Anaerobic Digestion

Anaerobic digestion (AD) is not a new process; it is well established in the waste water industry and more recently it has been used for the treatment of food waste. The process works by bacteria, which thrive in the absence of oxygen, breaking down the bio-degradable fraction of the waste to produce a liquor and biogas.

There are two types of AD - wet and dry and the difference is in the amount of solid in the liquor.  For wet systems this is typically 5 to 15 percent dry matter (DM) and for dry systems over 15 percent DM.  Wet systems operate on a pumping/stirring basis and dry is based on a stackable basis (similar to composting).

There is a portal that has some general information on anaerobic digestion.  This also has a map indicating where the AD sites are.

AD

Process

AD is carried out in a closed vessel(s) and the amount of time the waste spends in the digester is known as the resident (hydraulic retention time HRT) time and this can vary from 18-30 days. There are a number of factors which can affect the HRT; some of the key aspects include the feedstock. The longer a feedstock is processed the more biogas is extracted but there has to be a balance over throughput and biogas output, to make the facility viable.  Too much lignin-type material in the feedstock can take a long time to process and may not produce sufficient biogas.  It is important to control levels of ammonia produced from protein based feedstock as elevated levels can inhibit methanogenesis and the subsequent output of biogas.

Temperature is an important factor when using the AD process. There are two optimal temperatures for the different types of microbes. There are the mesophillic temperatures and this can vary from 30oC-40oC (37oC optimal) and the thermophillic temperatures which vary from 50oC-70oC (65oC optimal). Optimal for the microorganisms is not optimal for pasteurisation and the Animal By-Product Regulations (ABPR). The ABPR only apply if a facility is accepting food waste (for more information on the ABPR (see Animal By Products Regulations 2013  - as well as Technical Briefing Note No. 004 v1).

A number of outputs are produced once the organic material has been through the anaerobic digestion process. All of the outputs can be affected by the quality of the feedstock which is put into the AD digester. The removal of contaminants (e.g. plastics) is important as the AD  process can be hampered by such contraries. After debagging (if this stage is needed) the use of a shredder generates a uniform size of the material (12mm required for ABPR) going into the digester this can help aid the digestion of the waste.

Outputs

The typical outputs which are produced from the AD process are:

Gases: including methane, carbon dioxide and other trace gases which make up biogas. The biogas can be collected and cleaned and then used as a renewable energy source. The biogas which is produced can be used for heat and power which can then be used to run the plant and/or it can be sold to the national grid. AD Strategy and Action Plan 2011

Due to the electricity produced from the AD process being a renewable energy source, biogas qualified under the Renewable Obligations Certificates (ROCs) (new applications closed in 2017) and the Feed-in-Tarriffs (FITs) scheme, which closes to new applications in April 2019.  Renewable Heat Incentive (RHI) is the Government backed scheme that encourages take up renewable energy by businesses.  Northern Ireland has its own RHI scheme.

The amount of biogas produced in digestion facilities can vary between 30% and 75%. Dry AD plants normally produce less biogas than wet AD plants.

Digestate: the liquid output which is usually referred to as the digestate can be used as a liquid fertiliser.  The digestate can be filtered or dried to produce a concentrated liquid and a 'compost' which can be easily handled and spread on land.

Quality Protocol

There is a quality protocol for anaerobic digestion and a public available standard (PAS) 110 which sets out criteria for the production of quality digestate from anaerobic digestion.  The quality protocol identifies the point at which the digestate has reached end of waste and no longer needs to adhere to waste regulations.  This allows QP digestate to be spread without a permit.  It helps provide users with confidence that the digestate they use conforms to an approved standard and helps protect the environment and human health.

BSI PAS 110

PAS 110 was prepared and published by the British Standards Institution (BSI) and includes quality criteria for both the liquid and fibre fraction of the digestate.  It was first published in 2010 and updated in 2014 .  REAL are the certification body under the Biofertiiser Certification Scheme .

Quality Protocol Digestate

In England, Wales and Northern Ireland the quality protocol for the production and use of quality digestate from source-segregated biodegradable waste has been recognised by the environment regulator.

In Scotland if the digestate complies with both PAS110 and SEPA's regulatory position statement (RPS) on the Regulation of Outputs from Anaerobic Digestion Processes , it may be spread on land without waste management regulatory control.  This RPS set out standards in relation to plastic contamination levels which are phased in over a number of years:

  • From 1 Dec 2017 plastic limit 50% of current PAS 110
  • From 1 Dec 2018 plastic limit 25% of current PAS 110
  • From 1 Dec 2019 plastic limit 8% of current PAS 110.

Legislation

Waste facilities including digesters fall under a number of regulations, including Waste Permitting,  the Industrial Emissions Directive (IED) and ABPR (if the plant is accepting food waste).

Permits are required for operating on-farm facilities using farm waste only (new applications from October 2018 and existing facilities from January 2019) and for operating AD as a recovery operation (this is for new applications after October 2018 and existing facilities after January 2019).  Both of these permits include the use of the resultant biogas.  These permits are for facilities designed to take less than 100 tonnes a day.

AD is covered by the BREF on Waste Treatment and SEPA (Scottish Environmental Protection Agency) have produced a technical guidance note for anaerobic digestion.

For environmental guidance in Scotland and Northern Ireland check NetRegs .  SEPA have guidance on licensing of anaerobic digestion.
Cre - The Composting and Anaerobic Digestion Association for Ireland have issues guidelines on planning, permitting and health and safety.