Technologies

A wide variety and mix of waste treatment technologies are likely to be proposed by bidders for the residual and food waste treatment contracts.  There are some stipulations as to the scale of the different types of technology the Scottish Government will allow local authorities to use, such as no more than 25 per cent of all municipal waste can be treated by energy from waste processes.

Both Councils will only consider tried and tested technologies and at the same time have a determination to encourage as much waste reduction, reuse and recycling as possible in order to minimise the amount of residual waste that needs to be treated.

Our current intention is that the planning application in principle to be submitted for the project site near Millerhill in Midlothian, will look for permission to build a mix of technologies, specifically;

• an anaerobic digestion facility for food waste (30,000 tonnes per annum),
• an MBT facility (200,000 tonnes per annum) and
• an energy from waste facility (100,000 tonnes per annum).

We have completed an extended period of pre-application public consultation.  The application is likely to be submitted to Midlothian Council's Development Management section for consideration in early 2011.

Once contractors have been appointed they will then submit detailed planning applications which will contain much more information on the exact types and capacities of technologies they propose to build.  They will include a lot more design details about the facilities, including, for example what the exteriors will look like.

The following is an outline of the main types of waste treatment technology available:

Anaerobic Digestion (AD)

This is an accelerated composting process which takes place in the absence of oxygen in a sealed vessel like a large silo.

It is generally used to treat food waste, and produces a biogas that can be used as a fuel for vehicles or to generate renewable electricity and heat. It also produces a liquid that can be used as a fertiliser, and a composted fibre.

Mechanical Heat Treatment (MHT) - Autoclave

This is a rapid sterilising process that uses steam to treat pre-sorted mixed waste inside a large pressurised drum. It produces a light, cellulose fibre 'flock' from the paper and other organic parts of the waste which has a range of potential uses, but primarily as a fuel.

Mixed plastics, metals, glass and aggregates are separated off during the process.

Mechanical Biological Treatment (MBT) - Hydro-mechanical

This is a process in which pre-sorted mixed waste is immersed in large water-filled vessels to separate the recyclables  from the organic waste, which is then composted.  As with anaerobic digestion, it produces a valuable biogas, but in this case since the liquid and fibre outputs originate from mixed waste, they cannot be used on agricultural land.

Mechanical Biological Treatment (MBT) - Bio-drying

There are many combinations of processes that can be termed MBT.  They involve the separation of recyclables from mixed waste either by hand and/or mechanically, either  before or after partial in-vessel composting.

They all take place under controlled conditions inside a large industrial building.  The  main output is a stabilised residue suitable as low grade landfill cover or as secondary fuel for energy-from-waste plants.

Energy from Waste (EfW) - Incineration

This is the most widely used thermal treatment and involves the controlled burning of mixed waste or refuse-derived fuel in a large industrial furnace, leaving an ash from which metals can be extracted.

Most of the ash can also be recycled as aggregate. The heat is normally used to produce steam that can be used to generate electricity and/or hot water for district heating networks. Where the heat is used for both, this is known as Combined Heat & Power (CHP)

 

Energy from Waste (EfW) - Pyrolysis/Gasification

These can be either separate or combined thermal processes which take place inside smaller, but still industrial scale, plants.

Refuse-derived fuel (RDF) or solid recovered fuel (SRF) is broken down by heat either in the absence of oxygen (pyrolysis) or in a controlled oxygen atmosphere (gasification).

They mainly produce gas which can be burned to generate electricity and char (a solid residue) which can be recycled.

 

For more detailed information on each of these you can visit either of the websites listed below.

SEPA (the Scottish Environmental Protection Agency) is the body responsible for waste regulation in Scotland.  It will be their responsibility to review any applications for new waste treatment facilities and to apply rigorous conditions that ensure their design incorporates high standards of environmental control.

Once any plant is operational SEPA will monitor the facility to make sure it is operated within the strict limits of its pollution prevention and control (PPC) permit.

 

Defra (Department for Environment, Food and Rural Affairs) is the English equivalent and has lots of useful, regularly updated, information on its website.