How does anaerobic digestion work?
The methanization process is a technology that produces biogas from organic matter, thanks to a biological degradation process known as anaerobic digestion. Here’s a detailed explanation of the anaerobic digestion process:
The principle: anaerobic digestion
Anaerobic digestion is a biological process in which micro-organisms break down organic matter in the absence of oxygen. This breakdown takes place in four main stages: hydrolysis, acidogenesis, acetogenesis and methanogenesis.
- Hydrolysis : Complex organic molecules are broken down into simpler ones by enzymes secreted by hydrolytic micro-organisms. For example, proteins are broken down into amino acids, lipids into fatty acids and carbohydrates into simple sugars.
- Acidogenesis : The simple molecules resulting from hydrolysis are then broken down into volatile fatty acids and alcohols by acidogenic bacteria.
- Acetogenesis : Acetogenic bacteria transform the products of acidogenesis into acetate, carbon dioxide and hydrogen.
- Methanogenesis : Methanogenic archaea, a specific group of microorganisms, transform acetate, carbon dioxide and hydrogen into methane and water.
Various types of organic matter can be used as substrates, whose composition and degradability influence the yield and quality of the biogas produced. The main substrates used in anaerobic digestion can be classified into several categories, each with specific yields and profitability depending on their characteristics and availability.
The main substrates used in anaerobic digestion can be divided into three categories: agricultural waste, industrial waste and municipal waste. The yield and profitability of anaerobic digestion depend on the type of substrate, its chemical composition and dry matter concentration, and the anaerobic digestion technology used.
- Agricultural waste
- Manure and slurry (cattle, pigs, poultry): These wastes have a relatively low methanogenic potential due to their low concentration in dry matter and organic matter. However, they are often available in large quantities and can be mixed with other substrates to improve yield and profitability.
- Crop residues and straw : These have average methanogenic potential and are also available in large quantities. The profitability of their use depends on the distance between the fields and the anaerobic digestion plant.
- Energy crops (corn, beet, sorghum): These crops have a high methanogenic potential and are grown specifically for biogas production. They are profitable, but can be controversial due to competition with agricultural land for food production.
- Industrial waste
- Agri-food by-products (fats, coffee grounds, brewery waste): These wastes have a high methanogenic potential and are generally profitable to methanize. Their availability depends on the local industry.
- Industrial effluents (slag, vinasse): The methanogenic potential of these effluents varies according to their composition. Their profitability depends on the concentration of organic matter and on transport and treatment costs.
- Municipal waste
- Sewage sludge: Sewage sludge has a moderate to high methanogenic potential. Its profitability depends on transport and treatment costs, as well as local regulations on the spreading of digestate.
- Kitchen and canteen waste : This waste has a high methanogenic potential, but collection and treatment can be costly. Profitability depends on transport costs, local legislation and public awareness of waste separation.
The yields and profitability of anaerobic digestion therefore vary according to the substrates used, their availability and composition, and the anaerobic digestion technologies employed. Anaerobic digestion is generally more profitable when several substrates are mixed together, thus optimizing the process and improving the quality of the biogas produced.
Optimum conditions for anaerobic digestion
To ensure an efficient and stable anaerobic digestion process, certain conditions must be met:
- Temperature : anaerobic digestion can take place at different temperature ranges, generally classified into two categories: mesophilic digestion (between 35 and 40°C) and thermophilic digestion (between 50 and 60°C). Temperature must be kept constant to ensure that the micro-organisms work properly.
- pH : A neutral or slightly alkaline pH (between 6.8 and 7.5) is ideal for anaerobic digestion. Significant variations in pH can disrupt the process and inhibit biogas production.
- Hydraulic retention time (HRT ): HRT is the time required for a substrate to pass through the anaerobic digestion system. A longer HRT can increase biogas production, but can also increase operating and investment costs.
- Agitation : Regular agitation of the digester is necessary to homogenize the mixture and prevent the formation of crusts or deposits. However, excessive agitation can disrupt the anaerobic digestion process and consume a significant proportion of the energy produced.
Methanization is a biological process that requires specific equipment to be carried out efficiently. Here are the main types of equipment required for anaerobic digestion:
- The anaerobic digestion reactor (digester): This is the heart of the anaerobic digestion system, also known as the digester or biodigester. It’s a closed tank or container where organic matter is mixed and decomposed in the presence of bacteria to produce biogas. The choice of reactor depends on the quantity and type of organic matter to be treated.
- Feeding system: Organic matter must be fed into the anaerobic digestion reactor in a controlled manner to ensure efficient anaerobic digestion. Feeding equipment can vary according to the size of the reactor and the nature of the organic matter.
- The heating system: anaerobic digestion is a process that requires a constant temperature to ensure optimal anaerobic digestion. A heating system may be required to maintain the temperature in the anaerobic digestion reactor at an optimum level.
- Biogas recovery system: Biogas is the main product of anaerobic digestion. A biogas recovery system enables the gas produced in the reactor to be recovered and used as a source of energy.
- Biogas storage and distribution system: The biogas produced must be stored and distributed for later use. The biogas storage and distribution system may include a storage tank and pipes to convey the gas to the equipment that will use it.
- Control equipment: Control equipment such as temperature, pressure and pH sensors are needed to monitor and regulate the anaerobic digestion process.
- Safety equipment: Safety equipment such as alarm systems and relief valves are needed to ensure the safety of people and equipment in the event of malfunction or emergency.
Biogas and digestate production
The biogas produced during the anaerobic digestion process is mainly composed of methane (CH4, 50-75%) and carbon dioxide (CO2, 25-50%), with small quantities of other gases such as hydrogen sulfide (H2S) and nitrogen (N2). Biogas can be used as a renewable energy source to produce heat, electricity or both (cogeneration), or it can be purified to obtain biomethane, which can be injected into the natural gas distribution network or used as a fuel for vehicles.
The solid and liquid residue from the anaerobic digestion process, known as digestate, contains essential nutrients such as nitrogen, phosphorus and potassium, which can be used as fertilizers or soil improvers. Digestate can also be processed to extract value-added products, such as fibers for the production of building materials or compost for soil improvement.
Uses for biogas
Biogas from anaerobic digestion has many possible applications, notably in the fields of energy production, mobility and waste recovery. Here are some of the main applications for biogas:
- Electricity generation: Biogas can be used to power gas engines or gas turbines that generate electricity. This electricity can be used on site or fed into the electricity grid to power other facilities.
- Heat production: Biogas can be used to power boilers or cogeneration systems, which produce both heat and electricity. The heat produced can be used to heat buildings, greenhouses or industrial processes requiring heat.
- Mobility: Biogas can be purified to produce biomethane, a gaseous fuel alternative to natural gas. Biomethane can be used as fuel for natural gas-powered vehicles such as cars, trucks and buses, helping to reduce greenhouse gas emissions and dependence on fossil fuels.
- Injection into the natural gas grid: Purified biomethane can also be injected into the natural gas grid, where it mixes with conventional natural gas. This diversifies energy sources and reduces greenhouse gas emissions.
- Waste recovery : The anaerobic digestion process also makes it possible to recycle organic waste, producing a residue known as digestate. Digestate is rich in nutrients and can be used as an agricultural fertilizer, contributing to sustainable waste management and improved soil quality.
Advantages and disadvantages of anaerobic digestion
- Production of renewable energy : Methanization produces biogas, a renewable energy source that can be used to generate electricity, heat or fuel for vehicles, reducing our dependence on fossil fuels.
- Reducing greenhouse gas emissions?: By recovering organic waste that would otherwise have been landfilled or incinerated, methanization helps reduce emissions of methane, a powerful greenhouse gas, and CO2.
- Waste management: Methanization enables organic waste from agriculture, the food industry and households to be treated and recovered, contributing to more sustainable waste management.
- Fertilizer production: Digestate from methanization is rich in nutrients and can be used as an agricultural fertilizer, improving soil quality and reducing dependence on chemical fertilizers.
- Investment costs: Building and setting up an anaerobic digestion plant can require substantial investment, which can be an obstacle for some businesses or local authorities.
- Operational complexity : anaerobic digestion requires rigorous process management and monitoring to maintain optimal conditions
- Substrate availability and logistics : Collecting, transporting and storing organic waste to feed anaerobic digestion plants can be complex and costly. In addition, substrate availability can vary according to season and region.
- Competition with land use: Growing energy crops for anaerobic digestion can compete with land use for food production, raising ethical and sustainability issues.
- Olfactory and visual nuisance : Anaerobic digestion plants can generate an olfactory nuisance due to the odours emitted by organic waste and the degradation process. In addition, some facilities can be considered unsightly, provoking negative reactions from local residents.
- Environmental risks: Although anaerobic digestion reduces greenhouse gas emissions, it can also present environmental risks, such as gas or liquid leaks, which can contaminate soil, groundwater or air. Rigorous monitoring and management of facilities are necessary to minimize these risks.
Methanization deployment worldwide
The deployment of anaerobic digestion around the world has accelerated in recent years due to growing awareness of the need to produce renewable energy and manage organic waste in a sustainable way. Several countries have adopted favorable policies and financial incentives to encourage the installation of methanization units.
Europe is a leader in the field of methanization, with countries such as Germany, the UK, France, Italy and the Netherlands having thousands of plants in operation. Germany is the most advanced country, with over 10,000 methanization units, mainly fuelled by agricultural waste and energy crops.
In Asia, China and India have also invested heavily in methanization, mainly to treat agricultural waste and livestock effluent. National programs to promote domestic biodigesters have also been set up in these countries.
North America is also seeing growth in the deployment of anaerobic digestion, with the USA and Canada investing in anaerobic digestion projects to treat agricultural waste, food waste and sewage sludge.
In Latin America, Africa and the Middle East, anaerobic digestion is still at an early stage of development, but pilot projects and local initiatives are underway to promote the technology.