How Anaerobic Digestion is Transforming Waste into Renewable Energy

Food waste is a global issue. WRAP estimates that in 2021, 10.7 million tonnes of food was thrown away in the UK, and the United States produces more than 70 million tons of it every year, which has knock-on effects. What are we to do with all this waste? Isn’t there something better that we can do with it than send it off to landfills and just let it rot? 

Well, there is. Anaerobic digestion can solve this issue by converting food waste into biogas, a form of renewable energy. 

What is Anaerobic Digestion?

Anaerobic digestion (AD) is a biological process in which microorganisms break down organic matter in the absence of oxygen, producing biogas (a mixture of methane and carbon dioxide) and digestate (a nutrient-rich slurry) as a result.

AD is driven by a series of microbial processes occurring in four main stages:

Hydrolysis

Complex organic compounds, like carbohydrates, proteins, and fats, are broken down into simpler molecules, such as sugars, amino acids, and fatty acids, by hydrolytic bacteria. This stage is crucial for making the organic matter accessible to other microorganisms.

Acidogenesis

Acidogenic bacteria further break down hydrolysis products into volatile fatty acids, ammonia, hydrogen, and carbon dioxide. This step is essential for converting larger molecules into smaller compounds that can be easily utilised in subsequent stages.

Acetogenesis

Acidogenic bacteria produce acetic acid, carbon dioxide, and hydrogen from volatile fatty acids. This stage bridges the gap between the breakdown of complex molecules and the final production of methane.

Methanogenesis

Methanogenic archaea convert acetic acid, hydrogen, and carbon dioxide into methane and water. This final step produces biogas, which can be captured and used for energy production.

When AD occurs, microorganisms such as hydrolytic, acidogenic, acetogenic, and methanogenic archaea work together in a symbiotic relationship, breaking down food waste and other organic materials without oxygen and transforming them into biogas through a series of biochemical reactions. 

Anaerobic Digestion of Food Waste

Food waste is rich in organic matter, making it an excellent feedstock for AD. It contains carbohydrates, proteins, and fats that microorganisms can easily break down to produce biogas. Additionally, food waste is often readily available from households, restaurants, and the food processing industry, providing a consistent supply for AD facilities.

This is already happening right here in the UK. Somerset County partnered with Viridor Waste Management to recycle household food waste into methane gas, generating clean energy for homes and businesses. The facility also produces fertiliser for local agriculture, showcasing a successful model of food waste to energy conversion.

The Real Junk Food Project is a Brighton-based global movement which intercepts food waste from hotels, restaurants, and other sources to prepare meals in its cafés and pop-up stalls. The project reduces food waste and generates energy through AD, contributing to a circular economy.

The benefits of these projects are clear. AD reduces greenhouse gas emissions by diverting food waste from landfills and preventing the release of methane, a powerful greenhouse gas. At the same time, it converts food waste into biogas, which can be used for electricity and heat, while the digestate produced is a valuable biofertiliser, enhancing soil health and reducing the need for chemical fertilisers. It even has economic benefits; reducing food waste and generating energy can lead to cost savings for homes, businesses and municipalities who participate in it.

The Anaerobic Digestion Process

There are seven steps to the anaerobic digestion process: 

1. Feedstock Loading: Organic waste is collected and loaded into the anaerobic digester.

2. Anaerobic Digestion: Inside the digester, microorganisms break down the organic matter in an oxygen-free environment: hydrolysis, acidogenesis, acetogenesis, and methanogenesis. This process converts the waste into biogas and digestate.

3. Biogas Collection: As biogas is produced, it rises to the top of the digester. The digester has a gas collection dome or a floating gas holder to capture the biogas.

4. Gas Piping System: The biogas is then transported from the digester through a network of pipes specifically designed to handle it and prevent leaks.

5. Gas Purification: Before storage, the biogas undergoes purification to remove impurities such as hydrogen sulphide, moisture, and carbon dioxide. 

6. Gas Storage: The purified biogas is ultimately stored in gas storage tanks. Depending on the intended use, the tanks can be either low-pressure or high-pressure.

7. Energy Conversion: The stored biogas can be converted into electricity or heat through combustion in gas engines or turbines or upgraded to biomethane for use as a vehicle fuel or injection into the natural grid.

But how does it measure up in comparison with other waste-to-energy methods? Well, it’s more efficient for wet waste. It produces fewer harmful emissions than incineration, but it can’t handle the breakdown of non-organic compounds in the same way that pyrolysis or gasification can. Each method has its strengths, with AD being particularly effective for wet organic waste because of its significant environmental benefits.

Food Waste to Energy: Environmental and Economic Benefits

AD significantly reduces landfill waste and greenhouse gas emissions by converting organic waste into renewable energy. This process not only diverts waste from landfills, reducing methane emissions but also generates biogas that can be used for electricity and heat. Businesses can harness this technology to turn food waste into energy, offering a sustainable disposal method and simultaneously reducing reliance on fossil fuels.

Financial incentives such as the Renewable Heat Incentive and Feed-in Tariffs have supported AD projects in the UK. Globally, policies like the European Union's Renewable Energy Directive promote using AD to meet renewable energy targets.

Anaerobic Digestion Biogas: A Sustainable Energy Solution

Biogas is combusted in gas engines or turbines to generate electricity, which can be fed into the grid or used on-site. The heat produced during the combustion process can be used for space heating, water heating, or industrial processes. It can also be upgraded to biomethane and used as a renewable fuel for vehicles. 

This has significant real-life applications and benefits. In rural areas, small-scale biogas plants convert household organic waste into biogas for cooking and lighting. At the same time, businesses such as food processors and breweries can and do use biogas systems to manage waste and generate energy.

It's even increasingly used in municipal projects. Cities such as Bristol have implemented biogas projects, using organic waste from households and businesses to produce electricity and heat for their communities. Biogas production even supports circular economies by turning organic waste into valuable energy and byproducts. It reduces landfill waste, lowers greenhouse gas emissions, and provides a renewable energy source, contributing to a more sustainable energy future.

Future of Anaerobic Digestion and Renewable Energy

Recent advancements in AD systems have included improved conversion efficiency, process stability, and economic feasibility. Innovations like co-digestion and pre-treatment strategies, as well as the use of nanomaterials, have enhanced biogas production. 

Predictions for AD's growth as a renewable energy solution are promising. With increasing global waste and the push for sustainable energy, AD is expected to play a significant role in energy recovery and waste management.

Integrating AD with other technologies, such as gasification, could further boost its efficiency and adoption. Long-term sustainability goals, such as net-zero emissions by 2050, align well with this potential, making it a crucial component in achieving global climate targets.

Anaerobic digestion (AD) converts food waste into renewable energy, reducing landfill waste and greenhouse gas emissions. It produces biogas for electricity and heat and digestate for fertilising soil, promoting a circular economy. Both businesses and consumers should embrace AD as part of their sustainability efforts. Reduce waste, generate renewable energy, and support environmental health by integrating AD into their operations and daily practices!