Energy is neither created nor destroyed; it transforms from one form of energy to another.
Everybody is well-versed with this law of thermodynamics. So, it leads to a number of questions about what happens to organic matter when it decomposes.
What can be done to harness the energy that is created owing to organic matter decomposition?
To put it simply, energy production from organic matter decomposition is the ideal outcome that can be expected from any best-out-of-waste strategy!
When an organic material undergoes decomposition, it follows either aerobic digestion or anaerobic digestion pathways depending upon the availability of air and environmental conditions.
Aerobic means in the presence of oxygen, and anaerobic means devoid of oxygen.
Being two completely different types of processes, both the process of digestion is carried out by different sets of bacteria and results in different end products.
The energy released during biodegradation of the organic waste can be harnessed and transformed into a useful product.
The controlled aerobic digestion of organic material can provide us with a nutrient-rich fertilizer known as compost.
Similarly, the process of anaerobic digestion can lead to biogas production, which can be an extremely useful energy source directed towards a myriad of applications.
The natural process of anaerobic digestion occurs in the stomachs of ruminants or in swamps.
In an anaerobic process, microorganisms degrade the organic material to produce biogas, which is a mixture of methane (CH4), carbon dioxide (CO2), hydrogen sulfide (H2S), carbon monoxide (CO), Hydrogen (H2), nitrogen (N2) and water vapor.
Of all the end products obtained from anaerobic digestion, methane is the most interesting gas due to its characteristics. Methane generated from biogas is combustible and can be used as a source of energy.
The process of biogas production is a multistep process carried out by a different set of bacteria at each step.
The organic material undergoes a chain of degradation steps, viz. hydrolysis, acidogenesis, acetogenesis, and methanogenesis.
At the end of this chain of processes, the organic matter is converted into methane and other end products, and this process is known as biogas production.
The organic material added to anaerobic digesters is a complex polymer by nature. The hydrolysis process essentially breaks down the organic macromolecule into smaller compounds.
The hydrolytic bacteria secrete enzymes like extracellular enzymes like amylase, cellulose, protease, and lipase that break down carbohydrates into simple sugars like monosaccharides and disaccharides, and proteins are converted to amino acids, while lipids are converted to different chains of fatty acids.
The optimum pH for hydrolysis is 6 to 7, and a temperature range of 30 to 50 C. This is the first step towards biogas production and is critical because the higher the breakdown of complex matter, the more available substrate for biogas production.
The hydrolysis step provides small molecular compounds as its end product that can pass through the cell membrane of acidogenic bacteria.
The acidogenic bacteria convert precursor compounds into intermediary compounds known as volatile fatty acids (VFAs), alcohols like methanol, ethanol and aldehydes, and CO2.
VFA consists of organic acids such as acetates and larger organic acids like propionate, butyrate, and pentanoate in varying proportions.
Acetate is directly used as a substrate for methanogenesis, but this is not the case with higher organic acids.
Acetogenesis is a process by which higher organic acids such as propionate and butyrate are converted to acetates and hydrogen by acetoclastic bacteria.
From the acetogenesis process, the production of acetates is around 25%, while that of hydrogen is around 11%.
Methanogenesis is the final step of anaerobic digestion. The methanogenic bacteria convert the intermediates from previous steps, like acetates, hydrogen, methanol, and methylamine, into methane and H2O.
Acetate is consumed by acetoclastic methanogens and converted to methane.
Another set of hydrogenotrophic methanogens reduces CO2Â using hydrogen to produce methane during the acetogenesis step and convert it into methane.
The process of methanogenesis happens at a higher pH. 2/3 of the methane produced is generated through acetoclastic methanogenesis, while 1/3 is generated through hydrogenotrophic methanogenesis.
Hydrolysis process is performed by a group of anaerobic bacteria like Streptococcus and Enterobacterium, also known as fermentative bacteria.
They secrete extracellular enzymes, which are adsorbed on the waste subjected to digestion. The hydrolyzed waste is further converted to organic acids by genera of bacteria like Pseudomonas, Bacillus, Clostridium, Micrococcus, or Flavobacterium.
Depending on the diverse population of microorganisms, the acidogenesis process may be carried out in either a hydrogenated process, where the direct end products are acetates, hydrogen, and CO2, or a dehydrogenated process, where the waste is converted to higher organic acids.
Another function of facultative acidogenic bacteria is to create a working environment for obligate anaerobes of the following processes.
The facultative anaerobes utilize any oxygen present in the digester, creating a complete anaerobic condition.
The process of acetogenesis is performed by the genera of Syntrophomonas and Syntrophobacter, which convert butyrate propionate into acetates and hydrogen, and some bacteria like Methanobacterium suboxydans convert the pentanoic acid to propionic acid, while Methanobacterium propionicum converts propionic acid to acetate.
The hydrogen released as a result of acetogenesis is inhibitory to acetogenic bacteria; in such a case, the hydrogenotrophic bacteria work symbiotically to convert the hydrogen released to methane.
This process is also called syntrophy. The final step of methanogenesis is also carried out by two different sets of bacteria depending upon the substrate used for methane generation.
Acetotrophic bacteria such as Methanosarcina and Methanosaete are the most common genera of acetotrophic bacteria that convert methane from acetate, whereas genera Methanobacteriaceae and Methanoculleus are common hydrogenotrophic bacteria utilizing hydrogen as an intermediate for methane generation.
In general, anaerobic digestion is a sensitive process; hence, a specific kind of material should be used. The feed that is used for humans and animals or their byproducts can be used as raw material for biogas production.
In order to decide the feedstock for biogas production, it is necessary to consider the water content, total solids, and volatile solids of that feedstock.
Any material will have water and dry matter.
The dry matter is further classified as total solid content, which is the total mass of solid, and volatile solid, which is the total amount of degradable organic material.
It is recommended to use material with high volatile solid content.
Food byproducts can be used as raw material for biogas production, which can be easily available as market waste, food processing, and restaurants for low cost. Food byproducts can be easily degraded.
It is advisable to use one consistent source of food supply for biogas production in order to have a consistent material for feedstock.
Manure is undigested animal feed with additional water and bedding. It is also a good source of microorganisms.
Cattle manure is mostly preferred, while swine and poultry manure contain high nitrogen content; hence, it can lead to a low C: N ratio of the reaction system.
Fresh manure works best for anaerobic digestion.
The addition of energy crops as secondary feedstock can increase the biogas output. Energy crops include grasses, corn silage, and haylage.
The energy crops usually need to be purchased from the market; hence, the energy output should cover the cost of raw materials.
An anaerobic digester is a processing unit where organic material is anaerobically digested under controlled conditions to aid the formation of digestate and biogas production.
An anaerobic digester is an enclosed, air-tight structure. It has an inlet pipe to ensure the addition of raw material to the digester.
The inlet pipe reaches the digester. The digester is where all the anaerobic digestion takes place.
The digester volume is divided to maintain the required amount of slurry and head space for gas accumulation during biogas production.
On top of the digester, there is a gas pipe with a valve for controlling the gas outflow once the biogas production begins.
There is an outlet area and an overflow area that accommodates excessive slurry and allows it to overflow when required.
The most commonly used operating parameter for anaerobic digestion & biogas production is a one-stage, continuous system functioning under mesophilic conditions where the wet waste is ideally preferred as feedstock.
Considering these parameters and depending on scalability, ideal technology for biogas production are fixed dome reactor, Floating drum reactor and balloon type reactor.
A fixed dome digester is an enclosed system built out of bricks and cement. The digester area is built underground, and the inlet, gas controller valve, and outlet are situated above the ground.
This type of digester has a comparatively long life span among the reactors used for biogas production. Underground construction saves space and negates the temperature fluctuations happening outside.
A floating drum works on a similar principle as that of a fixed dome, but the main difference is that the digester tank is constructed underground while the gas-accumulating drum is floating above the ground.
The drum moves upwards as the gas pressure increases during biogas production, and when the gas is consumed, it comes down due to its own weight floating on a water jacket or slurry.
Gas-holding drums can be constructed using metal, plastic, or fiberglass.
It is a low-cost biogas digester used for biogas production that seems like a tubular balloon that is made from plastics or rubber. The bottom of the digester is placed below the ground in a pit-like area.
The inlet and outlet are made from the same material and are adjoined to the balloon.
Though it is a cost-effective solution, it has a shorter life span, being susceptible to mechanical damage.
Only a great infrastructure is not enough to give you satisfactory methane production. Being a complex system several parameters should be maintained from start to end in order to get a good biogas production.
An organic waste fraction of the feedstock should have high volatile solids. The particle size of the feedstock should be very small at around 2 cm.
A shredder can be used to obtain a feedstock of uniform size, as a large surface area enhances rapid degradation.
These microorganisms carry out the process of biogas production. The addition of cow dung manure slurry, along with specialized microbial consortiums, helps in efficient biogas production.
A minimum of 10 to 15 % of the reactor volume of cow manure is essential to initiate the biogas production process. The mesophilic microorganism takes a longer commissioning time as compared to thermophilic microbes.
The quantum and purity of methane produced in biogas production depend upon the C: N ratio of the input material. The optimum C: N ratio for biogas production is 30: 1.
That is, for every 30 units consumed by bacteria, it requires 1 unit of nitrogen as a source of food.
A correct ratio of carbon and nitrogen can give optimum biogas production via anaerobic digestion.
Organic loading rate means the mass of substrate to be added per day depending on the volume of the reactor to ensure optimized biogas production.
OLR is measured as a kilogram of total volatile solids per cubic meter per day. An ideal OLR for an unstirred tank reactor is <2 kg VS/ m3 per day, whereas a stirred tank reactor requires around 4 – 7 kg VS/ m3 per day.
The favorable temperature range for mesophilic bacteria is between 30 – 40C, and for thermophilic bacteria, it lies between 45 -60C. The colder temperature below 15 C slows down the process. Biogas production is known to be hampered in colder weather.
The optimum temperature for biogas production is neutral, which is from 6.5 to 7.6. The process of acidogenesis lowers the pH to acidic.
At a pH value of 7 to 7.5, the CO2 in the gas phase provides the alkalinity required for buffering the shift in pH caused by VFAs.
In case of pH shock load by acidogenesis, the OLR should be reduced, or the addition of lime or sodium hydroxide can elevate the pH to normal. The pH of the system is the most critical parameter for optimal biogas production.
The duration for which the material stays in the reactor depends upon the size of the reactor and the rate of degradation. A mesophilic reaction process usually takes between 10 to 40 days, depending, while a thermophilic process takes around 10 to 25 days.
Biogas production may just seem like a process where the organic materials can be converted to methane that can be used as a source of fuel, but the process of biogas production has a larger impact environmentally and financially.
In conclusion, Biogas production can turn the cost of biological solid waste management into a revenue-generation opportunity for countries.
Biogas production can lead to alternatives for heating, transportation fuel, and electricity generation and can, therefore, reduce greenhouse gas generation and dependency on fossil fuels and create jobs in otherwise increasingly volatile economies around the world.
Not just this, the digestate created as a result of biogas production can be used as an alternative to chemical inputs used to fertigate soil in agriculture.
Considering its many advantages, biogas production is the key to a greener and better future.
