What is Anaerobic Digestion?

Anaerobic digestion feedstock

Globally we generate millions of tons of waste annually, a part of which is organic material or waste. Organic material includes food, drink, sewage, garden, farm and agricultural waste. However, less than 20% of waste is recycled annually, with vast quantities still going to landfill sites. Furthermore, unrecycled organic material emits harmful Greenhouse Gases (GHG) into the atmosphere, causing unpleasant odours and taking up copious amounts of landfill. Consequently, an anaerobic digestion (AD) plant, also known as a biogas plant, can recycle organic waste, prevent the release of GHG and produce biogas and a nutrient-rich organic fertiliser.

The AD process can digest a variety of organic waste. Food waste is an excellent feedstock for anaerobic digestion and is typically acquired from households through municipal food waste recycling programmes. In addition, food waste comes from the hospitality industry, food manufacturers and supermarkets. Homeowners and businesses are encouraged to segregate their food waste into caddies or bins ready for collection by a local authority's food waste service. A truck collects the waste from homes and businesses and takes it to a pre-treatment centre.

Another excellent feedstock is agricultural waste such as manure, slurry, crop residues and purpose-grown energy crops or break crops, for instance, maize.

Sewage sludge, also known as biosolids, is another excellent feedstock for anaerobic digestion. Sewage sludge is the residue of treated materials entering a wastewater treatment plant.

Anaerobic digestion plant

Certain feedstocks, such as food waste, require complex pre-treatment before anaerobic digestion; others, such as agricultural waste and sewage sludge, need simpler pre-treatment processes.

Food waste pre-treatment

First, the food waste truck delivers the food waste to the reception building, where it is placed in a bunker. Next, depackaging technology shreds and separates the packaging and any other contamination from the food. Then a small amount of liquid is added to the substance, which is usually waste fluids sourced from food and drink manufacturers. De-gritting takes place in most operations before the material enters the digesters. Finally, the clean, packaging-free food waste, known as substrate, is ready to be fed into the digester tanks.

Agricultural waste pre-treatment

Certain agricultural wastes such as straws and manures benefit from grinding to increase surface area, thus enabling more effective digestion. Straws are often associated with stone contamination, and grinding is often used as a pre-treatment to protect machinery.

Wastewater pre-treatment

Sewage sludge from wastewater treatment is usually cleaner than substrates derived from food waste and can often be digested with little pre-treatment.

What is an anaerobic digester?

Anaerobic digestion occurs in a sealed, oxygen-free tank or vessel called an anaerobic digester. An anaerobic digester is typically constructed using a combination of materials that ensure structural integrity, gas tightness, and resistance to the corrosive nature of the process. The specific materials used may vary depending on factors such as the digester's scale, the feedstock being processed, and local regulations.

The primary structure of the digester is usually a tank or vessel that holds the organic material and provides a sealed environment. The tank can be made of various materials, including concrete, steel, or reinforced fibreglass.

Insulation is often used to regulate and maintain the temperature inside the digester. Common insulation materials include foam insulation boards or spray foam applied to the tank's exterior.

Biogas produced during anaerobic digestion need to be stored safely. Gas storage systems can include flexible gas holders, rigid gas storage tanks, or reinforced PVC or high-density polyethylene (HDPE) gas bags.

Piping and valves are used to transport the feedstock, biogas, and digestate within the digester system. These components are typically made of stainless steel, PVC, or high-density polyethylene (HDPE), resistant to corrosion and gas leaks.

Many anaerobic digesters require mechanical agitation or mixing to enhance microbial activity and ensure uniform distribution of the feedstock. Agitators are commonly made of stainless steel or coated with materials that resist corrosion. Gas mixing can also be used, whereby the biogas is transferred from the roof space of the digester and transferred to the base through a network of pipes.

Gas handling equipment includes gas meters, compressors, blowers, and gas purification systems. These components are often made of materials suitable for handling gases, such as stainless steel or corrosion-resistant alloys.

Inside the anaerobic digester

For anaerobic digestion to occur, specific physical and chemical conditions are necessary to activate the micro-organisms and start anaerobic digestion biology. For this reason, the digester tank must be sealed with no oxygen. Then, once the substrate has been added to the anaerobic digester tank, it is gently and consistently stirred to prevent any inedible crusts or sediment from forming on top. Furthermore, temperature plays an important role in anaerobic digestion. The temperature of the digester affects the activities of the anaerobic bacteria. For a mesophilic anaerobic digestion plant, a micro-organism’s optimal temperature ranges between 35°C and 42°C, whilst thermophilic processes generally operate between 48-51°C.

From here, biology takes over, and a culture of micro-organisms develops and physically digests the materials inside the tank. Subsequently, there are four main biological stages of anaerobic digestion: hydrolysis, acidogenesis, acetogenesis and methanogenesis, which happen over a period of 15 to 60 days. Specific groups of bacteria are more active than others at various stages, and the results from one phase are the input for the subsequent phase of digestion. The digester tank must be continually monitored, and all specific conditions must be maintained throughout the anaerobic digestion process.

The Four Stages of Anaerobic Digestion

• Hydrolysis: In this initial stage, complex organic compounds are broken down into simpler ones, such as sugars and amino acids. This is done by hydrolytic bacteria.
• Acidogenesis: The simple compounds are then converted into volatile fatty acids and other by-products like alcohol, hydrogen, and carbon dioxide by acidogenic bacteria.
• Acetogenesis: In this stage, the volatile fatty acids and other by-products are further converted into acetic acid, hydrogen, and carbon dioxide by acetogenic bacteria.
• Methanogenesis: Finally, methanogenic bacteria convert the acetic acid, hydrogen, and carbon dioxide into methane and water.

The anaerobic digestion process produces two products. The first is biogas captured at the top of the tank, and the second is a by-product known as digestate.

Pasteurisation

Pasteurisation is required when processing food waste. The digestate is fed into pasteurisation tanks and heated to 70 degrees for one hour to destroy pathogens, including E-Coli and Salmonella. The pasteurisation ensures that no harmful bacteria pose a biosecurity risk when digestate is applied to the land. Agrivert ensures all food waste derived digestate is PAS 110 accredited, meaning it has passed and maintained stringent standards regarding digestate quality. Pasteurisations can take place pre or post-digestion.

Bioproducts of anaerobic digestion

Biogas

Biogas is a renewable gas created directly from anaerobic digestion and includes methane (CH4), carbon dioxide (CO²), water vapour, and trace amounts of other gases.

Biomethane (CH4)

Biomethane is biogas that has had the CO² and trace gases removed. It is renewable gas with the same composition as natural gas but classified as a green alternative to fossil fuel. Biomethane can be injected into the national gas grid for homes and businesses to use for cooking, heating and for fuelling power stations that deliver electricity to homes and businesses.

BioLPG or Biopropane

The uses of bioLPG are similar to those of conventional LPG, which include: residential heating and cooking. BioLPG can be used for heating homes and buildings, as well as for cooking purposes. It provides a cleaner and more sustainable option compared to traditional LPG or other fossil fuels. BioLPG can be used as fuel for various industrial processes, such as heating, steam generation, and drying. Industries that rely on LPG for their operations can transition to using BioLPG to reduce their carbon footprint. Additionally, BioLPG can also be used as vehicle fuel, especially those that are equipped to run on LPG. BioLPG can be used in power plants to generate electricity. Its use in power generation can contribute to reducing greenhouse gas emissions and promoting renewable energy sources. BioLPG can be used for recreational activities such as camping, outdoor cooking, and heating.

The adoption and use of BioLPG are driven by environmental concerns, energy security, and the need to reduce greenhouse gas emissions. As technologies for producing and using renewable energy sources continue to advance, BioLPG is expected to play a significant role in the transition towards more sustainable energy systems.

Carbon capture (CO²)

When the CO² is separated from the biomethane, it can be captured and sent for use in the production of fire extinguishers, inflating life rafts and life jackets, encouraging the growth of plants in greenhouses, for carbonated beverages, food manufacturing, abattoirs and medicine preserve.

Combined heat and power (CHP)

Biogas can be used to power a combined heat and power engine (CHP). The engine burns the biogas and uses the energy to turn a generator, thus simultaneously generating electricity and heat. The heat can be used to heat digestion tanks and in the pasteurisation process. The electricity produced is used to power the AD plant, hence making the plant self-sufficient. In addition, the surplus electricity is supplied to the National Grid to power local homes and businesses.

Digestate

Digestate is rich in organic matter and nutrients such as nitrogen, phosphate and potash. Digestate helps return nutrients to the soil, improving its structure and water retention and reducing soil degradation. Nutritious and stable soil is essential to maintain agricultural efficiency and grow healthy crops.