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Small biogas plants utilize various organic materials such as, waste generated at home, agricultural residues, animal manure such as cow, goat and chicken dung, food waste and energy crops like maize and grass. All these materials are broken down by microorganisms in anaerobic conditions to produce biogas and digestate.
Small biogas plants produce energy, hence they must be sustainable, efficient and well-thought-out to match their intended role. Here is a type and design based on its size and operation based on small biogas plants:
Modular biogas plants
These small biogas plants are meant for easy installation and operation. This kind of plant can be built in stages according to the capacity needs. Modular plants tend to be aspheric and easy to expand when necessary. They are mostly used in small farms or households.
Floating drum digester
This design has a floating gas holder which moves up and down as the gas accumulates and decreases. The manufactured gas is stored in the upper part of the drume, while the slurry is contained in the bottom chamber. The design is very simple and sometimes a bit hard to operate because the gas storage elements are difficult to seal appropriately. The digester is usually constructed from bricks and cement or steel.
Biodisk digester
This is a relatively new small biogas plant design that is used in the production of biogas. It has been observed that such a small biogas plant produces a large amount of energy. This digester is based on a rotating biodisk system. This system looks like two disk-shaped containers placed one inside the other. These disks are constructed from materials that support the growth of microorganisms.
Fixed dome biogas plants
This is a traditional design that has a dome-shaped gas holder and a fixed, airtight fermentation chamber. This method is very suitable for rural areas. The digester is mostly constructed from bricks, cement or stone. Such a biogas plant is used to convert waste into energy.
Plug flow digester
This digester is long and narrow, and organic materials are inserted from one end and broken down and biogas is obtained at the other end. This digester works well with solid materials that are crushed and mixed with water so that they can fit into the digester. The digester is mostly constructed from concrete or steel.
Small biogas plants offer an effective means for different industries to manage waste, reduce environmental impact and produce renewable energy. Some of the key industrial applications of small biogas plants include:
Renewable energy production
Small biogas plants which are used to produce energy produce energy in the form of electricity and heat. The generated electricity can be utilized for own consumption or be fed into the electrical grid. This is especially valuable in areas where there is no electricity supply. Produced heat can also be used in heating systems or for cooking.
Agriculture and rural development
Small biogas plants are used on farms to convert animal waste, such as manure to energy. These then reduce odor, minimize pathogens and provide digestate as a nutrient-rich fertilizer. This supports crop production. Small biogas plants in rural areas also help decrease the dependence on firewood and fossil fuels, which reduces deforestation and carbon emissions. This keeps air pollution at a minimal to zero level.
Waste management and treatment
Small biogas plants help treat organic waste such as food waste, agricultural residues and livestock manure. This reduces the amount of waste that goes to landfills and reduces greenhouse gas emissions. Methane is a common gas that is released from waste in landfills and it is very potent. By utilizing waste in biogas production, it reduces the smell, minimizes pathogens and offers a productive use of resources.
Decentralized energy solutions
In remote or off-grid areas, small biogas plants provide a reliable source of electricity and heat. They support communities, especially in developing countries, for lighting, cooking, and powering small machines. This reduces reliance on imported fossil fuels.
Food and beverage industry
The food and beverage industry generate a lot of organic waste, from food scraps, peelings, spent grains and wastewater. Some biogas plants convert this waste into biogas. This not only helps to manage waste but also offsets energy costs. This energy can be in form of heat or electricity.
Wastewater treatment plants
Biogas is produced during the breakdown of organic matter in conventional anaerobic bacteria in the sludge. This is done by capturing methane and carbon dioxide. This biogas can be used to generate electricity, which can be used to offset the energy needs of the treatment facility.
Biogas production
Biogas is the main output of small biogas plants. Its production varies based on the type of feedstock and digester used. It can range from 0.4 to 0.8 cubic meters per kilogram.
Digester volume
The digester volume or biogas plant chamber size also varies based on the system. It can range from 1 cubic meter to more than 20 cubic meters.
Retention time
The hydraulic retention time or HRT is the time that the feedstock stays in the digester. It can range from 15 days to several weeks, depending on the type of digester and feedstock used.
Digestate
The remaining material after biogas production in small biogas plants is called digestate. It is rich in nutrients and used as fertilizer for crops.
Choose the site
When choosing where to locate a small biogas plant, it is very important to consider accessibility for feedstock and water, proximity to infrastructure and impact on local community and environment.
Build the biogas digester
The type of digester selected is dependent on the scale of the biogas production, the available space and the budget. Building a biogas digester requires basic construction skills and materials.
Setup the biogas collection system
The digester needs to have a gas collection system which comprises a gas hose or a gas bag. The gas holder should be connected to the digester with a gas outlet hose.
Install a gas management system
A biogas plant should have a gas management system that comprises valves and pressure gauges. These allow for monitoring and controlling the biogas within the system.
Prepare the feedstock
Small biogas plant feedstock should be prepared before mixing and loading into the digester. The feedstock can be chopped into smaller pieces to make it easier to digest.
Maintain the digester
One thing that should be done is to make sure the digester is sealed properly from any type of water or air entry. Water and air can disrupt the biogas production process. The biogas area should be cleaned to prevent accumulation of waste or hazards.
Maintain gas system
The gas collection system, which comprises the hose or gas bag should be examined regularly. Look out for any type of holes, cracks or damage in the gas bag.Keep the gas hose away from sharp objects to prevent leakage.
Maintain the gas management system
Gauges and valves should be monitored for any type of wear and tear. A pressure gauge shows the biogas levels so it must be checked for any damage, while a valve needs to be maintained to ensure it is not leaking gas.
Digestate management
The nutrient-rich liquid that is left in the digester after the biogas has been produced is called digestate. It needs to be removed at least once a year in annual maintenance.
Biogas plant production must meet certain quality requirements to make them efficient and safe for operation.
Biogas composition
Even though the biogas that is produced in small biogas plants normally contains composition proportions of 60% methane, 40% carbon dioxide, and trace gases such as hydrogen sulfide and ammonia, the gas composition should always be analyzed. This is because variations in biogas composition can greatly affect the energy value and cause the biogas plant to corrode or foaming to occur.
Feedstock quality
Users should only add high-quality organic matter to the small biogas plant digester. This is because low-quality feedstock will produce less biogas. Also, avoid feedstock that has high lignin or cellulose content because it is hard to digest. Low nutrient availability in the feedstock should also be considered.
Pneumatic safety
Avoid open flames, smoking, or using any type of device that generates sparks around the biogas plant area. Biogas which contains methane is highly flammable. Also, methane can accumulate to dangerous levels therefore it needs to be ventilated properly.
Digester sealing and maintenance
The digester lid should be sealed properly to prevent ammonia gas emission. Ammonia is a colorless gas that has a strong smell and causes irritation to the eyes, nose, and throat. This can also be potentially harmful to human health.
Hydrogen sulfide removal
Hydrogen sulfide commonly occurs in biogas. It is a toxic and dangerous gas that poses serious health and safety hazards to workers when exposed to in high concentration. The most common preventive method to deal with the gas is by installing a proper ventilation system in the biogas plant area to prevent its accumulation.
Use protective equipment
Protective equipment such as gloves, respiratory protection, and safety glasses should be utilized when working on small biogas plants. This reduces or eliminates exposure to harmful gases, chemicals or potential hazards.
A1. Biogas plants are mostly used to convert waste into renewable energy. They generate electricity and heat from organic materials such as agricultural residues, food waste, and livestock manure.
A2. Some foods that should not be added in small biogas plants include citrus fruits like oranges and lemons, oily foods like grease, ammonium-based foods such as eggs, dairy products and meat because they are hard to break down.
A3. Biogas systems are very cost-effective because they are used to produce energy. They also reduce greenhouse gas emissions and provide renewable energy for heating, cooking, electricity generation, and even vehicle fuel.
A4. A typical biogas plant produces around 160 m³ of biogas every single day. This is an average that is calculated over the course of many years. In optimal conditions, the biogas plant can produce up to 230 m³ of biogas each day.
A5. A biogas plant dimension is based on the number of people staying in the household. For an average family size of 4, a small biogas plant of 1 cubic meter is sufficient.
