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 biodegradable municipal waste (BMW). The process works by bacteria, which thrive in the absence of oxygen, breaking down the bio-degradable fraction of the waste to produce a stable residue.

AD

AD is carried out in a closed vessel or vessels 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. For example you do not want too much woody material present in your feedstock as this is often hard to break down and therefore can take a long time to degrade and may cause blockages in the system. For the AD process to be effective there needs to be a good carbon to nitrogen ratio in the feedstock, for instance if there is too much nitrogen it can lead to there being elevated levels of ammonia in any effluent.

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 2005  - as amended).
A number of outputs are produced once the organic material has been through the anaerobic digestion process, as illustrated in the figure below. All of the outputs can be affected by the quality of the feedstock which is put into the AD digester. This is why AD may be used alongside other treatment methods, such as after a material sorting facility (MRF). The MRF could be used to remove the non-organic fraction of the waste (e.g. plastics) as a digester would not be able to process these. This stage often includes the use of a shredder to uniform the size of the material (12mm required for ABPR) going into the digester this can help aid the digestion of the waste. When sorting technologies, such as MRFs and biological technologies like AD and composting are combined in this manner, the systems are termed mechanical biological treatment (MBT). For more information on MBT please see Mechanical Biological Treatment

An example of the anaerobic digestion treatment process with a MRF at the front end

Anerobic_Digestion_Picture

Outputs

The typical outputs which are produced from the AD process as shown above:
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 qualifies under the Renewable Obligations Certificates (ROCs) and the Feed-in-Tarriffs (FITs) scheme.   

Ofgem Information on ROCs  , Renewables Obligation (ROCs) Feed-in-Tarriffs

The amount of gas that is able to be converted to biogas is also affected by the type of plant used, HRT and again the feedstock. For example, dry AD plants normally produce less biogas than wet AD plants. The amount of methane in the biogas produced in waste digestion facilities can also vary between 30% and 75%.

Residual Digestate; the fibre, which is usually referred to as the digestate cake or solid digestate. It can be separated from the residual digestate and, once it has been made aerobic which can be done by bio-drying (i.e. blowing air through it to add oxygen and remove the moisture), it can be composted and sold on as a high carbon soil conditioner fertiliser. The liquor can also be separated from the residual digestate and can be sold on as a liquid fertiliser. The main factor affecting the quality of both the liquid and solid fertiliser produced is the feedstock.

Legislation

Waste facilities including digesters fall under a number of regulations, this can depend on the facility being operated and may include Waste Regulations, the Integrated Pollution and Control Directive (IPPC), the Industrial Emissions Directive (IED) and the ABPR if the plant is accepting food waste. For more information in;

England and Wales see http://www.environment-agency.gov.uk/subjects/waste/
Scotland see Scottish Environment Protection Agency
Northern Ireland see Northern Ireland Environment Agency's Waste Regulation
Republic of Ireland see Environmental Protection Agency Information on Waste