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Technical Information, and Developments in Mechanical Biological Treatment (MBT)

Mechanical Biological Treatment is a generic term for an integration of several processes commonly found in other waste management processes such as Materials Recovery Facilities (MRFs), Refuse Derived Fuel (RDF), sorting and composting plant. At present (Autumn 2006) about half a dozen MBT facilities are currently under construction in the UK.Click on image for list.

There are a wide variety of systems promoted to the UK municipal waste management market and the principle of the MBT plant is to stabilise and separate the residual waste stream into less harmful and / or more beneficial output streams. MBT Plants normally incorporate a number of different process technologies. Given the large and growing number of technologies and proprietary variants available in the market it has been suggested that there are now in excess of 50 different combinations, each of which would be classified as an MBT Plant.

The processes are designed to handle raw 'black bag' municipal waste (after any source segregated recycling and composting has taken place) and tend to involve a recyclate recovery element (typically metals and glass) and drying / partial composting of the remaining waste to produce a more stabilised residue.

The recyclable component may be extracted either prior to or post 'stabilisation'. The remainder of the waste is screened / sorted and homogenised to produce either a feedstock for another treatment process (e.g. a refuse derived fuel for energy recovery in a gasification, co-incineration, or Energy from Waste plant) or may be sent to landfill as a partially stabilised residue.

Some systems may screen the waste to produce a compostable stream appropriate for in-vessel composting processes. Where the waste is to be used as a secondary fuel there is usually a minor non combustible element sent for disposal to landfill. An alternative MBT approach is the extraction of recyclables followed by homogenisation of the residue prior to processing in an Anaerobic Digestion plant. Whole waste composting with some materials extraction may also be regarded as simple MBT to provide a residue for landfilling / low grade land restoration type applications / subsequent thermal treatment.

Schematic diagram showing the Inputs and Outputs of a typical Mechanical Biological Treatment (MBT) process

Mechanical Biological Treatment (MBT) process  schematic

Some systems may include facilities to screen the waste to produce a minor compostable stream appropriate for in-vessel composting processes.

Where the waste is to be used as a secondary fuel there is usually a minor non-combustible element which remains after processing, which is sent for disposal to landfill.

An alternative MBT approach is the initial extraction of recyclables, followed by homogenisation of the residue prior to processing in an Anaerobic Digestion (AD) plant.

Whole waste composting with some materials extraction may also be regarded as simple MBT to provide a residue for landfilling / thermal treatment.

The facilities are usually housed and kept under negative pressure with biofilters to help address any odour / emissions to air.

A Technical  Specification for a "typical" MBT Facility

Typical  capacity:

25,000 to  200,000 tpa (usually multiple modular units are used on larger sites)

Land  requirements:

0.9  to 2Ha for small facility (25,000 to 60,000tpa), 3-4Ha for large facility  (~180,000tpa)

Capital  Costs:

(See full report.)

Operating  costs: 

(See full report.)

Staffing  requirements:

Staffing  levels, including technical competence training, management and administrative resources will vary depending on the size and technology adopted.


 

 

 

 

 


Other  Issues

Key issues with regard to Mechanical Biological Treatment  include the impacts of legislation upon the products of the process. The  output of an MBT process will be classified as biodegradable under the EU Landfill Diversion Targets, unless additional processing takes place such as composting.

The composting of the output  would be subject to the Animal By-products Regulations (ABPR).

Other treatment processes  (for example co-incineration of the fuel in cement kilns, or power stations) will incur a gate fee, which is currently subject to market testing in the UK.

Some systems may include facilities to screen the waste to produce a minor compostable stream appropriate for in-vessel composting processes.

Where the waste is  to be used as a secondary fuel there is usually a minor non-combustible element  which remains after processing, which is sent for disposal to landfill.

An alternative MBT approach is the initial extraction of recyclables, followed by homogenisation of  the residue prior to processing in an Anaerobic Digestion (AD) plant.

Whole waste composting with some materials extraction may also be regarded as simple MBT to provide a residue for landfilling / thermal treatment.

All facilities are usually housed and kept under negative pressure for the avoidance of odour nuisance.

Strengths & Weaknesses of MBT

Strengths

Weaknesses

Reduces the mass of the input waste through stabilisation  / composting processes (by ~20%)

Landfill of residue will still count as Biodegradable  Municipal Waste (BMW) with regard to the Landfill Directive unless further treated. It would also pay the full Landfill Tax.

Designed to extract additional recyclate from the  residual waste stream

The system is reliant on other treatment / disposal  processes for the residues

Based on combinations of existing proven  technologies

No plant currently in operation in the UK (although some plant due for commissioning over the next two years)

Homogenises residual waste feedstock for use as fuel in other processes (e.g. cement kilns, power stations, other waste recovery processes)

Lower value for recyclate derived from a mixed residual  stream

Increases calorific value of waste through drying /  separation

Potential contamination issues over products from  composting or Anaerobic Digestion of waste limiting potential  applications

Is designed to be part of an integrated  system

Residue is likely to be subject to Animal By-Products  Regulation requirements for any additional  composting.

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