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Anaerobic Digestion

Anaerobic Digestion is a biochemical process during which complex organic matter is decomposed in absence of oxygen, by various types of anaerobic microorganisms. The process is similar to that found in nature as in the stomach of ruminants such as cows. Anaerobic digestion is a purely bacterial process. In a biogas installation, the outcome of the Anaerobic Digestion process is the biogas and the digestate. If the feedstock for Anaerobic Digestion is a homogenous mixture of more than one feedstock types such as animal slurries and organic wastes from food processing factories, it is known as “co–digestion” and is the most common biogas applications today.
AD is a microbiological process of decomposition of organic matter in absence of oxygen. The main products of this process are biogas and digestate. Biogas is a combustible gas, consisting primarily of methane and carbon dioxide. Digestate is the decomposed substrate, resulted from the production of biogas.
During AD, very little heat is generated in contrast to aerobic decomposition i.e. in presence of oxygen, like is the case of composting. The energy, which is chemically bounded in the substrate, remains mainly in the produced biogas, in form of methane. The process of biogas formation is a result of a series of linked steps, in which the initial material is continuously broken down into smaller units. Specific groups of micro-organisms are involved in each step. These organisms successively decompose the products of the previous steps. The four main process steps are hydrolysis, acidogenesis, acetogenesis, and methanogenesis.

The process steps run parallel in time and space, in the digester tank. The speed of the total decomposition process is determined by the slowest reaction of the chain. During hydrolysis, relatively small amounts of biogas are produced. Biogas production reaches its peak during methanogenesis.


Hydrolysis is categorized as the first step of AD, during which the complex organic matter namely polymers is decomposed into smaller units. During hydrolysis, polymers like carbohydrates, lipids, nucleic acids and proteins are converted into glucose, glycerol, purines and pyridines. Hydrolytic microorganisms excrete hydrolytic enzymes, converting biopolymers into simpler and soluble compounds.
A variety of microorganisms are involved in hydrolysis, which is carried out by exoenzymes, produced by those microorganisms which decompose the particulate material which has not been dissolved. The resultant products from hydrolysis are further decomposed by the microorganisms involved and used for their own metabolic processes.


During acidogenesis, the products of hydrolysis are converted by acidogenic or fermentative bacteria into methanogenic substrates. Simple sugars, amino acids and fatty acids are degraded into acetate, carbon dioxide and hydrogen as well as into volatile fatty acids and alcohols.


Acidogenesis products that cannot be directly converted to methane by methanogenic bacteria are converted into methanogenic substrates during acetogenesis. Volatile fatty acids and alcohols are oxidized into methanogenic substrates like acetate, hydrogen and carbon dioxide. Volatile fatty acids with carbon chains longer than two units and alcohols, with carbon chains longer than one unit, are oxidized into acetate and hydrogen. The production of hydrogen increases the hydrogen partial pressure. During methanogenesis, hydrogen is converted into methane. Acetogenesis and methanogenesis usually run hand in glove with each other.


The production of methane and carbon dioxide from intermediate products is carried out by methanogenic bacteria. 70% of the formed methane originates from acetate, while the remaining 30% is produced from conversion of hydrogen and carbon dioxide, Methanogenesis is a critical step in the entire anaerobic digestion process, as it is the slowest biochemical reaction of the process. Methanogenesis is greatly influenced by the operation conditions. Composition of feedstock, feeding rate, temperature, and pH are some of the factors influencing the methanogenesis process. Overloading of the digester, temperature changes or excessive entry of oxygen can result in termination or reduction of the production of methane.

Factors Influencing AD

It is crucial that appropriate conditions for anaerobic microorganisms are provided for the efficiency of AD. The growth and activity of anaerobic microorganisms is significantly influenced by conditions such as exclusion of oxygen, constant temperature, pH-value, nutrient supply, stirring intensity and amount of inhibitors such as ammonia. The methane bacteria are fastidious anaerobes, hence the presence of oxygen in the digestion process must be avoided.


The AD process can take place at different temperatures, divided into three temperature ranges. Namely Psychrophilic i.e. below 25 degree C, Mesophilic i.e. 25 degree C to 45 degree C , and thermophilic i.e. 45 degree C to 70 degree C. The temperature stability is a major factor governing AD. The operation temperature is chosen considering the feedstock used. The necessary process temperature is provided by floor or wall heating systems, inside the digester.

Many modern biogas plants operate at thermophilic process temperatures as the thermophilic process provides many advantages, compared to mesophilic and psychrophilic processes such as reduced retention time, making the process faster and more efficient, improved digestibility and availability of substrates, better degradation of solid substrates and better substrate utilization, better possibility for separating liquid and solid fractions.

pH-values and optimum intervals

The pH value of the AD substrate influences the growth of methanogenic microorganisms and affects the dissociation of some compounds of importance for the AD process (ammonia, sulphide,organic acids). Experience shows that methane formation takes place within a relatively narrow pH interval, from about 5,5 to 8,5 , with an optimum interval between 7,0-8,0 for most methanogens. The pH-value in thermophilic digesters is higher than in mesophilic ones, as dissolved carbon dioxide forms carbonic acid by reaction with water. The value of pH can be increased by ammonia, produced during degradation of proteins or by the presence of ammonia in the feed stream, while the accumulation of Volatile fatty acids decreases the pH-value.

Volatile fatty acids (VFA)

The stability of the AD process is reflected by the concentration of intermediate products like the Volatile fatty acids VFA. The VFA are intermediate compounds, produced during acidogenesis. In most cases, AD process instability will lead to accumulation of VFA inside the digester, which can lead furthermore to a drop of pH-value. However, the accumulation of VFA will not always be expressed by a drop of pH value, due to the buffer capacity of the digester, through the biomass types contained in it. Animal manure e.g. has a surplus of alkalinity, which means that the VFA accumulation should exceed a certain level, before this can be detected due to significant decrease of pH value. At such point, the VFA concentration in the digester would be so high, that the AD process will be already severely inhibited.
Practical experience shows that two different digesters can behave totally different in respect to the same VFA concentration, so that one and the same concentration of VFA can be optimal for one digester, but inhibitory for the other one. One of the possible explanations can be the fact that the composition of microorganism populations varies from digester to digester. For this reason, and like in the case of pH, the VFA concentration cannot be recommended as a stand-alone process monitoring parameter.


Ammonia (NH3) is an important compound, with a significant function for the AD process. NH3 is an important nutrient, is normally encountered as a gas, with the characteristic pungent smell. Proteins are the main source of ammonia for the AD process. High ammonia concentration inside the digester, especially free ammonia, is considered to be responsible for process inhibition. This is common to AD of animal slurries, due to their high ammonia concentration, originating from urine.

Macro- and micronutrients (trace elements) and toxic compounds

Trace elements like iron, nickel, cobalt, selenium or tungsten are equally important for the growth and survival of the AD microorganisms as the macronutrients carbon, nitrogen, phosphor, and sulphur. Insufficient provision of nutrients and trace elements, as well as too high digestibility of the substrate can cause inhibition and disturbances in the AD process.

Parameters affecting Operations
Organic Operating load

The Operation of a biogas plant is a combination of economical and technical considerations. Obtaining the maximum biogas productivity, by complete digestion of the substrate, would require a long retention time of the substrate inside the digester and a correspondingly large digester size. In practice, the choice of system design of the digester size and type or of applicable retention time is always based on a compromise between getting the highest possible biogas yield and having justifiable plant economy.

Hydraulic retention time (HRT)

An important parameter for dimensioning the biogas digester is the hydraulic retention time (HRT). The HRT is the average time interval when the substrate is kept inside the digester tank. HRT is correlated to the digester volume and the volume of substrate fed per time unit, Thus, increasing the organic load reduces the HRT. The retention time must be sufficiently long to ensure that the amount of microorganisms removed with the digestate is not higher than the amount of reproduced microorganisms. Knowing the targeted HRT, the daily feedstock input and the decomposition rate of the substrate, it is possible to calculate the necessary digester volume.

Parameter list

A number of parameters can be used for evaluation of biogas plants and for comparing different systems. In order to evaluate the performance capabilities of a biogas plant a multi-criteria analysis should be performed. Evaluations based on a single parameter can never do justice to the process. In order to determine if a biogas plant can provide a return on investment, in an acceptable time frame, economic parameters must always be included.


To make the world a better place to live in by the development of economic, efficient and environmental friendly energy technology solutions.


To create environmental sustainability and energy independence through the growth of the biogas industry thus reducing the dependence n fossil fuels.


Biogas is the one of the only technologically fully established

renewable energy source that is capable of producing heat, steam, electricity, vehicle fuel and other valuable byproducts. It is, in the true sense of the word, a dynamic energy source. Biogas has become a fuel to be looked forward to. This growth has begun over the last two decades.

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