Anaerobic digestion
The word 'anaerobic' (or anoxic) means oxygen deficient (or oxygen depleted).
Anaerobic digestion (AD) is the microbial degradation of organic material known as ‘feedstock’ (such as farm waste, food waste and energy crops) to produce biogas. This process is sometimes referred to as ‘biomethanation’.
Typically, anaerobic digestion takes place in sealed, insulated tanks (digesters) in the absence of oxygen. It requires a heat source, either a mesophilic process at temperatures of 35 to 40°C or a thermophilic process at 50 to 60°C. The mesophilic process tends to be used for low-solid wastes such as animal slurry, while the thermophilic process tends to be used for high-solid materials such as a garden waste.
The products of anaerobic digestion are; biogas (sometimes called biomethane or renewable natural gas (RNG)), water and fermented organic material (digestate).
The digestate can be used as a high-quality fertiliser.
After a relatively striaght-forward clean-up process, biogas can be used as a fuel. Biogas is mainly composed of methane and carbon dioxide, but may also contain small amounts of nitrogen, hydrogen and carbon monoxide, as well water vapour and contaminants such as hydrogen sulphide and siloxanes.
Biogas is often used to fuel combined heat and power (CHP) plant that produces both electricity and heat. The heat can be used for the anaerobic digestion process and can also be used to pasteurise animal-derived waste so that it can be used as fertilisers.
Combined heat and power plant can feed electricity into the national grid, and so biogas installations may qualify for payments under the renewable heat incentive (RHI) scheme (other than biogas from landfill).
Anaerobic digestion is recognised by the UK government and by the United Nations as having significant potential to help achieve sustainability targets as it produces local energy and fertilizer and makes use of local waste products that might otherwise be difficult to dispose of.
Combined heat and power plant can feed electricity into the national grid, and so biogas installations may qualify for payments under the renewable heat incentive (other than biogas from landfill).
Biogas can be fed into the national gas network if it is cleaned of carbon dioxide.
NB BG 34/2021, The Illustrated Guide to Renewable Technologies, 2nd edition, written by Michelle Agha-Hossein and David Bleicher and published by BSRIA in 2021, states: ‘Anaerobic digestion is a process that produces gas from organic material with a high methane content, such as agricultural, household and industrial residues and sewage sludge. In addition to methane gas, a solid residue similar to compost and a liquor are also produced. The former can be used as a soil conditioner while the latter can be used for fertiliser (depending on the level of contamination). The solid residue can also be burnt as a fuel or gasified. The methane gas can be used to power a gas engine or turbine to generate electricity and heat. This process is used in very large applications.’
[edit] Related articles on Designing Buildings
- Biogas (repeats some of the text in this article).
- Biomass.
- Combined heat and power.
- Feed in tariff.
- Renewable energy.
- Renewable heat incentive.
- Zero carbon homes.
- Zero carbon non-domestic buildings.
Featured articles and news
The current and future global market dynamics of boilers
Significant challenges but adaptation to sustain for longer.
Designing sustainability and performance into buildings
Specifying and selecting sustainable resilient timber products.
Modifying wood to improve resistance to decay and movement.
A last minute, long look for built environment professionals.
The architecture of creative reuse. Book review.
Installing solar panels on listed structures.
Sustainable development global goals, history in progress?
"Unless we act now, the 2030 Agenda will become an epitaph for a world that might have been."
Mike Kagioglou FCIOB named CIOB President
'Sustainable Development Goals must be focus for construction'
BSRIA training; a look at what's on offer
From energy management to compliance training.
TESP video warns to beware of rogue trainers.
Highlighting the slippery tactics of non-approved providers.
New Building Safety Wiki launched
Boosting awareness and understanding of the new fire safety regime.
New playbook on AI in construction published by CIOB
How to get to grips with, and the best from AI.
Digital Construction Report NBS
BIM, cloud, off-site, immersive tech, AI, twins and sustainability.