Anaerobic digestion plants are simply the process facilities in which anaerobic digestion takes place. When people talk of anaerobic digestion plants (AD Plants) they are usually referring to commercial-scale Anaerobic Digestion Facilities. The smaller “low technology” house-and-community food waste type digesters popular in the developing nations are usually referred to as biogas digesters.

How does anaerobic digestion work?

Anaerobic digestion is a process that breaks up toxins in the environment such as food waste from animal faeces or wastewater. Anaerobic digestion for biogas production occurs in closed vessels known as reactors, which are designed and constructed in different forms depending on site condition and food feedstock (learn about AD systems engineering and technology).

These reactors are characterized by the action of complex microorganisms that disintegrate waste into a digestion product – biogas – which are discharged from the surface of digester tanks.

Anaerobic digestion

Anaerobic digestion is a process through which microorganisms break down biodestructable materials when oxygen is unavailable. It uses a natural industrial process for the production of materials.

Many industrial fermentations for food products and beverage products and home fermentation utilize anaerobic digestion.

Anaerobic digestion occurs naturally in many soils and sediments in lakes or marine basins. The methane emitted from marshes has been identified by Alessandro Volta since 1776.

Organic residues and waste from the municipal collection of organic waste, green waste, animal husbandry and breeding, as well as agriculture and arable farming, are all used in the anaerobic digestion process.

Furthermore, feed materials from the food industry and gastronomy are converted into clean, renewable energy, valuable organic fertiliser, and compost for healthy soils.

This process accomplishes all of this while adhering to the principles of a sustainable cycle. It's an untapped resource that exists in abundance all over the world.

What Happens in an AD Plant?

In an anaerobic digestion plant, normally occurring micro-organisms digest the biomass, which results in an output of a methane-rich gas (biogas) that can be made use of to generate sustainable heat and power. This tends to reduce fossil fuel usage. It also lowers greenhouse gas emissions. What is left over after the process is complete is a material (digestate) rich in nutrients, so it can be used as a natural fertiliser which is also renewable.

What are AD Plants Fed With?

Several kinds of feedstock are appropriate for use in anaerobic digestion plants. These include food waste, slurry as well as manure, in addition to plants and plant residues. Nevertheless, woody biomass can not be utilised in these plants since the micro-organisms can't break down the lignin. It is the lignin that gives wood its woodiness and makes wood hang around a long while because it is hard to digest.

A typical commercial AD Plant processes about 10,00

Naturally occurring micro-organisms digest the biomass, which releases a methane-rich gas (biogas) that can be used to generate renewable heat and power; this helps cut fossil fuel use and reduce greenhouse gas emissions. The remaining material (digestate) is rich in nutrients, so it can be used as a fertiliser.

The average commercial AD plant ranges in throughput tonnages, from 5,000 to 30,000 tonnes annually of organic feedstock.

The most common form of CSTR AD plant comprises:

1. A reception facility where incoming wastes are offloaded checked for the presence of impurities or “contraries” and fed into the plant and AD reactor vessel(s) material processing and supply system
2. Pre-treatment facilities as necessary to the feed materials before entering the AD reactor vessel(s) with pre-treatment being provided for such purposes as removing all recyclables, cutting up and mixing, and optimising the reactor feed materials for maximum biogas production
3. The reaction stage in which the “fermentation” takes place in single or multiple tanks, in series or in parallel circuits
4. The post-treatment stage where both solid and liquid phase outputs will be further treated to suit the specified provision of a product wherever possible. In most cases, there will be a final maturation stage  where the solid fibrous “compost” in the output will be allowed to aerobically decompose further
5. Ancillary structures such as a biogas storage facility (typically round or oval in shape and distinctive looking), storage tanks to hold liquid digestate etc.
6. Equipment for generating electricity from the biogas and is usually a large reciprocating engine that utilizes the methane gas as its fuel.
7. A treatment facility for any excess liquid digestate if this is unsuitable for sale. Where such a facility is needed it will typically be an oxidation stage reactor where the air is passed through the water in sequencing batch reactors or a reverse osmosis unit
8. Related buildings etc
9. Site access roads, drainage and electric connection to the local grid, plus any other services. installations.

The Agri-Food and Biosciences Institute (UK) has a good page about Anaerobic digestion plants in agriculture, and we quote their statement:

Anaerobic Digestion Plants for Agriculture

Anaerobic digestion (AD) plants can be on-farm units, designed to deal with manures and other organic materials produced at farm level. Alternatively, AD plants can be designed as centralised units to deal with products from a number of farms, along with co-digestion of organic materials from other industries…

and,

The most appropriate way… to realise this energy potential is through centralised anaerobic digestion (CAD) schemes (Frost, 2005). In addition to energy production, there is considerable potential for CAD to assist in centrally managing the distribution of plant nutrients in manures, together with minimising bio-security risks (pathogen kill) .. . Whilst CAD has potentially a major role… and offers the most appropriate and immediate way forward, there is also significant potential for on-farm AD.

Still, on the same page, they describe the increasingly popular idea of the centralised Anaerobic Digester:

Centralised anaerobic digestion

The main feedstocks for typical agriculturally based centralised AD (CAD) plants are farm products (livestock manures and crops), as well as other organic material from, for example, food processing.

Co-digestion can provide an additional source of income through gate fees while also increasing biogas yield per unit of feedstock input.

Thermophilic or mesophilic CAD plants are possible. CAD plants are larger (0.1-1.0 MW electricity) than typical on-farm plants, provide economies of scale, and provide better market opportunities for heat (for local industry and/or district heating) and fibre production.

A number of farms within a 10-kilometer radius of the plant may be involved in CAD schemes. All agriculturally based CAD schemes return digestate to agricultural land, typically the supplying farms'.

Plant nutrients abound in raw slurry and digestate (nitrogen, phosphorus and potassium). Digestate must be applied to agricultural land in accordance with crop nutrient needs. When determining the feasibility of any AD scheme, nutrient management is a major factor to consider.

CAD schemes have a great deal of potential for assisting in the management and re-distribution of plant nutrients in slurry. It is critical to ensure biosecurity when redistributing digestate to farms. All CAD schemes should include material sterilisation prior to redistribution.

So there you have it, and this page is hopefully all you needed in order to understand what an Anaerobic Digestion Plant is.

More About Anaerobic Digestion Plants

Plants can be generally be categorised as:

• Agricultural — plants that make use of mainly farming feedstock such as manures, slurries, crops and plant deposits.

Or,

• Waste – plants that utilize primarily local, industrial and also hazardous waste streams as feedstock.

Each is, after that, categorised by the end-use of the biogas:

• Heat and/or Power (CHP) — an anaerobic digester generating biogas that is burned on-site to generate heating hot water, power or both.
• Biomethane to Grid (BtG)— an anaerobic digester generating and updating biogas, to derive biomethane for a shot into the nationwide gas grid

How much energy could anaerobic digestion generate in the UK?

AD could generate 10-20 TWh of heat and power per year by 2020. To put this in context, the United Kingdom's largest power station Drax sold 27.1 TWh of electricity in 2012. To put it another way, AD could represent 3.8-7.5% of the renewable energy we estimate will be required in 2020. via biogasinfo

What are the Disbenefits of AD?

AD plants are 24-hour operations and because of this, they need to be fed on a regular basis. Pumps and other machinery also require to be maintained to guarantee production is not disrupted.

There can be noise made by these plants especially of vehicles reversing. They can also be dirty, as well as there could possibly be leakages.

The potential for smells also exists and also environmental contamination from accidental emissions of digestate fluids.

Nonetheless, these issues are all easily avoided if managed to comply with ecological regulations, so will not occur, or if they do the plant operator will be prosecuted.

The fluid component of the digestate consists of nitrates and various other chemicals which should not be released to water but which can in most locations safely be spread out on the land or refined to make a more “in-demand fuel”.

Stages of the AD Process

Acidogenesis: the most important acidogenetic fermentations, with acetate as the main end product. Along with carbon dioxide and hydrogen, volatile fatty acids are produced.

Acetogenesis: is the process by which volatile acids are broken down into acetate and hydrogen.

Methanogenesis: The process by which acetate, hydrogen, and other substances are converted to methane and carbon dioxide.

In fermentation processes, two temperature ranges are distinguished: mesophilic temperatures ranging from 25° to 35° C and thermophilic temperatures ranging from 49 to 60 ° C. The vast majority of agricultural biogas plants run at mesophilic temperatures.

Design Configurations

Anaerobic digestion plants can be designed and engineered to operate using a number of different configurations. These can be categorized into:

• batch vs. continuous process mode,
• mesophilic vs. thermophilic temperature conditions,
• high vs. low portion of solids, and
• single stage vs. multistage processes.

Continuous processes require a more complex design, but still, it may be more economical than batch processes. That's because a batch process requires more initial building money and a larger volume of the digesters (spread across several batches) to handle the same amount of waste as a continuous process digester. via Wikipedia

The future of small scale anaerobic digestion plants

A new method has emerged to produce biogas using small-scale digestion plants (micro-scale digestion plants or granular digestion plants) to reduce carbon and other wastes.

About 130 microscale digesting plants were operational across Europe in 2019. Smaller and more economical, easily self-sufficient production systems attract farmers.

In total the world is home to almost 20,000 AD systems. The total project number is growing by more than 4,000 per annum.

[Article first published in December 2011.]