Microorganisms in the Anaerobic Digestion

Microorganisms in the Anaerobic Digestion

Aastha Valecha

Undergraduate Student Mechanical Department, IIT Delhi

Abstract Anaerobic digestion can provide an alternative energy source. In the modern world scenario, such a non- conventional source of energy is highly valuable. Micro- organisms play a very crucial role in these anaerobic digestors. The cooperation of four different groups of microorganisms, namely fermentative, syntrophic, acetogenic, and methanogenic bacteria, allows for the conversion of complex organic compounds. The report discusses the classification of the anaerobic co-digestion process based on the application of various microorganisms.

Keywords Anaerobic, methanogenic, digestors, microorganisms

INTRODUCTION

Anaerobic digestion is a waste treatment approach that aims to reduce waste's dangerous effects on the environment. The mutualistic behavior of numerous anaerobic microorganisms causes the decomposition of complicated natural materials into simple, chemically strong compounds, primarily methane. Anaerobic respiration is a complicated biochemical reaction importantly related to some steps and is performed with the aid of anaerobic microorganisms that require very little oxygen to live on pollutant waste substances. For the duration of the method, gas methane (CH4) and carbon dioxide are produced. a few bacteria can't live in presence of oxygen. Among them, the most feasible and easy technique of producing energy is anaerobic respiration. In this process, Ethyl alcohol is produced, which is utilized in various industries. The procedure is applied in Lactic Acid fermentation. Anaerobic digestion is a sequence of procedures by means of which microorganisms break down biodegradable material in the absence of oxygen. The technique is used for industrial or domestic functions to control waste or to supply fuels.

1. STAGES OF ANAEROBIC DEGRADATION OF

   ORGANIC WASTES

   The microbiology of the anaerobic degradation of the natural wastes consists of acid-forming, hydrolytic, acetogenic, and methanogenic bacteria that produce CO2 and CH4 as the primary merchandise for the digestion process. Each and every process is accountable for the degradation of a distinct type of compound. It is a multi- species technique that involves various micro-organisms at different levels of the process.

   • Hydrolysis

     Biomass consists of large organic polymers, these chains must first be broken down into their smaller constituent parts so as to get readily used by bacteria. The process of breaking of the chains and dissolving their smaller molecules into a

     solution is called hydrolysis. Acetate and hydrogen produced within the first stages can be used at once via methanogens. other molecules, inclusive of volatile fatty acids (VFAs) with a sequence period more than that of acetate need to first be catabolized into compounds that may be directly utilized by methanogens. Hydrolysis approach the cleavage of bonds by using the addition of water molecules. Hydrolysis is usually the rate-restricting step in biodigestion. Microorganisms used in this process are Bacteriodes, Clostridium, and Acetivibrio.

   • Acidogenesis

     Acidifying (Fermentive) bacteria convert water-soluble chemical substances, such as hydrolysis products, to short- chain organic acids (formic, acetic, propionic, butyric, and pentanoic), alcohols (methanol, ethanol), aldehydes, carbon dioxide, and hydrogen during this stage. The hydrogen, carbon dioxide, acetic acid and water will pass to the third stage, acetogenesis, and be utilized without delay by the methanogenic microorganism in the final stage.

   • Acetogenesis

     Acetogenesis is a method via which acetate is produced by the reduction of CO2 or by the reduction of organic acids. Simple molecules developed by the means of acidogenesis phase are further digested by acetogens to produce largely acetic acid, as well as carbon dioxide and hydrogen. Syntrophobacter genera, convert acid phase products into acetates and hydrogen, as they can be used by methanogenic bacteria. Here two groups of microorganisms play a key function: the acetogenic microorganism produce hydrogen binding using the products of acidogenic bacteria as sublayer for this reason giving rise to acetate, hydrogen, and carbonic anhydride after which additionally the homoacetogene microorganism that synthesize acetate beginning from carbonic anhydride and hydrogen.

   • Methanogenesis

     This is the terminal stage in which methane is produced from substrates such as acetic acid, H2, CO2, and formate, as well as methanol, methylamine, or dimethyl sulphide, which are products of previous phases. The microorganisms chargeable for this segment are physiologically united as methane producers in anaerobic digestion . Waste stabilization is then completed whilst methane gas and carbon dioxide are produced.

     Fig. 1. Process of Anaerobic Digestion

2. CLASSIFICATION OF BACTERIA USED IN ANAEROBIC DIGESTER

   Various types of bacteria play an important role in determining the organic dynamics of an anaerobic digestor. The classification of bacteria on the basis of their function and purpose in the four stages of anaerobic digestion is fermentative, syntrophic, acetogenic, and methanogenic bacteria.

   • Fermentative

     Anaerobic fermentation has been applied to many important industrial fermentations, such as ethanol production by yeasts, lactic acid preservation of foods, anaerobic digestion of organic matters in ruminant cultivation and waste treatment. This group of microorganism is answerable for the primary level of anaerobic digestion – hydrolysis and acidogenesis. These bacteria are both facultative or strict anaerobes.

   • Acidogenic

     Fermenting bacteria and acetogenic microorganism are in syntrophic relations clean elucidation of the interspecies electron transfer and energetic mechanisms amongst syntrophic microorganism is vital for optimization of acidogenic. Acidogenic bacteria like Streptococcus lactis, Leuconostos mesenteroides, Lactobacillus casei, Lactobacillus plantarum produces organic acids as well as hydrogen, CO2, different alcohols, and a small amount of acetic acid out of organic material.

   • Acetogenic

     The oxidation of VFAs through syntrophic bacteria isn't always a thermodynamically favorable process and calls for that they're closely associated with methanogens or with non- methanogenic hydrogenotrophs. Syntrophic association of acetogenic organisms with methanogenic Hydrogen consuming bacteria helps lower the concentration of Hydrogen below inhibitory level so that propionate degrading bacteria are not suppressed by excessive Hydrogen level.

     Sulfate-reducing bacteria also are found in anaerobic digesters along with acetateforming bacteria and methane- forming bacteria. If sulfates are present, sulfatereducing bacteria such as Desulfovibrio desulfuricans multiply. Their multiplication or reproduction often requires the use of hydrogen and acetatethe same substrates used by methane- forming bacteria.

     Acetate-forming (acetogenic) bacteria grow in a symbiotic relationship with methane-forming bacteria. Acetate serves as a substrate for methane-forming bacteria. For example, when ethanol is converted to acetate, carbon dioxide is used and acetate and hydrogen are produced. When acetate-forming bacteria produce acetate, hydrogen also is produced. If the hydrogen accumulates and significant hydrogen pressure occurs, the pressure results in termination of activity of acetate-forming bacteria and lost of aceate production.

     Methanogenic

     As a part of the waste treatment gadget, methanogenic bacteria are used in the anaerobic decomposition of wastewater. Sedimentation strategies are also used to stabilise primary and secondary sludge produced all through the aerobic wastewater remedy manner. Although methane- forming bacteria are oxygen sensitive, this is not a significant disadvantage.

     Methane-forming bacteria are found in habitats that are rich in degradable organic compounds. Methanogens can only process a small number of simple organic substrates, the most common of which are acetic acid, hydrogen, and carbon-di-oxide. Although some species process Acetic Acid, and Formic Acid, most methanogenic bacteria can use Hydrogen and Carbon-di-oxide to grow.

     Methanogens use the intermediate products of the previous levels and convert them into methane, carbon dioxide, and water. these components make up the majority of the biogas emitted from the system.

     Methane-forming bacteria are classified in the domain Archaebacteria because of several unique characteristics that are not found in the true bacteria or Eubacteria. These bacteria obtain energy by reducing simplistic compounds or substrates such as carbon dioxide and acetate.

     The unique chemical composition of the cell wall makes the bacteria sensitive to toxicity from several fatty acids. Also, many methane-forming bacteria lack a protective envelope around their cell wall. Surfactants or hypotonic shock easily lyse methane-forming bacteria that do not have this envelope.

     There are three principal groups of methane-forming bacteria. These groups are-

   • Hydrogenotrophic methanogens

     By converting carbon dioxide to methane, these organisms help to maintain a low partial hydrogen pressure in an anaerobic digester that is required for acetogenic bacteria.

   • Acetotrophic methanogens

     The acetotrophic methanogens divide acetate into methane and carbon dioxide. The carbon dioxide produced from acetate may be converted by hydrogenotrophic methanogens to methane. The acetotrophic methanogens reproduce extra slowly than the hydrogenotrophic methanogens and are adversely affected by the accumulation of hydrogen. Therefore, the renovation of a low partial hydrogen strain in an anaerobic digester is favorable for the interest of not most effective acetate-forming micro organism however additionally acetotrophic methanogens. under an exceedingly high hydrogen partial pressure, acetate and methane production are reduced.

   • Methylotrophic methanogens

     The methylotrophic methanogens grow on substrates that contain the methyl group . Examples of these substrates include methanol and methylamines. The use of different substrates by way of methane-forming bacteria outcomes in extraordinary power gains via the bacteria. As an instance, hydrogen-eating methane production results in greater energy advantage for methane-forming bacteria than acetate degradation.Although methane production the usage of hydrogen is the extra effective system of energy seize by means of methane-forming micro organism, much less than 30% of the methane produced in an anaerobic digester is through this technique. About 70% of the methane produced in an anaerobic digester is derived from acetate. The cause for that is the confined supply of hydrogen in an anaerobic digester. In general, methane acquired from acetate is produced by way of two genera of acetotrophic methanogens, Methanosarcina and Methanothrix.

     Below optimal situations, the range of generation times of methane-forming micro organism may be from a few days to numerous weeks. therefore, if solids retention time (SRT) is short or quick-circuiting or early withdrawal of digester sludge occurs. The population length of methane-forming bacteria is significantly reduced. these conditions lower the time to be had for reproduction of methane-forming bacteria, this is, the bacteria are eliminated from the digester quicker than they can reproduce. This outcomes in negative digester performance or failure of the digester. Most methane-forming bacteria are mesophiles or thermophiles, with a few micro organism growing at temperatures above a hundred°C .

     Mesophiles are those organisms that develop best in the temperature range of 3035°C, and thermophiles are the ones organisms that grow satisfactory within the temperature range of 5060°C. Some genera of methane-forming bacteria have mesophilic and thermophilic species.

   • Difficulties in growing Methane forming Bacteria

   It is tough to grow methane-forming micro organism in pure culture. fashionable laboratory enumeration strategies are not appropriate for methane-forming micro organism. This issue is because of –

   1. their intense obligate anaerobic nature and the probability that they are killed hastily by extraordinarily quick time exposures to air compared with other anaerobes

   2. their limited wide variety of substrates.

3. INOCULUM AND EFFECT OF INOCULUM SOURCE

   Distinct stages of anaerobic digestion contain a huge range of symbiotic microorganisms, which can be divided into categories:acidogens and methanogens. The inoculum utilized in anaerobic digestion impacts the way it degrades natural count and produces biogas. The startup of biodigesters includes the accumulation of some consortium of microorganisms that assist in the process of anaerobic digestion. For a faster startup and a quicker biogas building up, the methanogenic bacteria (methanobacteria) are isolated and cultured to evolve to the feed and the operating conditions of the biodigester, this is referred to as inoculum. Although the use and application of inoculum have advantages, the cost of preparing and culturing the inoculum remains a setback to the method.

   Using a highly active anaerobic inocula or animal inoculum waste in full-scale batch digesters will significantly reduce the experimental time or the amount of inocula required, and thus the digester volume,

   in the case of anaerobic biodegradability of solid waste.

   The rate-determining step in the anaerobic digestion of organic solid waste is cellulose solubilization. The overall efficiency of the anaerobic digestion process should improve as the rate of solubilization increases.

4. IMPORTANCE OF MICROORGANISMS IN ANAEROBIC DIGESTION

   Many microorganisms have an effect on anaerobic digestion, which includes acetic acid-forming bacteria (acetogens) and methane-forming archaea (methanogens). These organisms promote a number of chemical techniques in changing the biomass to biogas. Gaseous oxygen is excluded from the reactions by way of physical containment. The microbiome residing in anaerobic digesters drives the anaerobic digestion system to convert various feedstocks to biogas as a renewable supply of energy.

   The microbial communities play a vital function at the overall performance and balance of food waste. The conversions of complex organic compounds to and are feasible because of the cooperation of four distinct groups of microorganisms, that is, fermentative, syntrophic, acetogenic, and methanogenic bacteria. Microbes undertake numerous pathways to keep away from the detrimental conditions inside the anaerobic digester.

5. CONCLUSION

Under anaerobic conditions, anaerobic digestion is a complex reduction manner related to a number of biochemical

reactions. This digestion procedure converts biomass to energy, which is then used to recycle

organic waste and decrease harmful environmental outcomes. Anaerobic digestion is split into four degrees: hydrolysis, acidogenesis, acetogenesis, and methanogenesis.

Fermentative, syntrophic, acetogenic, and methanogenic bacteria are some of the microorganisms that make biogas feasible. Inocula play an important role in anaerobic reactor startup by balancing the populations of syntrobacter and methanogens. This balance makes syntrophic metabolism thermodynamically feasible in aaerobic digestion. The source of inocula not only affects the amount of biogas production but also influences the kinetics of the process of anaerobic digestion. Different molecular techniques, such as DGGE and FISH, are being used to investigate microbial dynamic changes in anaerobic digester microbial communities.

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