Mechanical biological treatment
MBT is a generic term used to describe a number of different residual waste treatment processes that involve both mechanical and biological treatment of municipal solid waste (MSW).
MBT plants are used to separate mixed waste streams, typically from MSW, into a range of dry products (typically ferrous and non-ferrous metals and glass), high calorific value refuse derived fuels (RDF) suitable for incineration, and wet biodegradable slurries suitable for either composting or anaerobic digestion (AD).
MBT technology uses the naturally generated heat produced during biological degradation to dry and stabalise MSW. There are two main approaches and the difference between them is the stage at which the biological part of the waste is treated – either before the mechanical separation process or after it has taken place.
MBT itself is a two stage process where on receipt at a processing plant MSW will be, after mixing and shredding (required to ensure waste is evenly mixed and sized), separated into different fractions using a range of different processes such as :
- Screening – removal of larger pieces of waste;
- Magnetic separation – removal of ferrous materials such as tin cans;
- Eddy current separation – removal of non-ferrous metals such as aluminium cans;
- Optical separation – separation of certain types of plastic; and
- Air classification – removal of light materials such as paper.
As discussed above, once separated the materials can go on to be further recycled and the RDF used in energy production. The quality of the end products will depend upon the processed that is used. Dry materials are generally of poor quality and only some types can be recycled eg glass is used as an aggregate in road construction. The residual biological material is generally of quality that is only suitable for landfill restoration.
Further information is available from: