The Biology Topics of biotechnology involve using living organisms to develop new products or solve problems.
Microbial Responses to Different Operating Practices for Biogas Production Systems
Microbes in the Production of Biogas
Biogas typically refers to a gas produced by the breakdown of organic matter in the absence of oxygen. It is a renewable energy source like solar and wind energy. It is environmentally friendly. Biogas or gobar gas is methane-rich fuel gas produced through anaerobic breakdown and fermentation of biomass.
Composition of Biogas
It contains 50%-70% methane, 30%-40% CO2, traces of hydrogen, hydrogen sulphide, and nitrogen. Half of the energy present in the organic wastes can be converted to methane gas with a calorific value of 23-28 MJ/m3. The residue is used as manure. It is rich in lignin, part of cellulose, and minerals. Gobar gas generation has been taken up in India on a large scale. There are over a million individual and several thousand community biogas plants operating in the country. It also contains cyclohexene and humidity.
The bacteria involved in the conversion of biogas to methane are-
1. Hydrolytic bacteria:
Bacillus, Clostridium, Escherichia and Ruminococcus. These bacteria break breakdown cellulose and protein to produce hydrogen, CO2, biurea, fatty acid, and ethanol.
2. Acidogenic bacteria:
Desulphovibrio, Syntrophomonas. The products formed by the hydrolytic bacteria are transformed into acetate by these bacteria.
3. Methanogenic bacteria:
Methanobacterium formicicum, M. sohngenii. They converted acetate to methane. The raw materials used in biogas plants are cattle dung, night soil, farm refuse, water weeds (e.g., Eichhornia crassipes), and other organic wastes. These are converted into slurry with 90% water content and fed to the digester. Cattle dung contains Methanobacterium and other methanogens which are normally present in the rumen of cattle for aiding in the digestion of cellulose.
Amount of organic wastes and production of biogas as a source of energy:
|1. 10-20 kg cowdung||1 cubic meter gas (m3)|
|2. 25-40 kg human faeces||1 cubic meter gas (m3)|
|3. 8-15 kg drooping of chicken||1 cubic meter gas (m3)|
|4. 4-10 kg vegetable and outer covering of the fruit||1 cubic meter gas (m3)|
In India, China, and the USA, Eichhornia is used as biogas. Biogas can be used as a fuel in any country for heating purposes. It can also be used in anaerobic digesters where it is typically used in a gas engine to convert the energy in the gas into electricity and heat. A biogas plant has a large air-tight cylindrical tank which is called a digester. A digester is made up of concrete bricks and cement or steel. It has a side opening (charge pit) into which organic materials are incorporated for digestion. Above the digester, there lies a cylindrical container to collect the gas.
Anaerobic Digestion System
An anaerobic digestion system can provide an optimal environment for controlled anaerobic digestion. A typical system consists of liquid manure handling equipment, a heated anaerobic digester, gas utilization equipment, safety equipment, and effluent storage and handling systems. The anaerobic digestion system is an addition to the manure handling scheme a step for manure processing between the barn and the storage facility. It does not replace any part of a typical manure handling system.
Liquid Manure Handling System:
A liquid manure handling system (such as the system used to transport liquid manure from a barn to a storage facility) transports manure from the animal housing facility to the anaerobic digester, and from the digester to the storage facility or spreader. When possible, the use of gravity flow is encouraged to reduce the energy consumption and complexity of the handling system. A bypass line routes manure around the digester when the manure is unsuitable for digestion or the digester is not operating.
Anaerobic Digester: An anaerobic is a sealed, heated tank that provides a suitable environment for naturally occurring anaerobic bacteria to grow, multiply, and convert manure to biogas and a low-odour effluent. Typical digesters have been insulated, squat, silo-like structures or in-ground rectangular or round concrete tanks. Rigid or flexible covers have been used. They are designed to hold about 20 days of manure and a small supply of biogas. Manure, added daily to the digester, remains inside for about 20 days, the retention time, before flowing to the storage facility or spreader. Because there is no volume reduction with anaerobic digestion, the same amount of material added daily to the digester is also removed daily.
While manure is flowing through the digester, the bacteria convert organic matter to biogas and effluent. During the retention time, lightweight material such as bedding or animal hair can float to the top of the digester, forming a crusty scum, and heavy or insoluble material such as dirt can settle to the bottom. Settling reduces the effective volume of the digester and can cause incomplete digestion and odour problems, while crusting can keep gas from escaping the surface of the digesting manure. To control settling and scum formation, material in the digester can be agitated by a slurry pump, a mechanical stirrer, or strategic placement of the heating pipes. Slurry pumps are an effective way to keep material in the digester well-mixed.
Mechanical mixing adds complexity to the system but can aid thermal uniformity, reduce settling, and break up crust formation. Mechanical mixing may be necessary for certain manure handling systems such as flush systems where solid and liquid portions may separate easily into distinct layers within the digester. Strategic placement of the heating pipes will encourage thermal circulation and reduce settling problems.
The heating system is a critical part of the anaerobic digester. Heating pipes in which hot water circulates must be able to heat all material entering the digester to 95°F and to resist corrosion from manure. Adding manure to the digester as soon as possible after it is excreted from the animal will help minimize heating requirements. In this enclosed structure the anaerobic microorganism converts the organic matter into biogas which is utilized for energy.
Key Process of Anaerobic Digestion:
Anaerobic digestion takes place in three steps – Solubilisation, Acidogenesis, and Methanogenesis.
It is the initial stage when raw material is solubilized in water to make a slurry. Raw materials (organic wastes) are composed of lipids, proteins, cellulose, hemicellulose, and lignin. In the first stage of biogas generation, facultative anaerobic decomposers are active. They secrete hydrolytic enzymes – lipases, cellulases, proteases, peptidases etc. They break down the complex organic components into simpler and soluble substances. Simple and soluble substances are also called
In this stage, monomers are acted upon by fermentation-causing microbes. Monomers are converted to organic acids. Organic acids, especially acetic acid, are acted upon by methanogenic bacteria in the final stage.
Methanogens act on various components of microbial digestion and fermentation. Methane, carbon dioxide, and water are formed. The primary reaction is CO + H2O → CO2 + H2
The secondary reaction takes place in the presence of sufficient hydrogen, where carbon dioxide reacts with Hydrogen and form methane.
CO2 + 4H2 → CH4 (methane) + 2H2O
Other reactions showing methane formation from various substrates are
The biogas thus formed is stored in tanks for supply.
Different Factors in the Production of Methane
- Slurry: Liquefied organic substance, water, and solid particle in 1 : 1 ratio.
- Seeding: Slurry is mixed with sludge as a digester because sludge contains acetogenic and methanogenic bacteria.
- pH: 6 – 8.
- Temperature: For thermophilic digestion 50°C-60°C and for mesophilic digestion 30°C-40°C.
- Carbon and Nitrogen ratio: 30 : 1.
- For anaerobic respiration or fermentation, the digester must be airtight.
- Addition of Algae: If cowdung is mixed with algae, the production of biogas increases.
Advantages of Methane Usage
- Methane is insoluble.
- Can be separated from the fermenter system.
- Can be collected for storage.
- Directly inflammable.
- Provide both energy and manure.
- More economical.
- Utilization of agricultural residue as a source of energy.
- Reduce environmental pollution.
- Utilized for various purposes, e.g., looking and lighting in the domestic sector, and operating diesel engines for water pumps.
- The overall thermal efficiency is high.
- Can be manufactured locally and requires very little maintenance
Role of Bacteria in Biogas Production
Methane-forming bacteria are more sensitive to their environment than acid-forming bacteria. Acid-forming bacteria can survive under a wide range of conditions while methane-forming bacteria are more demanding. Under the conditions typical of liquid manure storages, more acid-forming bacteria can survive than methane-forming bacteria. Therefore, acids are formed and are not converted to biogas. This excess of volatile acids can result in a putrid odour. In a controlled, optimum environment, methane-forming bacteria survive and convert most of the odour-producing volatile acids into biogas. Conditions that encourage the activity of both acid and methane-forming bacteria include:
- An oxygen-free environment.
- A relative constant temperature of about 95°F.
- A pH between 6.6 and 7.6.
- A consistent supply of organic matter to “feed” upon.
For consistent operation of an anaerobic digester, the manure that “feeds” the bacteria should be:
- A flowable liquid, about 12% solids or less (for pump or flow requirements).
- Not frozen.
- Free from excess amounts of medication, feed additives, or chemical washes.
- Supplied fresh to the digester at least twice a day.
- A uniform slurry of manure that does not separate easily, such as:
- Dairy manure from scrape systems which can include small amounts of fine, organic bedding such as sawdust, waste feed, milking center waste, or dilution water.
- Swine manure from pull-plug or scrape systems.
- Poultry manure was diluted to about 10% solids with the grit settled out.
Acid-forming bacteria can Survive:
- With temperature fluctuations.
- In a wide range of pH conditions.
- With or without oxygen.
- On a broad range of organic compounds as a food source.
Methane-forming bacteria can survive only:
- If the temperature is held relatively constant.
- In a narrow band of pH conditions.
- Without oxygen.
- On simple organic acids as a food source.
Anaerobic digestion is simply a continuation of the animal’s digestive system – a process to turn manure into energy and effluent, just like an animal turns feed into energy and manure.
Applications of Biogas
Biogas is generally produced in anaerobic conditions, by the breakdown of organic matter and giving rise to methane, and carbon dioxide. Those gases along with hydrogen generally crammed with oxygen. Nowadays biogas is used to run different types of heat engines to generate mechanical power. Biogas is basically a type of biofuel that is renewable unlike fossil fuels and is originated from biogenic material.
1. When biogas is used, many advantages arise. In North America, utilization of biogas would generate enough electricity to meet upto 3% of the continent’s electricity expenditure. In addition, biogas could potentially help reduce global climate change. Normally, manure that is left to decompose releases two main gases that cause global climate change – nitrogen dioxide and methane. Nitrogen dioxide (NO2) warms the atmosphere 310 times more than carbon dioxide and methane 21 times more than carbon dioxide.
2. By converting cow manure into methane biogas via anaerobic digestion, the millions of cows in the United States would be able to produce one hundred billion kilowatt hours of electricity, enough to power millions of homes across the United States. In fact, one cow can produce enough manure in one day to generate 3-kilowatt hours of electricity; only 2.4-kilowatt hours of electricity are needed to power a single one hundred-watt light bulb for one day. The following flow chart of anaerobic digestion and its applications are as follows.
Advantages of Biogas
- The most economical benefits are minimizing environmental pollution and meeting demands.
- Sludge is used as fertilizer.
- It has other applications other than in cooking and lighting.
- This can be stored and used more efficiently and economically.
- It minimizes the chances of the spread of faecal pathogens.
- Sanitation and health care, therefore, improved.
- Water weeds like Eichhornia can be utilized profitably.
- The cost of production is less.