methanothrix. However, these two types of methanogenic bacteria are more common in anaerobic reactors, especially the latter, because acetic acid is the main methanogenic substrate in anaerobic reactors, and generally about 70% of methane comes from the oxidation and decomposition of acetic acid. Typical methane production reaction: Methanogenic bacteria have various forms, common ones include: ① Methanogenic bacteria; ② Methanogenic bacteria; ③ Methanogenic eight fold bacteria; ④ Methanogenic mycelium; Wait a minute. Methanogenic bacteria are strictly anaerobic bacteria, requiring an oxidation-reduction potential of -150-400mv. Oxygen and oxidants have a strong toxic effect on them; The proliferation rate of methane producing bacteria is very slow, with a long reproductive generation time of up to 4-6 days. Therefore, methane production reaction is generally the rate limiting step of anaerobic digestion.
3、 The influencing factors of anaerobic biological treatment are methane production reaction, which is the control stage of anaerobic digestion process. Therefore, generally speaking, when discussing the influencing factors of anaerobic biological treatment, the main focus is on the various factors that affect methane producing bacteria; The main influencing factors include temperature, pH value, redox potential, nutrients, F/M ratio, toxic substances, etc.
1. Temperature: Temperature has a particularly significant impact on anaerobic microorganisms; Anaerobic bacteria can be divided into thermophilic bacteria (or thermophilic bacteria) and thermophilic bacteria (mesophilic bacteria); Correspondingly, anaerobic digestion can be divided into high temperature digestion (around 55 ° C) and medium temperature digestion (around 35 ° C); The reaction rate of gasification is about 1.5-1.9 times that of medium temperature digestion, and the gas production rate is also high, but the methane content in the gas is relatively low; When treating wastewater or sludge containing pathogenic bacteria and parasitic insect eggs, high-temperature digestion can achieve good hygiene results, and the dewatering performance of the digested sludge is also good; With the development, research, and application of new anaerobic reactors, the influence of temperature on anaerobic digestion is no longer very important (the biomass inside the new reactor is large), so it can be carried out at room temperature (20-25 ° C) to save energy and operating costs.
2. PH value and alkalinity: pH value is the most important influencing factor in anaerobic digestion process; Important reason: Methanogenic bacteria are very sensitive to changes in pH value, and it is generally believed that their optimal pH range is 6.8~7.2. When<6.5 or>8.2, methane producing bacteria will be severely inhibited, further leading to the deterioration of the entire anaerobic digestion process; The pH value in anaerobic systems is influenced by various factors, such as influent pH value, influent water quality (organic matter concentration, type of organic matter, etc.), biochemical reactions, acid-base balance, and dissolution balance between gas, solid, and liquid phases; The anaerobic system is a pH buffering system, mainly controlled by the carbonate system; Generally speaking, the increase (accumulation) of fatty acid content in the system will consume? HCO3, causing a decrease in pH; But the action of methane producing bacteria not only consumes fatty acids, but also produces? HCO3 causes the pH value of the system to rise. Alkalinity was once considered a crucial influencing factor in anaerobic digestion, but in reality, its main function is to ensure that the anaerobic system has a certain buffering capacity and maintains an appropriate pH value; Once an anaerobic system undergoes acidification, it takes a long time to recover. 3. Oxidation-reduction potential: A strict anaerobic environment is the basic condition for methane producing bacteria to carry out normal physiological activities; Non methane producing bacteria can grow and operate normally in an environment with an oxidation-reduction potential of+100~-100mv; The optimal oxidation-reduction potential for methanogenic bacteria is -150~-400mv. In the initial stage of cultivating methanogenic bacteria, the oxidation-reduction potential cannot be higher than -330mv; 4. Nutritional requirements: Anaerobic microorganisms have slightly lower requirements for nutrients such as N and P than aerobic microorganisms, with a COD: N: P=200:5:1 requirement; Most anaerobic bacteria do not have the function of synthesizing certain necessary vitamins or amino acids, so sometimes they need to be added: ① Metal salts such as K, Na, Ca, etc.; ② Trace elements such as Ni, Co, Mo, Fe, etc.; ③ Organic trace substances: yeast extract, biotin, vitamins, etc. 5. F/M ratio: Anaerobic biological treatment has a better organic matter load, and aerobic biological treatment has a higher load, generally up to 5-10kgCOD/m3. d, and even up to 50-80kgCOD/m3. d; No restrictions on oxygen transfer; Can accumulate higher biomass. The reaction rate in the acid production stage is much higher than that in the methane production stage, so it is necessary to choose the organic load very carefully; The prerequisite for high organic volumetric load is high biomass, while the corresponding low sludge load; A high organic volumetric load can shorten HRT and reduce reactor volume. 6. Toxic substances: Common inhibitory substances include sulfides, ammonia nitrogen, heavy metals, cyanide, and certain organic compounds; ① Sulfides and sulfates: Sulfates and other sulfur oxides are easily reduced to sulfides during anaerobic digestion; When soluble sulfides reach a certain concentration, they will have an inhibitory effect on the anaerobic digestion process, mainly methane production; Adding certain metals such as Fe can remove S2-, or blowing off H2S from the system can alleviate the inhibitory effect of sulfides Ammonia nitrogen: Ammonia nitrogen is a buffering agent for anaerobic digestion; But if the concentration is too high, it will have a toxic effect on the anaerobic digestion process; The inhibitory concentration is 50-200mg/l, but after domestication, the adaptability will be enhanced Heavy metals: - disrupt the enzyme system of anaerobic bacteria. ④ Cyanide: ⑤ Toxic organic matter:
4、 The main characteristics of anaerobic biological treatment
1. The main advantages of anaerobic biological treatment process are: ① greatly reduced energy consumption and the ability to recover bioenergy (biogas); ② The sludge production is very low—— The proliferation rate of anaerobic microorganisms is much lower than that of aerobic microorganisms. The yield of acid producing bacteria Y is 0.15~0.34kgVS/kgCOD, the yield of methane producing bacteria Y is about 0.03kgVS/kgCOD, and the yield of aerobic microorganisms is about 0.25~0.6kgVS/kgCOD Anaerobic microorganisms may degrade or partially degrade some organic compounds that aerobic microorganisms cannot degrade; ④ The reaction process is relatively complex - anaerobic digestion is a continuous microbial process in which multiple microorganisms with different properties and functions work together;
2. The main drawbacks of anaerobic biological treatment process are: ① sensitivity to environmental factors such as temperature and pH; ② The water quality of the treated effluent is poor and further aerobic treatment is needed; ③ Has a strong odor; ④ The removal effect of ammonia nitrogen is not good.