Anaerobic digestion

Anaerobic digestion

Anaerobic digestion (AD) processes plant materials, known as biomass, into gas to be used for heating and power; the resultant gas is called methane or biogas and is created by bacteria which digest biomass and produce methane as a by-product.

Biomass includes anything that is plant-derived; municipal solid waste, manure, crop residues, compost, food waste, paper and waste water. Crops can be grown specifically for use in AD, as a supplementary feedstock, or a stabilising material. Biogas has been used in the UK since 1895 when gas from sewage was used in street lamps by the city of Exeter.

Looking at this in more detail, anaerobic digestion is a natural process where plant and animal materials are broken down by micro-organisms in the absence of air as follows:

  • The AD process begins when biomass is put inside a sealed tank or digester
  • Naturally occurring micro-organisms digest the biomass which releases a methane-rich gas, known as biogas that’s used to generate renewable heat and power. The process helps cut fossil fuel use and reduce greenhouse gas emissions
  • The remaining material, known as digestate, is rich in nutrients and can be used as a fertiliser

Many forms of biomass are suitable for AD including food waste, slurry and manure, as well as crops and crop residues. However, woody biomass cannot be used because the micro-organisms can't breakdown the lignin - the compound that gives wood its strength.

AD is not a new technology, it has been used in the UK since the late 1800s, but an increasing number of AD plants are being built to generate clean renewable energy. AD is also used to treat waste produced in homes, farms, supermarkets and industry which helps divert waste from landfill.

The products of AD are referred to as biogas and digestate. Biogas is a mixture of 60% methane, 40% carbon dioxide and traces of other contaminant gases. The exact composition of biogas depends on the type of feedstock being processed.

Biogas can be combusted to provide heat and/or electricity. Alternatively, the biogas can be upgraded to pure methane, often called biomethane, by removing other gases. This pure stream of biomethane can then be injected into the mains gas grid or used as road fuel.

Digestate is made from left-over indigestible material and dead micro-organisms. It contains valuable plant nutrients such as nitrogen and potassium. It can be used as a fertiliser and soil conditioner.

Typical biogas/anaerobic digestion plant

Balmoral Tanks wishes to thank NNFCC, The Bioeconomy Consultants, for their kind assistance in preparing the copy for this page.


Different types of AD

The term AD covers several different processes; the most common are shown below and can be classified according to whether they are:

  • Mesophilic, 25-45°C; or thermophilic, 50-60°C
  • Wet, 5-15% dry matter in the digester; or dry, over 15% dry matter in the digester
  • Continuous flow or batch
  • Single, double or multiple digesters
  • Vertical tank or horizontal plug flow

Mesophilic or thermophilic
Mesophilic systems operate at 25-45°C while thermophilic systems operate at 50-60°C or above. Thermophilic systems have a faster throughput with more rapid biogas production per unit of feedstock and digester capacity and there is greater pathogen kill. However, the capital costs of thermophilic systems are far greater as more energy is needed to heat them and they generally require more management.

Wet or dry
The difference between what is considered a wet process and a dry process is quite small. Effectively, in wet AD the feedstock is pumped and stirred and in dry AD it can be stacked. Dry AD tends to be cheaper to run as there is less water to heat and greater gas production per unit feedstock. However, wet AD has a lower set-up capital cost.

Continuous or batch flow
Most digesters are continuous flow as opening the digester and restarting the system from cold every few weeks is a management challenge. They generally provide more biogas per unit feedstock and operating costs are lower. Some dry systems are batch flow and, to overcome peaks and troughs in gas production, multiple batch digesters with staggered changeover times are specified.

Single or multiple digesters
AD occurs in several stages; some systems have multiple digesters to ensure each stage is as efficient as possible. Multiple digesters can provide more biogas per unit feedstock but at higher capital and operating costs with a greater management requirement. Most digesters are single or double digesters.

Vertical or horizontal plug flow
Vertical tanks simply take feedstock through a pipe on one side of the tank whilst digestate flows out through a pipe on the other side. In horizontal plug-flow systems a more solid feedstock is used as a 'plug' that flows through a horizontal digester at the rate it is fed in.

Vertical tanks are simple and cheaper to operate, but the feedstock may not reside in the digester for the optimum period of time. Horizontal tanks are more expensive to build and operate but the feedstock will neither leave the digester too early nor remain within for an uneconomically long period.

The optimum system will be determined by what feedstocks are available, the output required, ie, energy production or waste mitigation, space and infrastructure.



Biogas/anaerobic digestion plant install


Planning an AD project

A typical AD project can be divided into three planning phases; design, build and operation. The first stage is to identify a suitable site and determine what the feedstock will be.

Upon choosing a site the following needs to be evaluated:

  • Site layout and space to ensure sufficient space and access for construction, operation and storage
  • Feedstock supplies and maintenance access
  • Visual impact in terms of topography and natural screening
  • Regulatory and environmental requirements regarding buildings and water course proximity

Planning permission is necessary for most anaerobic digestion installations. Small scale digesters using only on-farm waste may be passed as ‘Permitted development’, but local authorities must be consulted in the early stages of planning for confirmation.

Any installation accepting third party waste will need full planning permission.


Balmoral Tanks AD products

Steel digester tanks
Balmoral’s steel tank product range is the most comprehensive in the industry. Designed to service a wide range of applications Balmoral delivers tailored solutions which are suitable for all types of anaerobic digestion systems.

efusion epoxy coated tanks represent the very latest developments in steel tank coating technology. This has provided significant engineering and cost benefits to users across a range of applications since the early 2000s.

The benefits of epoxy coated steel tanks against more traditional tank coatings include higher durability to impact, lower cost manufacturing, lower carbon footprint and more efficient design principles.

The epoxy is electro-statically applied and thermally bonded in a two-coat process. This ensures all efusion panels have the most durable and resistant coating possible and all products are tested for discontinuities and thickness prior to dispatch.

Balmoral Tanks guarantees that only 100% defect-free panels leave its factory.

efusion is tested to similar stringent standards as glass coated steel and meets or exceeds the following internationally comparable coating requirements:
ISO 12944
WIS 4-25-01
ISO 28765:2008
EEA 7.20

Epoxy coated steel tank for anaerobic digestion systems



Concrete tanks
Balmoral is the UK's first choice for large scale reinforced concrete digester tanks. Company personnel have completed numerous demanding construction projects of this type since the early 2000s. Digestore logo The Balmoral digestore is a reinforced concrete, post-tensioned digester tank offering the following benefits:

  • OSHAS 18001, 14001 and 5-Star British Safety Council accreditations
  • Leak free structure using post-tensioning technology
  • Unique Balmoral design and build process offers a tank with low carbon footprint
  • Lifecycle more than twice that of steel tanks: 60 years v steel tanks’ lifecycle of 25 years
  • Highly experienced and trained construction teams
  • Dedicated design team headed up by chartered Civil Engineer with over 30 years’ experience
  • Established track record with high levels of customer satisfaction and repeat business
  • Highly experienced contracts team

Design and build
With reinforced concrete tanks offering the most durable solution to the AD sector, Balmoral Tanks supports its clients with end to end service.

The company provides a full build approach working with highly trained employees. Balmoral installation teams always work to the latest Health & Safety standards whilst maintaining the leanest build durations.

In addition to providing tanks, Balmoral offers civils packages including excavation, piling and related activities.

Cast in-situ reinforced concrete tanks
Balmoral reinforced concrete tanks are designed to all appropriate standards including British Standards (BS) and Eurocode.

From large scale anaerobic digesters to on-farm slurry stores, and using its proprietary formwork system, Balmoral offers build times of the shortest possible duration and does this without compromising quality or health and safety.


Concrete tank for anaerobic digestion systems