Composting and anaerobic digestion

Food and other organic wastes can be treated by composting and anaerobic digestion to produce valuable resources such as fertiliser and biogas.

Composting and anaerobic digestion are increasingly being used as an alternative to landfill and we have an important role to play in ensuring these are correctly managed and regulated.

What is composting?

Composting is the autothermic (self-heating) and thermophilic (between 40 to 80 degrees Celsius) biological decomposition and stabilisation of biodegradable waste under controlled conditions, including the presence of oxygen.

This process produces a stable, sanitised material that can be applied to land for the benefit of agriculture, horticulture or ecological improvement.

Composting can range from the very simple ‘open windows’ (large heaps), to sophisticated computer controlled in-vessel systems (IVCs).

What is anaerobic digestion?

Anaerobic digestion (AD) is the biological decomposition and stabilisation of biodegradable waste in the absence of oxygen.

This process produces a stable, sanitised material that can be applied to land for the benefit of agriculture, horticulture or ecological improvement. It also generates biogas which can be used directly in place of natural gas or used to generate electricity.

AD is generally done on an industrial scale using sophisticated computer systems. Smaller scale plants can be used to treat manures and slurries generated on farms.

What is mechanical biological treatment?

Mechanical biological treatment (MBT) is another technology associated with composting and AD.

The terms 'mechanical biological treatment' or 'mechanical biological pre-treatment' relate to a group of solid waste systems that combines a sorting facility with a form of biological treatment such as composting or AD. The organic output may have a higher level of contaminants (for example, plastics and glass) than other biological treatment processes which only treat source segregated wastes.

MBT plants are normally designed to process mixed wastes, but can also process source segregated waste.

The ‘mechanical’ element is usually an automated mechanical sorting stage involving factory-style conveyors, industrial magnets, eddy current separators, trommels, shredders and other systems. These systems remove recyclable elements from a mixed waste stream (such as metals, plastics, glass and paper) or process them.

Some MBT plants process the waste to produce a high calorific fuel called Refuse Derived Fuel (RDF). RDF can be used in cement kilns or power plants and is generally made up from plastics and biodegradable organic waste.

How are these processes regulated?

The regulation of composting and AD varies depending on the size of the operation, the technology used and types of waste processed.

Most biological waste treatment processes are subject to regulatory control through either the Waste Management Licensing (Scotland) Regulations 2011 (WML) or the Pollution Prevention and Control (Scotland) Regulations 2012 (PPC).

If they treat food waste they will also be subject to the Animal By-Products (Scotland) Regulations 2011[bookmark] which are enforced by the Animal and Plant Health Agency (APHA) .

  • Activities regulated by the Waste Management Licensing (Scotland) Regulations 2011

Some composting and AD activities are exempt from the requirement to have a full waste management licence.

There is an exemption for composting activities that meet the requirements detailed in Paragraph 12 of the Waste Management Licensing (Scotland) Regulations 2011 .

These requirements relate to the types of waste that can be composted and whether this requires special buildings or concrete surfaces with sealed drainage systems.

AD processes that meet the requirements in Paragraph 51 do not require a full waste management licence.

To  be classed as an exemption, the activity must not:

  • endanger human health;
  • harm the environment by presenting a risk to water, air, soil, plants or animals;
  • adversely affect the countryside or places of special interest.
  • cause nuisance through noise or odours;

Exemptions are either simple or complex – our guide to exempt activities will help you to identify your activity and its classification.

Thereafter, you can apply for registration online or by downloading and completing the relevant paper form. We then have 21 days to decide whether or not to grant registration.

However, even if an activity is exempt from waste management licensing, it is still subject to statutory controls to prevent environmental pollution and harm to human health and must be registered.

  • Activities regulated by the Pollution Prevention and Control (Scotland) Regulations 2012

The biological treatment of animal waste, including catering waste, is regulated by the Pollution Prevention and Control (Scotland) (PPC) Regulations 2012 .

Our section on PPC regulations contains more information about how these are imposed.

  • Activities regulated by the Animal By-Products (Scotland) Regulations 2011

The Animal By-Products (Scotland) Regulations 2011 set out the statutory requirements governing the processing and disposal of animal by-products, including composting. These regulations are enforced by the Animal and Plant Health Agency (APHA).

The Scottish Government’s guidance gives more detailed information the regulations are applied.

When do the regulations apply to me?

You will need a PPC permit if you carry out the following activities:

  • composting or AD of food waste in a process with a capacity of greater than 10 tonnes per day
  • composting any other waste in a process with a capacity of greater than 75 tonnes per day
  • AD of any other waste in a process with a capacity of greater than 100 tonnes per day

You will need a Waste Management Licence if you carry out a composting or AD process on any waste below the PPC thresholds. However, some composting or AD activities may be registered under an exemption if they fit within these thresholds:

  • compost up to 400 tonnes of green waste or kitchen and canteen waste (all food waste must be treated in an enclosed process), or
  • compost on a farm up to 1000 tonnes of green waste arising from within the farm business
  • the AD plant has a capacity to treat up to 100 tonnes per day of waste arising from agriculture and distillery processes, and
  • the process makes biogas, which is used to generate energy, and a fertiliser

We do not regulate household composting systems. Zero Waste Scotland has produced a number of guides to home composting which will be of use to households who wish to compost food or garden waste.

Contact us

For more information on any aspect of waste disposal through composting or the regulations governing it, please contact us.