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Typical anaerobic digestion plant


The potential for anaerobic digestion, and the benefits of the biogas process are massive. Discover mankind's sustainable renewable energy future here! This website is an independent venture, by people who are simply fascinated by the subject (read About Us). We want to help more people see our vision for a vibrant low-carbon future, and become part of the AD technology movement. You are very welcome to spend time on this website, and we hope that you will get involved by commenting, and maybe even joining our campaign for better AD awareness, as well.

What is Anaerobic Digestion?

Anaerobic digestion (AD) is a biological process similar to composting, but without air. As in composting, micro-organisms break down organic matter into simpler smaller compounds and reduce its bulk or “mass”.

However, unlike composting, which overall always consumes energy, anaerobic digestion can be used to create energy. That is because it produces a gas known as biogas, just over half of which is methane. Everyone that lives in a developed nation and has cooked anything on a kitchen stove, using gas piped in from the street, has used methane, and knows how cleanly it burns, and how wonderfully hot the flame is, because "natural gas" is pure methane.

In addition to biogas, the process also produces "digestate", which is a solid fibrous residue, and any liquid which is left over.

Exciting Era for Innovation and Expansion in AD!

The AD process is rapidly gaining global recognition for the benefits it can bring at a wide range of levels. In August 2014 the US Department of Agriculture (USDA) published a Biogas Opportunities Roadmap of "voluntary actions to reduce methane emissions and increase energy independence, which includes a target for 11,000 additional agricultural anaerobic digestion (biogas) plants by 2020.  That's more than 2,000 new AD plants a year for anaerobic digestion companies to design, build and commission, just in the agricultural waste sector, not to mention plants which will be built to process Food Waste, and a score of other waste categories which can similarly benefit from the technology.

But it's not just in the US that many new AD plants will be built, other nations involved in large expansions in AD before 2020 are, for example:

  • Most EU nations - to comply with policy changes (large commercial plants)
  • India, China, Pakistan, Philippines (mostly small or community sized, self-help craftsman-built plants)
  • Most African states (especially Kenya), Vietnam, Nepal, Indonesia (ditto),
  And, that's to name just a few!

Benefits of Anaerobic Digestion

a) Environmental Benefits

  • Reduces carbon emissions and can contribute to national renewable energy targets
  • Only biogas (and some carbon dioxide) is taken out, so valuable nutrients are recycled back into the soil
  • Digestate may also be used as a feedstock for other biofuel production and for innovative fibre building materials
  • AD creates skilled ‘green’ jobs and contributes to growth in the local economy
  • When used in conjunction with segregated municipal waste collection, it reduces waste sent to landfill
  • Sanitizes waste output providing a reduced public health hazard/ reduces pollution risk.

b) Benefits to Those That Own and Operate AD Plants

The Biogas Plant Renaissance

Until the early 2000s there had been a long period of cheap fossil fuel sourced power. While power was so cheap, the anaerobic digestion process was largely ignored in the west. China and India and some other nations realized its value especially to subsistence farmers, and developed early versions of the now increasingly popular buried, fixed dome, household and small community biogas digester systems which they invented. Ever larger numbers of these affordable energy plants are now being built and operated, so that by 2016 literally millions of people will be benefiting from the "biogas renaissance". In the western nations during the first half of the 20th century, anaerobic digestion had been popular among the water boards, and municipal corporations in charge of the safe disposal of sewage sludge, but almost all of those had, by then, been replaced by incinerators. Energy was so cheap that even those running those incinerator plants failed to truly realize that they were using energy, to destroy energy! That was an expensive waste, which is now being put right as large numbers of sewage sludge incinerators are being shut-down and replaced with biogas plants, the latest of which can be so efficient in making power that they can generate enough electricity to run the entire wastewater treatment works (sewage works) solely using the sludge from the works as their bio-fuel! These are just two examples of the way that the anaerobic digestion and biogas industry is now growing. The word is out now, and even if fluctuating oil prices slow growth a little through 2015, expect to see more and more biogas plants wherever you go!

Quick Facts About The Process

1. There are four key biological/ chemical reaction stages of anaerobic digestion: 2. Almost any organic matter can be digested anaerobically. Not all will produce enough biogas to be viable though. 3. Anaerobic digestion occurs in nature at all temperatures, but below 10 degrees centigrade the rate of biogas production is so slow that running a biogas process is usually considered not viable, for any form of biogas generation system. 4. There are two temperature ranges over which the many micro-organisms which produce biogas work most efficiently. These are known as the: Temperature ranges, with the optimum temperature usually considered to be at the top of the range. 5. The process is truly ancient:
  • Fermentation for the production of alcohol was known to the Ancient Egyptians and is recorded  as far back as 2,000 BC.
  • The methane producing stage of anaerobic digestion is performed by a type of micro-organism known as archaea. It's not a bacteria. In fact in comes from a distinctly different branch of the phylogenetic tree of life to bacteria.

Types of Anaerobic Digestion Plants – From the Smallest to the Largest

DIY and Micro-Biogas Systems

Individuals and educational establishments
Type: Desk-scale, laboratory and yard
-bottle and flask reactors -balloons and car type inner tubes
More Info:
Watch YouTube videos for demos.

Household and Community Biogas Plants

Users: Households and community groups Type: Fixed dome and "Arti" Equipment: -stone built buried chambers -arched dome, floating drum More Info: YouTube videos, "arti" website etc.

On-Farm and Central Biogas Plants

Users: Farm businesses, co-operatives etc. Type: Usually wet-process Equipment: -circular tank reactors -  biogas holders - flexible dbl. membrane covers More Info: US Agstar, UK WRAP websites etc.

Large Waste & Commercial AD Facilities

Public bodies, large waste companies.
Type: WasteWater Sludge Treatment Facilities, Large Farms, MBT Plants, Food Waste
-Wet & Dry AD Processes -Various
More Info:
Commercial websites.

Types of Anaerobic Digestion Process

Wet AD Process (Most Common)

Feed Type:
All wastes, and crops.
-Wet & Dry feeds are mixed with water and pumped. -Large tanks. look like typical AD Plants
More Info:

Dry AD Process (Popularity Rising)

Not for DIY'ers or most farmers.
Feed Type: Fibrous Feed Materials.
-Material handled as a solid -Not pumped. -Innoculum spray introduces micro-organisms.
More Info:

Other Anaerobic Digestion Processes

Reasons for Building an AD Plant – Other Than for Energy

For Water Treatment

  • Although Wastewater Treatment Plants, which treat foul sewage, are routinely aerobic processes, some are anaerobic especially in South America.
  • Upflow Anaerobic Sludge Blanket (UASB) systems use the process and are primarily used for water treatment.

For Odour Abatement

  • Large intensive livestock farms that are situated close to housing may be able to obtain permission to extend their operations only if they install an AD plant to reduce the odour of "muck spreading".
  • Digestion of manure substantially reduces spreading odours, especially if ground injection of digestate, is used.

To Reduce Risk of Water Pollution (1)

  • High intensity livestock farms in high rainfall areas, and nitrate sensitive zones, may experience a particularly high risk of river pollution and prosecution from nitrate contaminated run-off, when heavy rain occurs soon after spreading manure.
  • Plant and soil uptake of the nutrients, from the digestate from a biogas plant, can be more rapid and reduce watercourse pollution risks.

To Reduce Risk of Water Pollution (2)

  • The quality of the seawater at Blue Flag bathing beaches was shown to be impaired after heavy summer rainstorm events, in an estuarine environment.
  • Installation of farm biogas plants to digest all livestock farm cattle slurry, reduced the faecal indicator organism counts on nearby beaches, avoiding damage to local tourism.

3 Top AD Plant Add-Ons and Upgrades

A Hydrolysis Stage

"Hard to digest" feed materials can benefit greatly by including a hydrolysis stage in which the substrate is subjected to high temperature and pressure, before digestion. Hydrolysis breaks up intact cells allowing the digestion micro-organisms to carry out a much more complete digestion/ higher gas yields/ reduced retention times and greater plant throughput.

Upgrade Biogas to Biomethane

  • There is a lot of talk about creating added-value for the raw biogas from AD Plants, by "upgrading".
  • Upgrading is the process of removing the carbon dioxide and other impurities from biogas, and compressing it.
  • It produces biomethane - a highly sought after product. It is sold as Renewable CNG as a vehicle fuel, or injected into the regional gas distribution network.

Improved Biogas Reactor Mixing and Avoidance of “Hard Crust”

(Applies to Wet AD Processes Only)
  • Many early plants will benefit from improved reactor mixing, which avoids "dead zones" within the digester tank.
  • Some mixing systems can also break-up any tendency for a crust to form on the reactor surface.
  • Higher biogas volumes may repay the cost of installation quite rapidly.