Anaerobic Digestion vs. Composting: Which is Better for Organic Waste?

What we do with our organic waste matters. If we do it correctly, it can significantly benefit the environment. There are two main ways of recycling organic waste–Anaerobic Digestion and Composting–and they work differently and are often used for different purposes. 

What is Anaerobic Digestion?

Anaerobic digestion is a biological process which, as its name suggests, breaks down organic matter in the absence of oxygen, producing biogas (methane and carbon dioxide) and digestate (nutrient-rich slurry). The process of anaerobic digestion comes in four stages:

1. Hydrolysis: Complex organic molecules are broken down into simpler compounds like sugars and amino acids.

2. Acidogenesis: These compounds are further broken down into volatile fatty acids, ammonia, and hydrogen.

3. Acetogenesis: Volatile fatty acids are converted into acetic acid, carbon dioxide, and hydrogen.

4. Methanogenesis: Methane-producing bacteria convert the acetic acid and hydrogen into methane and carbon dioxide.

Bacteria facilitate each step in this process, breaking down organic matter and converting it into biogas through a series of biochemical reactions. Common materials used may include a diverse range of food waste, such as peels, scraps, expired products, animal manure from farms, agricultural crop residues and straw, and organic matter from wastewater treatment. 

What is Composting?

Composting is the natural decomposition of organic materials into nutrient-rich compost. In contrast to anaerobic digestion, composting is an aerobic process which involves microorganisms breaking down this matter in the presence of oxygen, producing carbon dioxide, water, and heat.

Oxygen is crucial for aerobic composting because it supports aerobic bacteria, the primary decomposers. These bacteria break down organic material efficiently, generating heat and preventing the foul odours associated with anaerobic decomposition.

Green materials such as food scraps (vegetable peels, fruit waste, coffee grounds), grass clippings and fresh garden waste are excellent for composting because they are extremely rich in nitrogen. However, brown materials such as dry leaves, straw and hay, wood chips and sawdust, shredded cardboard, and paper are suitable because they are extremely carbon-rich.

Anaerobic Digestion vs. Composting: Environmental Impact

Anaerobic digestion processes organic waste in an oxygen-free environment, producing biogas. This biogas can be used for electricity generation, heating, or as vehicle fuel, reducing reliance on fossil fuels. 

AD mitigates greenhouse gas emissions and provides a renewable energy source by capturing methane that would otherwise escape into the atmosphere. The residual digestate can be used as a nutrient-rich fertiliser, promoting sustainable agricultural practices.

Composting enriches soil by improving its structure, water retention, and nutrient content, promoting healthy plant growth. It also aids in carbon sequestration by storing carbon in the soil, reducing atmospheric CO2 levels. Well-managed composting prevents methane emissions, making it an environmentally friendly practice that enhances soil health and helps mitigate climate change.

There are risks to all this. Improperly managed composting can become anaerobic, producing methane emissions, a potent greenhouse gas. This occurs when there's insufficient oxygen, causing anaerobic bacteria to thrive. But this is easily kept in check; regular turning, proper moisture levels, and balancing green (nitrogen-rich) and brown (carbon-rich) materials ensure aerobic conditions, minimising methane release.

Cost and Infrastructure

Anaerobic digestion facilities in the UK have high setup costs, ranging from £1.5 million for small farm-based digesters to £7.5 million for larger food waste facilities. Ongoing expenses include maintenance and feedstock costs, typically around £300-£1,000 annually. 

Home composting is much cheaper to set up, with bins starting at around £16. Ongoing expenses are minimal, mainly involving occasional bin cleaning and maintenance. Industrial composting setups are more costly than home setups but less than AD facilities, with setup costs of around £100,000-£500,000. Ongoing expenses include labour, equipment maintenance, and feedstock management.

In contrast, home and industrial composting have lower operational costs and typically require minimal investment. While there are fewer direct subsidies for composting, the reduced costs and environmental benefits make it an attractive option for organic waste management.

Anaerobic digestion (AD) facilities often benefit from government subsidies like the Green Gas Support Scheme (GGSS) in the UK, which provides financial incentives for biomethane production. These subsidies help offset high setup and operational costs, making AD a viable renewable energy source.

Energy Production and Soil Fertility

It’s important to remember that anaerobic digestion and composting aren’t just two different processes but result in two completely different outcomes. Anaerobic digestion generates renewable energy, such as electricity and heat and contributes to the circular economy by reducing waste and dependence on fossil fuels. Conversely, composting enhances soil fertility, improves soil health, and aids in carbon sequestration but does not generate energy.

Suitability for Different Types of Organic Waste

Anaerobic digestion is ideal for wet organic waste and industrial food waste. It breaks down waste in oxygen-free conditions, producing biogas for energy and digestate for fertilisers. This process efficiently handles large volumes of wet waste from industries and municipal sources.

Composting, on the other hand, works best for dry organic materials and small-scale household waste. Aerobic processes decompose materials like dry leaves, straw, and food scraps into nutrient-rich compost. This method is perfect for managing home garden waste and kitchen scraps, enhancing soil health and reducing landfill use.

Anaerobic Digestion vs. Composting: Which is Right for You?

Well, that depends on the amount of space you have and what type of waste you’re looking to work with. Anaerobic Digestion is best for wet organic waste and industrial food waste. It requires more space and infrastructure, so it’s ideal if you have access to large volumes of waste and the necessary setup for biogas production.

Composting is dry organic materials and small-scale household waste. It’s suitable for smaller spaces such as gardens or small community gardens such as allotments and 

works well if you generate moderate amounts of garden waste and food scraps and can manage a compost pile or bin. 

Anaerobic digestion and composting are two sides of the same coin. They have different requirements to work at their best and result in very different outcomes. As such, it’s not necessarily an either/or choice. You might even find that you can do both! With more nutrient-rich soil and conversion to fuel, you might find that you can do both as part of a holistic waste management plan.