What is Mechanical Biological Treatment (MBT)?

The EU Landfill Directive stipulates that: before waste can be sent to landfill it must be pre-treated; and, that the amount of biodegradable waste (green waste, food waste, paper/card waste) sent to landfill is reduced. The directive demands such actions in order to reduce the environmental impact of wastes that do get sent to landfill, and also to further increase the amounts of materials recycled.

Mechanical Biological Treatment (MBT) is a waste treatment process that involves both mechanical and biological treatment processes. MBT is used to treat residual waste (waste that has not been collected for recycling or composting and has been placed in wheelie bins/black bags), as well as reducing its biodegradable content. Ultimately, the aim of MBT is to reduce the impacts residual waste has on the environment when sent to landfill. This article will take a closer look at both the mechanical and biological elements of MBT, as well as taking into consideration its potential advantages and disadvantages.

The Mechanical Stage of MBT

In the mechanical stage, the first stage of MBT, residual waste undergoes preparation and separation in order to make it more suitable for biological treatment. Primary waste preparation involves the removal of large, bulky items of waste. This is followed by further mechanical waste preparation techniques used to prepare the materials for subsequent separation stages (DEFRA). The main purpose of such techniques is to either split open refuse bags in order to release the waste within them, or to shred waste into smaller particles sizes suitable for separation or further biological treatment (DEFRA). Shredders and rotating drums are most commonly used to further mechanically prepare residual waste. Following waste preparation, the remaining materials of the residual waste are then separated appropriately in accordance to their different end uses, for example, material recycling and biological treatment. There are a number of different techniques that can be used to separate the remaining materials, and most MBT facilities use a series of several different techniques in combination to achieve specific end use requirements for different materials (DEFRA). Trommel screens, magnetic separation, eddy current separation, and air classification are all types of waste separation techniques which utilize the different properties of the waste, properties such as shape, magnetism, electric conductivity and weight, in order to separate it.

The Biological Stage of MBT

In the biological stage of MBT, the biodegradable fraction of residual waste (green waste, food waste, paper/card waste) is biologically treated. Aerobic digestion/composting and anaerobic digestion are the two most common processes used to biologically treat the waste.

Aerobic digestion/composting is the process whereby organic material is decomposed into carbon dioxide (CO₂), water (H₂O), and heat through microbial respiration in the presence of oxygen leaving a stabilised residual solid material, compost (DEFRA).

This biological treatment of waste is known as aerobic composting and usually takes place in specifically designed vessels such as tunnels.

Anaerobic digestion is a naturally occurring process whereby anaerobic bacteria breakdown organic material, in the absence of oxygen to produce biogas and digestate.

Biogas produced from anaerobic digestion can either be used as a natural gas substitute, or more commonly, it is used to fuel boilers to produce heat (hot water and steam), or to fuel generators in combined heat and power (CHP) applications to generate electricity, as well as heat (DEFRA).

Advantages of MBT

• MBT is proven in Europe as a waste treatment technology used for treating large amounts of residual waste.
• MBT reduces both the volume and biodegradable content of residual waste, therefore, reducing the amount of landfill space taken and the production of landfill gases (methane) when the waste is sent to landfill. The reduced volume of waste also results in lower waste disposal costs for local authorities.
• Poor levels of waste management, particularly recycling, can see recyclable materials become part of the composition of residual waste. Such recyclable materials within residual waste can be captured by MBT and sent for reprocessing.
• Household items/materials that become residual waste due to there being no recycling systems in place for them, e.g. steel coat hangers, can be captured by MBT and sent for reprocessing.

Disadvantages of MBT

• MBT reduces the quality of recyclable material compared to that collected by kerbside collection schemes, in turn, lowering its market value.
• MBT plants bring in large volumes of waste from wide areas. This contradicts the proximity principle which suggest waste should be disposed of as near as possible to its place of generation.
• MBT plants that have long term contracts with local authorities and demand a fixed tonnage of waste from them could potentially undermine recycling and waste minimisation.
• It is possible that some local authorities will see MBT as a way to meet recycling rates without the need for separate kerbside collections of recyclable materials.