Energy is neither created nor destroyed, it transforms from one form energy to another. Every body 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 simple, energy production from organic matter decomposition is the most 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 pathway 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 set of bacteria and results in different end products. The energy released during biodegradation of the organic waste can be harnessed to and transformed into a useful product. The controlled aerobic digestion of organic material can provide us with nutrient rich fertilizer known as compost. Similarly the process of anaerobic digestion can lead to biogas production which can be an entremely useful energy source directed towards a myriad of applications.

The natural process of anaerobic digestion occurs in stomachs of ruminants or in swamps. In an anaerobic process, microorganisms degrade the organic material to produce biogas, which is mixture of methane (CH₄), carbon di oxide (CO₂), hydrogen sulfide (H₂S), carbon monoxide (CO), Hydrogen (H₂), nitrogen (N₂) and water vapor. From all the end products obtaining 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 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.

Hydrolysis

The organic material added to anaerobic digesters are complex polymers by nature. The hydrolysis process essentially breaks down the organic macromolecule into smaller compounds. The hydrolytic bacteria secretes enzyme like extracellular enzymes like amylase, cellulose, protease and lipase that breaks down carbohydrates into simple sugars like monosaccharides, disaccharides, proteins are converted to amino acids , while  lipids are converted to different chain of fatty acids. 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.

Acidogenesis

The hydrolysis step provide small molecular compounds as its end product that can pass through the cell membrane of acidogenic bacteria. The acidogenic bacteria converts precursor compounds into intermediary compound known as volatile fatty acids (VFA’s), 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.

Acetogenesis

Acetate is directly used as substrate for methanogenesis but this is not the case 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, production of acetates is around 25% while that of hydrogen is around 11%.

Methanogenesis

Methanogenesis is the final step of anaerobic digestion. The methanogenic bacteria convert the intermediates from previous steps like acetates, hydrogen, methanol, methylamine into methane and H₂O. Acetate is consumed by acetoclastic methanogens and converted to methane. Another set of hydrogenotrophic methanogens reduces CO₂ using the hydrogen to produce methane produced during acetogenesis step and convert it into methane. The process of methanogenesis happens at higher pH. The 2/3 of the methane produced is generated through acetoclastic methanogenesis while 1/3 is generated through hydrogenotropic methanogensis.

Hydrolysis process is performed by the 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 microorganism the acidogenesis process may be carried out in either hydrogenated process where the direct end products are acetates, hydrogen and CO₂ or dehydrogenated process where the waste is converted to higher organic acids. Another function of facultative acidogenic bacteria is to create working environment for obligate anaerobes of following processes. The facultative anaerobes utilize any oxygen present in the digester creating a complete anaerobic condition. The process of  acetogenesis are performed by the genera of Syntrophomonas and Syntrophobacter convert butyrate, propionate into acetates and hydrogen and some bacterias like and Methanobacterium suboxydans that converts 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 hydrogenotrphic bacteria works symbiotically to convert the hydrogen released to methane. This process is also called as 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 most common genera od acetotrophic bacteria which coverts methane from acetate whereas genera Methanobacteriaceae and Methanoculleus are common hydrogenotrophic bacteria utilizing hydrogen as intermediate for methane generation.

In general the anaerobic digestion is a sensitive process hence the specific kind of material should be used. The feed that are used for human and animals or their byproduct can be used as raw material for biogas production. In order to decide the feedstock for biogas production it 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 that is total mass of solid and volatile solid which is total amount of degradable organic material. It is recommended to use material with high volatile solid content.

Food by-products-

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- 

Manure is undigested animal feed with addition water and bedding.

It also a good source of microorganisms. Cattle manure is mostly preferred while swine and poultry manure contains high nitrogen content hence it can lead to low C: N ratio of reaction system. Fresh manure works best for anaerobic digestion.

Energy crops-

Addition of energy crops as secondary feedstock can increase the biogas output. Energy crops include grasses, corn sileage, haylage. The energy crops are usually needed to be purchased from the market hence the energy output should cover the cost of raw material.

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 addition of raw material to the digester. The inlet pipe reaches digester. Digester is where all the anaerobic digestion takes place. The digester volume is divided to maintain required amount of slurry and head space for gas accumulation during biogas production. On top of 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.

Fix dome digesters- 

A fixed dome digester is an enclosed system built out of bricks and cement. The digester area is built under ground and the inlet, gas controller valve and outlet are situated above the ground. This type of digester has comparatively long life span amongst the reactors used for biogas production. Underground construction saves space and negates the temperature fluctuations happening outside.

Floating Drum Digester- 

Floating drum works on the similar principle as that of 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 water jacket or slurry. Gas holding drum can be constructed using metal, plastic or fiberglass.

Balloon type digester- 

It is low cost biogas digester used for biogas production that seems like tubular balloon which 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 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 a satisfactory methane production. Being a complex system several parameters should be maintained from start to end in order to get a good biogas production.

Feedstock- 

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 large surface area enhances rapid degradation.

Microbial Inoculum- 

These microorganisms carry out process of biogas production. The addition of cowdung manure slurry along with specialized microbial consortium help in efficient biogas production. Minimum of 10 to 15 % of 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.

Carbon to nitrogen ratio (C: N) –

The quantum and purity of methane produced in biogas production is depended 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. Correct ratio of carbon and nitrogen can give optimum biogas production via anaerobic digestion.

Organic Loading rate ( OLR) – 

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 kilogram of total volatile solids per cubic meter per day. An ideal OLR for unstirred tank reactor is <2 kg VS/ m³ per day whereas for a stirred tank reactor it requires around 4 – 7 kg VS/ m³ per day.

Temperature – 

The favorable temperature range for mesophilic bacteria are 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.

pH – 

The optimium temperature for biogas production is neutral that 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 gas phase provides the alkalinity required for buffering the shift in pH caused by VFA’s. In case of pH shock load by acidogenesis the OLR should be reduced or 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.

Hydraulic retention time –

The duration for which the material stays in the reactor depends upon the size of the reactor and rate of degradation. A mesophilic reaction process usually takes between 10 to 40 days depending, while thermophilic process takes around 10 to 25 days.

1. Biogas is a renewable source of energy containing methane as combustible gas. Biogas can help in reducing the dependability on natural gas or fossil fuels. The biogas can be directly connected to household cooking stoves with some modifications.
2. The biogas can be used to generate electricity using generator set or combustion motors. The biogas is compatible with almost all combustion engines like gas engines, diesel engines, gas turbines etc. Biogas acts as fuel for combustion engine which converts it to the mechanical energy; this mechanical energy powers the electric generator to produce electricity.
3. New development of technologies allows the use of biogas in transportation. Compressed biogas can be used to run vehicle engines. Many European countries have started to use vehicles running on compressed biogas.

Biogas production may just seem as 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 larger impact environmentally and financially.

1. At first the process of biogas production reduces the load of solidwaste management. Many landfills have installed biogas digesters to dispose the organic waste with known high volatile solid content into an enclosed chamber and get useful end products from it. The segregation of organic waste from garbage can help reduce the load on landfills.
2. The biogas is a clean and renewable source of energy, where the energy can be continuously harnessed by addition of waste organic material. Methane combustion produces no green-house gases making it ecofriendly. Harnessing biogas for industrial processes & transportation could be key to reducing the dependency on fossil fuels.
3. For many developing countries it can be source if fuel for cooking. Many big canteens dispose their organic waste material into the biogas digester and the methane obtained from the process is used for cooking. Biogas production can help make the hotel and restaurant industry self-sufficient and help decrease costs.
4. The solid digestate remaining after the digestion process is a stable product that can be used in agriculture as organic fertilizer. The digestate is a highly nutritious slurry containing nitrogen, phosphorus, potassium and trace elements. Most of the nitrogen being available in the form of ammoniacal nitrogen, which are known to be directly assimilated by plants.
5. Many countries have developed huge power station grid that run on the process of biogas productionfor generation and supply of electricity to cities.
6. The process of biogas production can be used even for waste water treatment. Various technologies in waste water treatment like UASB allow biogas productionfrom effluents with high organic load.
7. Due to availability of digester in variable sizes, biogas productioncan be carried out at landfill sites to even household level.

1. Requirement of skilled labor- Biogas productionneeds a controlled environment to function efficiently. It needs to be constructed using skilled labor as slightest mistake can cause it to dysfunction.
2. Downstream processing- Even though biogas contains large portion of methane, it also contains other gases as the end product. The biogas needs to be purified in order to efficiently use methane gas. The water vapor needs to be trapped and separated using a T junction pipe. Desulphurization process helps in removing H₂S gas.
3. High capital investment-To ensure efficient biogas production with coupled with a higher life span for the bioreactor requires high capital investment. Therefore, unless constructed by government authorities, biogas treatment system is not affordable to most of the people in developing countries.
4. Maintenance-Constant monitoring and repairing is a must for any bio-digester tank. Gas leaks or any mechanical damage due to wear and tear or corrosive gases needs an early fixing in order to assure good operating conditions.
5. Presence of pathogens-The digestate used from mesophilic digester as organic fertilizer may contain some pathogenic bacteria or weed seeds unlike thermophilic system. Hence the digestate needs to be treated before addition as organic fertilizer to an agricultural field
6. Energy recovery-More development in technologies should be made for efficient use of methane for electric generation. Currently only 30% of the biogas energy is transformed as electricity.

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 gerneration, dependency on fossil fuels and create jobs in otherwise increasingly voltile 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.