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 & biodrying – 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
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 GE Jenbacher gas engines.
Gas engines can be integrated into MBT-AD plants in a number of 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. 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.