Biogas Production Process
Four steps of fermentation



Scheme 1. Metabolism products of the anaerobic fermentation

Bacteria decompose the organic matter in anaerobic environment. Biogas is an intermediate product of their metabolism.

The decomposition process can be divided into 4 steps (see scheme 1) each of those accompanied by different bacteria groups:

In the first stage aerobic bacteria reconstructs high-molecular substances (protein, carbohydrates, fats, cellulose) by means of enzymes to low-molecular compounds like monosaccharide, amino acids, fatty acids and water. Enzymes assigned by hydrolysis bacteria decompose substrate components to small water-soluble molecules. Polymers turn into monomers (separate molecules). This process called hydrolysis.

Then decomposition is made by acid-forming bacteria. Separate molecules penetrate into bacteria cells where further transformation takes place. This process is partially accompanied by anaerobic bacteria that consume rest of oxygen hence providing suitable anaerobic environment for methane bacteria.

This step produces:

  • acids (acetic acid, formic acid, butyric acid, propionic acid, caproic acid, lactic acid),
  • alcohols and ketones (methanol, ethanol, propanol, butanol, glycerin and acetone),
  • gases (carbon dioxide, carbon, hydrogen sulfide and ammonia).

The step is called oxidation.

Afterwards acid-forming bacteria form initial products for methane formation: acetic acid, carbon dioxide and hydrogen). These products are formed from organic acids.

For vital functions of these bacteria that consume hydrogen, stable temperature mode is very important.
 
  

The last step is methane, carbon dioxide and water formation. 90% of methane yield takes place at this stage, 70% from acetic acid. Thus acetic acid formation (3rd step) is the factor that defines the speed of methane formation .

One and two stages process


Scheme 2. One and two stages methane production process.
 
In most cases such processes take place simultaneously it means that there is no boundaries for place and duration of decomposition. Such technology is called two stages technology. For fermentation of rapidly decomposable raw materials in pure state two stage technology required. For example chicken dung, distillery slop shouldn’t be recycled in one digester.

In order to process those substrates hydrolysis reactor is needed. Such reactor allows control over the acidity and alkalinity level in order to avoid bacteria collapse and increase methane yield. (Scheme 2.)

For successful lifecycle of all microorganisms inside the digester special conditions must be secured. Mandatory factors for that are the following:

Anaerobic environment - active functioning of bacteria is possible only in oxygen-free conditions.
Biogas plant design takes that into consideration.

Humidity - bacteria can live, feed and propagate only in moist conditions.

Temperature - the optimum temperature for mode for all bacteria groups is 35-40о С range. Human is not able to control this, that is why it is done by automatic control system.

Fermentation period - The quantity of produced biogas is different within the fermentation period. In the beginning of fermentation it is more intensive then at the en of it. Then comes the moment when further biomass presence in the digester is economically unfeasible.

Our specialists rest upon long-term experience while calculating fermentation period efficiency.
   Middle scale biogas plant





pH level - hydrolysis and oxidation bacteria can live in acid environment with pH level 4.5-6.3 while methane and acetic acid formation bacteria can exist only in low alkalinity environment with pH 6.8-8. All the bacteria kinds have tendency to suspend their activity in case pH level is higher of the optimum hence the biogas production suspends as well. That is why the best pH level 7 should be maintained.

Even substrate feed - the by-products of each group of bacteria lifecycle are the nutrients for other bacteria group. The all work with different speed. The bacteria should not be overfeed as they hardly be able to produce nutrients for another group. That is why the substrate feed is calculated and programmed for each project carefully.

Nutrients supply - bacteria provided with all necessary nutrients that are contained in substrate so the only thing is needed is constant substrate supply. Substrate contains vitamins, soluble ammonia compounds, microelements and heavy metals in small quantities. Nickel, cobalt, molybdenum, wolfram and ferrum are required by bacteria for enzyme formation and are also present in substrates.

Particle size - The smaller the better rule is working here. Bacteria size 1/1000 mm the smaller the substrate particles the easier the decomposition made by bacteria. Fermentation period becomes shorter and biogas production faster. If necessary additional substrate disintegration should be done before substrate feed into reactor.

Mixing - is important not only to avoid floating cork and sediment formation but also for biogas extraction (mixers help bubbles to go up the digester). Mixers work constantly in a bacteria preserving mode.

Process stability - microorganisms are used to certain feed other modes.
Any changes should be done smoothly.

Avoid getting into reactor antibiotics, chemical and disinfection means, big quantities of heavy metals. Our specialists can advice you on that.

The end product of biological treatment are:

biogas (methane not less then 55%, carbon dioxide not more then 45%, hydrogen sulfide not more then 2%, hydrogen not more then 1%);
fermented substrate as fermentation residue, consisting of water, cellulose residues, small quantity of bacteria and organic nutrients (nitrogen, phosphorus, potassium etc.).
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