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Amechanical biological treatment (MBT) system is a type of waste processing facility that combines a sorting facility with a form of biological treatment such ascomposting oranaerobic digestion. MBT plants are designed to processmixed household waste as well ascommercial andindustrial wastes.
The termsmechanical biological treatment ormechanical biological pre-treatment relate to a group ofsolid waste treatment systems. These systems enable the recovery of materials contained within themixed waste and facilitate the stabilisation of thebiodegradable component of the material.[1][2] Twenty two facilities in the UK have implemented MBT/BMT treatment processes.[3]
The sorting component of the plants typically resemble amaterials recovery facility. This component is either configured to recover the individual elements of the waste or produce arefuse-derived fuel that can be used for the generation of power.
The components of the mixed waste stream that can be recovered include:
MBT is also sometimes termed biological mechanical treatment (BMT), 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 withmechanical heat treatment (MHT).
The "mechanical" element is usually an automated mechanicalsorting stage. This either removesrecyclable elements from a mixed waste stream (such as metals, plastics, glass, and paper) or processes them. It typically involves factory styleconveyors, industrialmagnets,eddy current separators,trommels,shredders, and other tailor made systems, or the sorting is done manually at hand picking stations. The mechanical element has a number of similarities to amaterials recovery facility (MRF).[4]
Some systems integrate a wet MRF to separate by density and flotation and to recover and 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 termedrefuse derived fuel (RDF). RDF can be used incement kilns or thermalcombustion 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.
The "biological" element refers to either:
Anaerobic digestion harnesses anaerobic microorganisms to break down thebiodegradable component of the waste to producebiogas andsoil improver. The biogas can be used to generateelectricity andheat.
Biological can also refer to a composting stage. Here the organic component is broken down by naturally occurring 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 and 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 awindrow composting facility.
By processing the biodegradable waste either by anaerobic digestion or bycomposting MBT technologies help to reduce the contribution ofgreenhouse gases toglobal warming.
Usable wastes for this system:
Possible products of this system:
Further advantages:
MBT systems can form an integral part of a region's waste treatment infrastructure. These systems are typically integrated withkerbside collection schemes. In the event that aRefuse-Derived Fuel is produced as a by-product then a combustion facility would be required. This could either be an incineration facility or a gasifier.
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, municipalities and councils to reduce the use of waste vehicles on the roads and keep recycling rates high.
Friends of the Earth suggests that the best environmental route forresidual waste is to firstly maximise removal of remaining recyclable materials from the waste stream (such as metals, plastics and paper). The amount of waste remaining should be composted or anaerobically digested and disposed of tolandfill, unless sufficiently clean to be used as compost.
A report by Eunomia[5] undertook a detailed analysis of the climate impacts of different residual waste technologies. It found that an MBT process that extracts both the metals and plastics prior to landfilling is one of the best options for dealing with our residual waste, and has a lower impact than either MBT processes producing RDF for incineration or incineration of waste without MBT.
Friends of the Earth does not support MBT plants that produce refuse derived fuel (RDF), and believes MBT processes should occur in small, localised treatment plants.[6]
