Mechanical biological treatment

A mechanical biological treatment system is a form of waste processing facility that combines a sorting facility with a form of biological treatment such as composting or anaerobic digestion. MBT plants are designed to process mixed household waste streams or similar mixed commercial and industrial wastes.

System outline
The terms 'mechanical biological treatment' or 'mechanical biological pre-treatment' relate to a group of solid waste treatment systems. These systems enable the recovery of materials contained within the and stabilisation of the biodegradable component of the material.

The sorting component of the plants resemble a materials recovery facility. This component is either configured to recover the individual elements of the waste or produce a refuse-derived fuel that can be used for the generation of power.

The components of the mixed waste stream that can be recovered include:


 * Metals
 * Plastics
 * Glass

Terminology
MBT is also sometimes termed BMT – biological mechanical treatment – however this simply refers to the order of processing, i.e. the biological phase of the system precedes the mechanical sorting. MBT should not be confused with MHT – mechanical heat treatment - which does not include any form of biological degradation or stabilisation.

Mechanical sorting


The "mechanical" element is usually an automated mechanical sorting stage. This either removes recyclable elements from a mixed waste stream (such as metals, plastics and glass) or processes them. It typically involves factory style conveyors, industrial magnets, eddy current separators, trommels, shredders and other tailor made systems. The mechanical element has a number of similarities to a materials recovery facility (MRF).

Some systems integrate a wet MRF to recover & wash the recyclable elements of the waste in a form that can be sent for recycling. MBT can alternatively process the waste to produce a high calorific fuel given the term refuse derived fuel (RDF). RDF can be used in cement kilns or power plants and is generally made up from plastics and biodegradable organic waste. Systems which are configured to produce RDF include the Herhof and Ecodeco Processes. It is a common misconception that all MBT processes produce RDF. This is not the case and depends strictly on system configuration and suitable local markets for MBT outputs.

Biological processing


The "biological" element refers to either:


 * Anaerobic digestion
 * Composting
 * Biodrying

Anaerobic digestion breaks down the biodegradable component of the waste to produce biogas and soil improver. The biogas can be used to generate renewable energy.

Biological can also refer to a composting stage. Here the organic component is treated with aerobic microorganisms. They break down the waste into carbon dioxide and compost. There is no green energy produced by systems employing only composting treatment for the biodegradable waste.

In the case of biodrying, the waste material undergoes a period of rapid heating through the action of aerobic microbes. During this partial composting stage the heat generated by the microbes result in rapid drying of the waste. These systems are often configured to produce a refuse-derived fuel where a dry, light material is advantageous for later transport combustion.

Some systems incorporate both anaerobic digestion and composting. This may either take the form of a full anaerobic digestion phase, followed by the maturation (composting) of the digestate. Alternatively a partial anaerobic digestion phase can be induced on water that is percolated through the raw waste, dissolving the readily available sugars, with the remaining material being sent to a windrow composting facility.

By processing the biodegradable waste either by anaerobic digestion or by composting MBT technologies help to reduce the contribution of greenhouse gases to global warming.

Consideration of applications
MBT systems can form an integral part of a region's waste treatment infrastructure. These systems are typically integrated with kerbside collection shemes. In the event that a refuse-derived fuel is produced as a by-product then a combustion facility would be required.

Alternatively MBT solutions can diminish the need for home separation and kerbside collection of recyclable elements of waste. This gives the ability of local authorities and councils to reduce the use of waste vehicles on the roads and keep recycling rates high.

Position of Environmental Groups
Friends of the Earth suggests that the best environmental route for residual waste is to firstly remove the remaining recyclable elements from the waste stream (such as metals, plastics and some paper). The small amount of waste remaining should be composted or anaerobically digested and unless sufficiently clean to be used as a compost should be disposed of in landfill. MBT plants that fit in with these statements could therefore play an increasing role in the environmental management of mixed streams.