Mechanical biological treatment – Anaerobic Digestion

Mechanical Biological Treatment with Anaerobic Digestion (MBT-AD)

Disposal and treatment of biological waste represent a major challenge for the waste industry. For a wide range of organic substances from agriculture, foodstuff of feed industries, anaerobic fermentation is a superior alternative to composting. Biogas – a mixture of methane and carbon dioxide – is created during anaerobic digestion and serves as a high-energy renewable fuel that can be used as a substitute for fossil fuels. Biogas-fuelled gas engines improve waste management while maximising the use of an economical energy supply.

Mechanical biological treatment is a waste treatment technology that processes waste mechanically in order to separate recyclable elements from the organic component of the waste. The organic component is then treated biologically to stabilise the material. Along with recyclable components it also produces a refused derived fuel (or solid recovered fuel) which is typically sent to an energy from waste plant. There are two forms of biological treatment for the organic fraction:

  • Aerobic and bio-drying – Here a form of composting takes place. This system does not produce any valuable biogas and is a net energy consumer
  • Anaerobic – Here the organic component is converted into biogas, a renewable fuel and a solid digestate. With biogas as a fuel for gas engines, it is a net energy producer

With biogas as a fuel for gas engines, the MBT plant can be a net energy producer.

Mechanical biological treatment with anaerobic digestion (MBT-AD) is an advanced form of waste treatment which can be combined with embedded generation with the utilisation of Jenbacher gas engines. Gas engines can be integrated into MBT-AD plants in several ways. Primarily the combustion of the biogas in gas engines produces a source of renewable power well in excess of the power requirements of the plant itself. The heat from the gas engines can be used to provide heat for the digestion tanks in a combined heat and power / cogeneration configuration, or directly to support the drying of the RDF.

Digestion systems need to be maintained at ~37°C for mesophilic and 55-65°C for thermophilic digestion. It can also be used in order to pasteurise the organic material so that it has pathogens removed and is able to be safely used on land as a soil improver.

The exhaust gas is also an attractive source of energy to dry outputs from the MBT-AD facility and can be fed directly into drum driers or belt driers. Drying of the material may be useful for two key reasons:

  • To remove water in order to increase the calorific value of the material, for utilisation as a fuel
  • To remove water in order to reduce the weight of the material and hence reduce costs and emissions associated with its transportation

Typically, siloxanes are rarely found to be a problem in food waste digestion projects. However due to the mixed waste source of the feedstock for MBT-AD plants, it is possible that this can include materials that contain volatile silica. When designing the facility correct gas pre-treatment should be installed according to the expected properties of the biogas.

Boigas CHP Schematic with Boiler for MBT-AD

Advantages of Biogas from MBT-AD

  • Electrical efficiency of up to 43%, and up to 90% efficiency in the case of combined heat and power.
  • Smooth operation despite fluctuations in gas composition and pressure.
  • Low weight containerised units that are easy to move and adjust to changing project capacity.
  • Basic design and support for gas conditioning, if required.
  • AIR: Integrated exhaust after-treatment solution complying with country-specific standards.
  • Flexible contractual maintenance agreements

 

More information

If you would like to learn more about the deployment of biogas engines or biogas upgrading plants alongside an MBT-AD facility, please contact your local Clarke Energy office.

 

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