Anaerobic digestion

What is anaerobic digestion?

Anaerobic digestion (AD) is the process of breaking down organic matter, commonly animal and food waste, into carbon dioxide, methane, water and bacteria. Anaerobic refers to the absence of oxygen, as the process can either occur naturally or in oxygen-free tanks called an anaerobic digester.

AD is predominantly used to produce biogas and biofertiliser, otherwise known as digestate. Biogas is the mixture of methane and carbon dioxide, and can be used to produce heat and electricity or mixed into vehicle fuel and gas grids. Biofertiliser is the nutrient-rich substance that can be applied to farmland or fed into ethanol production or even some building materials like fibreboard.

How does AD work?

There are four key stages to the AD process, which helps break down the organic matter into methane, carbon dioxide and water.

Biomass, which is any plant-derived item including municipal solid waste, manure, crop residue, compost, food waste and waste water are placed into the oxygen-free anaerobic digester tanks. Naturally occurring micro-organisms then start to break down the matter during the four stages.

Hydrolysis converts the fats, proteins and carbohydrates of the organic matter into fatty acids, amino acids and glucose molecules. Acidogenesis takes the sugars and acids and breaks them down further into alcohols and fatty acids, while producing by-products of carbon dioxide, ammonia and hydrogen sulphide.

Acetogenesis then converts the alcohols into carbon dioxide, hydrogen and acetic acid. Finally, methanogenesis takes place to convert any remaining hydrogen and acetic acid into more carbon dioxide and methane.

Once the process is completed, biogas can then be burned to produce both heat and electricity through combined heat and power (CHP) systems. The digestate is separated into a solid and liquid, which is pasteurised to kill pathogens and stored in tanks until it is used as biofertiliser. Any water treated within the AD process can be returned to watercourses.

What is the market size for AD?

Globally, revenues for biogas reached around $24.5bn in 2015 and are expected to double to just under $50bn by 2026. Just under half this revenue share will be accounted for from agricultural waste and electricity applications will continue to act as the predominant use.

The UK Government recognises AD as one of the best methods for dealing with food waste, farm waste and sewage sludge. Between 2014 and 2015, the number of AD plants quadrupled. There are now 401 operational AD plants in the UK, including 77 biomethane-to-grid plants, and a further 420 projects are under development. 

Currently, biogas is being produced by UK AD plants to power more than one million homes, according to the Anaerobic Digestion & Bioresources Association (ADBA).

UK AD capacity has reached 730MWe, with a recorded increase of 18% in the last 12 months along. Total energy generation has reached 10.7TWh annually and AD is currently reducing UK emissions by 1%, and employing more than 3,500 people.

What are the business benefits of using AD?

The business benefits of AD can be felt by companies that have contracts in place to either send waste to the facilities, or source the clean energy/fuel from AD specialists.

Biogas can be used to produce electricity and heat for onsite operations, which can reduce a company’s carbon footprint as a result. Any excess energy can also be sold to the grid at a profit.

In some cases, the biogas can be upgraded to biomethane, to create a transport fuel that will also help lower the carbon footprint of a businesses’ transport fleet. With the recent crackdown on older, diesel vehicles the use of AD to create a low-carbon transport fuel has become a much more attractive proposition.

According to ADBA, electricity generated from the grid is likely to emit 500g of CO2 on average, for each kWh. However, electricity from an AD has been measured as more than 45 times cleaner.

AD enables businesses to deal with waste without sending it to landfill. In the UK, businesses are responsible for around six million tonnes of food waste annually, which can cost more than £8bn to deal with. Companies can either treat this waste onsite at an AD plant or arrange to have it collected and sent to a facility.

Every tonne of food waste treated through the AD process, instead of sending it to landfill, prevents between 0.5 and 1 tonnes of CO2 escaping, which can create a big improvement on a company’s carbon footprint.

What are the costs?

As there is no standard size for an AD plant, the costs of installing one can vary greatly. Some outlets report that start-up costs for a facility with a capacity of around 275kWe can reach upwards of £1.2m. A 2MW plant that handles around 40,000 tonnes of food waste could cost three to five times more.

Upfront costs for self-funding AD typically range from £2,500-7,000 per kW of capacity, but factor in maintenance costs at 1-2% of capital cost. Average payback is 8-12 years, depending on the feedstock type and size of plant.

For businesses wishing to send waste to biogas plant operators, a gate fee will have to paid. Gate fees are essentially maintenance costs for the operators to treat business waste. Government statistics have shown that the average gate fee for AD has fallen from just under £60 per tonne to £40 tonne, well below the £100 per tonne average to send the same waste to landfill.

What are the business requirements?

Many businesses are unlikely to physically own an AD plant, which are better suited as onsite applications on farms, where the waste is usually generated and doesn’t have as far to travel.

Some farms will lease their land for specific AD plant use, while others will grow crops to be used primarily to produce biogas and biofuel.

Around 5,000 tonnes of feedstock per annum is required to feed a small commercial 250kW AD plant so businesses must ensure locally available feedstock can be sourced. If producing feedstock onsite, waste disposal off-site costs should also be examined. There must be adequate land available as even the smallest AD facilities can require up to an acre in space.

National and local planning policies will usually support AD application on appropriate sites, although these will be subjected to environmental standards. Most AD plants do require an Environmental Permit to operate.

What is the policy backdrop?

AD plants with less than 5MW capacity are supported by Feed-In Tariffs, and there are other incentives available such as the Renewable Heat Incentive and Renewable Transport Fuel Certificates (RTFC) that apply to AD generation.

The UK share for renewable market fuels had been held at 4.75% since 2012, but new amendments to the RTFC will increase it to 9.7% by 2020 and 12.4% by 2032. While this is beneficial for waste-based fuels such as cooking oil, the Government is introducing a crop cap that will reduce plant-based renewable fuels down to 2% by 2032.

Companies seeking to spread digestate and waste material to agricultural land will have to apply for permits, or achieve PAS110 accreditation – issued by WRAP.

AD plant operators also have to comply with Animal By-product Regulations that control the use of materials of animal origin that aren’t meant for human consumption. The regulation covers sources such as blood, uneaten eggs and feathers.

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