SIG 5
What does biogas contain?
Biogas contains mainly CH4 (methane, 55-75%) and CO2 (carbon dioxide, 25-40%). But that is not all, biogas contains smaller levels of CO (carbon monoxide), H2 (hydrogen gas), N2 (nitrogen gas), O2 (oxygen gas) and sulfur hydrogen. If the level of sulfur hydrogen rise above it´s maximum level of 1.5% it will most likely kill off all the bacteria in the process and therefore stop the process.
What are the different types of processes called?
Psykrophile, this process works at 10-20°C, this process can only occur if the bacteria is able to withstand these low temperatures. This process requires long retention time as well as a quite large volume.
Mesophile, this process works at 20-40°C, this process requires a much smaller volume compared to Psykrophile, the bacteria used in this process is called mesophiles, this is the most commonly used process today.
Thermophile, this process works at 50-60°C, this process either needs heating or works with warm wastewater, it is very useful when pathogenic organisms need to be neutralized.
What different materials would you start with in a biogas production?
Today the used materials are either Agricultural products, Domestic waste, Livestock waste, Algae, Industrial waste or Sludge. Biogas is most produced from decomposition of wastes in farming sewage treatment, the cleaning up of wastewater or taking care of wastes in all communities. This is good since the global production of waste has increased in the recent years (biogas review pdf).
What factors affect the biogas production?
Temperature determines what type of bacteria is used in the process and how long the process will take
pH-level determines in which form the acids are in COOH in low pH and this determines the conversions in the acetogenesis. The last step of the fermentation, methanogenesis where the methanogenic bacteria is involved, the pH-level is absolutely crucial and should be 6.5-7.5, nearly neutral.
Anaerobiosis determines whether the cells will grow or of fermentation actually will take place, the anaerobiosis tells us the lack of oxygen and lack of oxygen is crucial for fermentation as known.
Micronutrients are needed for the enzymes involved in the different processes
Toxic metals will gradually or totally stop the process.
Describe the 4 parts of the biogas production.
There are four parts of the fermentation process, hydrolysis, acidogenesis, acetogenesis and methanogenesis.
Hydrolysis is the first process where high-molecular compounds such as carbohydrates, lipids, and proteins, in a process mediated by saprophytic bacteria with the participation of extracellular enzymes to produce low molecular compounds such as volatile fatty acids, alcohols, aldehydes, CO2 and H2. These products of decomposition are then being used as energy and carbon sources for microorganisms found in the decomposed organic matter which mediate in the subsequent stage, acidogenesis. During hydrolysis, hydrolytic bacteria colonize the surface of the solid waste that is being used. It also releases enzymes on the surface of the material undergoing decomposition, this enables the degradation of the organic compounds under the influence of bacteria.
Acidogenesis is the second process of the fermentation, during this part acidogenic bacteria converts product from the hydrolysis process as well as water soluble products into alcohols (methanol and ethanol), aldehydes, CO2, H2, short-chain organic acids and acetic acids.
Acetogenesis is the third part of the fermentation process and is where acetic bacteria converts propionate to acetate, glucose to acetate and ethanol to acetate, these can then be used by methanogenic microorganisms as substrate. During this process, H2 is released which has a toxic effect on the organisms that carries out this stage which explains the importance of symbiosis of acetic bacteria with autotrophic methane microorganisms, as the latter can use hydrogen as substrate. Nearly 70% of methane are produced in the acetogenic process.
Methanogenesis is the fourth and last stage of the fermentation process and is where methanogenic bacteria converts hydrogen, carbon and acetic acid dioxide to methane. Acetic acid is a direct substrate for methanogenic microorganisms and therefore an improved conversion of acetic acid can strengthen this stage. This is the part of the fermentation which is most sensitive to changes in pH, if the pH drops below 6, the methanogenic microorganisms will die. Methanogenic microorganisms are strict anaerobic and even the slightest presence of oxygen can kill these microorganisms. These microorganisms are as called chemolithotrophic, which means that they can use CO2 as source for carbon.
How can hydrogen be metabolized biologically?
Hydrogen can be metabilzed in two different enzymes, either using the hydrogenases or the nitrogenases. The hydrogenases creates hydrogen from protons and electron, easy formula. The nitrogenases creates mainly ammonia, but as a rest product we get hydrogen, hard formula, includes N2, H+, e- and energy from atp. There is an enzyme called uptake hydrogenase which inhibits the production of hydrogen, it is in the same cell as nitrogenase, hence why we want to get rid of this uptake hydrogenase enzyme to produce more hydrogen.
What are the different organisms that can produce hydrogen?
Some eucaryotes, photosynthetic algae, photosynthetic bacteria, heterotrophic bacteria, nitrogen-fixing bacteria.
What is characteristic for photoproduction of hydrogen in algae?
There is no evolutionary reason for the plant to produce hydrogen in photoproduction, hence it has to be forced to produce hydrogen in some way. This is because it usually produces biomass instead, this is inhibited by including a hydrogenase in the chloroplast after the ferrodoxin. This disables the production of NADPH because the proton and electron used for NADPH is used for H2 instead.
What is characteristic for hydrogen in heterotrophic conditions?
The process needs carbohydrates but cannot produce any by themselves because no photosynthesis, hence in some way we need to provide the process with these carbohydrates.
How can hydrogen be produced by nitrogen-fixation?
The nitrogen fixation enables the process through the nitrogenase which creates hydrogen. However in the same cell, there is another enzyme that inhibits this production (or actually takes up the produced hydrogen), we need to get rid of this enzyme.
What is special with cyanobacteria?
Cyanobacteria is one of the only bacteria that can do photosynthesis, they are also able to fixate nitrogen and therefore are able to produce hydrogen.