image credits
 

Chartered Institution of Water and
Environmental Management (CIWEM)

106-109 Saffron Hill, London, EC1N 8QS  
Tel: 020 7831 3110 Fax: 020 7405 4967
 

Mechanical Biological Treatment of Waste

Purpose

This Policy Position Statement introduces the concept of Mechanical Biological Treatment (MBT) of waste, reviews its contribution and role in helping to deliver sustainable waste management, and considers the operational, policy and environmental issues relating to this increasingly high-profile technology.

CIWEM's Position on MBT:

  1. CIWEM considers that there are a number of potential benefits for waste management which may be offered by MBT.  For example:
    • MBT can assist in meeting targets for reduction of biodegradable municipal waste (BMW) sent to landfill, e.g. under the Landfill Allowance Trading Scheme (LATS), and as such is welcomed in principle.
    • MBT can also enhance recycling performance, even where kerbside recycling is already employed, by removing a further fraction of residual recyclable material.
  2. Consequently, CIWEM welcomes the ongoing development and improvement of such systems and considers that they can have a role to play in diverting waste from landfill to more sustainable functions.
  3. However, CIWEM considers that there remain a number of obstacles and uncertainties which are currently restricting the use of MBT, and mean that it is far from an ideal solution at present.  These include uncertainty regarding the strength of markets for MBT end-products such as refuse derived fuel (RDF) and biologically stabilised products for application to land.
  4. CIWEM also considers that there may not be any real environmental benefit in adding further energy consuming processes to a recovery/disposal route.  Furthermore, there is some doubt about the suitability of some of the unusable residues for landfilling and hence whether MBT does in fact contribute to sustainable development.  There is not necessarily a connection between sustainable development and meeting Landfill Directive diversion targets and government recycling targets, and CIWEM maintains it is important to consider the whole lifecycle of the waste.
  5. CIWEM considers that the Government must address the issue of markets for refuse derived fuel (RDF) and minimise the existence of obstacles to such markets if it wishes such technologies to develop sufficiently quickly to make a real contribution towards meeting Landfill Directive targets.  Making RDFs eligible for renewable obligation certificates would act as a possible spur to this market.
  6. CIWEM would support the development of quality standards for low-grade land applications for all compost-like outputs, including those from MBT of municipal solid waste.

Context

Mechanical Biological Treatment (MBT) has risen to prominence in waste management thinking over recent years as the demands of the European Landfill Directive have become clear to those authorities tasked with collecting and disposing of the waste we produce (particularly municipal waste).

MBT was originally conceived as a way to further treat residual municipal solid waste (MSW) - i.e. waste which had already been subject to source segregation, for example via kerbside recycling schemes - as a means to extract further value from the waste - including its latent energy.  MBT systems are now attracting attention from local authorities, as a means of achieving their increasingly stringent targets for recycling and diversion of waste from landfill in the coming years.

The term MBT is used to describe not one specific process, but rather a combination of different technologies brought together in an integrated process.  An MBT plant combines mechanical processes to separate out the dry recyclables such as glass and metals, with biological processes to drive out moisture and to handle the organic-rich fraction of the incoming waste.  In addition to the separation of dry recyclables from the incoming waste stream, the plant can be designed to produce:

  • an energy-rich refuse derived fuel (RDF) comprising paper, plastics and other combustible fractions, that can be combusted in an energy from waste plant or in an industrial furnace;
  • an organic-rich fraction that is suitable for composting or anaerobic digestion; or
  • a biologically treated material with its biodegradable content reduced to a level that, when landfilled, will allow the waste disposal authority to meet its biodegradable municipal waste (BMW) allowance under the Landfill Directive.

The residual component, depending on the type of treatment employed, is typically landfilled.

The capacity of MBT facilities ranges from very small plants treating 10,000 tonnes/year or less, to large scale integrated facilities with capacities of over 200,000 tonnes/year1.  Capital costs have been estimated at around £8 million for a 50,000 tonnes/year plant to £25 million for 220,000 tonnes/year(1).  The number of MBT plants in the UK is increasing, with up to 15 local authorities likely to be employing the technology by 20102.

A number of companies in Europe and elsewhere are able to deliver MBT systems and components. These suppliers range from equipment makers, supplying parts for simple combined shredding/separation and biological treatment operations, to project developers and turnkey suppliers delivering plants with an integrated energy recovery capability.  MBT is thus quite a generic term for a number of combined processing options, which means it is important that technical specification and capacity of plant including quality and quantity of outputs is understood when discussing different MBT cases.

Competing technologies such as mechanical heat treatment (MHT) are not considered within this statement although MHT plants and their outputs should be regulated in the same way as MBT plant.

Discussion of Key Issues

The MBT concept should more correctly be termed Mechanical Biological Pre-treatment (as it is described in Germany and Austria) because an MBT plant is not a final disposal solution such as a landfill.  The success of MBT plants rests on the availability of markets and/or suitable disposal outlets for the principal products and residues.

MBT and recycling performance

Even successful kerbside recycling schemes leave large quantities of material in the residual waste (i.e. that which is not collected separately for recycling) which could be recycled.  Some MBT systems are able to recover a further 15% to 20% from residual waste for recycling if metals, inert materials and plastics are removed.  Metals are the easiest materials to remove using these systems and could boost local authority recycling rates by around 5%. However individual recycling rates depend on the waste composition and the individual MBT technology used.

MBT plants that create a fuel

The main barriers to wider uptake of MBT have been the severe planning problems facing waste management service providers in constructing dedicated energy from waste plants to receive the fuel, and the lack of secure long-term markets for the fuel in the cement, power and other energy-intensive industries that would be open to trialing alternatives to fossil fuel.  There is also a question as to whether nationally, sufficient capacity exists in industrial furnaces to accept more than a small percentage of the RDF that would be produced if the UK took up the MBT option on a large scale.

One possible spur to the development of fuel markets would be for them to be made eligible for renewable obligation certificates (ROCs) - essentially approving them as a sustainable fuel under the government's Renewables Obligation.  This would require a more refined version of RDF, and the classification of the product as biomass fuel.  The recent consultation on the future of the Renewables Obligation by DTI has suggested a lowering of the threshold for energy content from non-renewable sources to 90%, for a waste-derived fuel to be eligible for ROCs.

MBT plants that create a biologically stable product for landfill

The Landfill Directive establishes targets for the diversion of BMW from landfill.  According to the Environment Agency,3 England currently landfills around 14.4 million tonnes of BMW per year.  By 2010 this amount must be reduced to 11.2 million tonnes and by 2020 5.2 million tonnes.  The Landfill Allowance Trading Scheme (LATS) was established under the Waste and Emissions Trading Act 2003 and gives each Waste Disposal Authority (WDA) targets to reach in reducing the amount of BMW it sends to landfill.  It is for each WDA to consider how it is best able to reach these targets, but MBT may provide an option.

A primary uncertainty relates to whether the product delivers the reductions required by a WDA's BMW landfill allowance.  A report by the consultancy Juniper4 states that "most MBT technologies will usually deliver high levels of performance against UK BMW diversion targets - even if all the bio-treated outputs from the plant are sent to landfill." However, MBT processes that cannot demonstrate a reduction in the biological content of the waste to the satisfaction of the Environment Agency will not be acceptable to Local Authorities, because the product cannot be offset against their landfill allowance under LATS.

A key factor in determining the contribution MBT can make toward diverting BMW from landfill is the methodology employed to test the reduction in the biodegradability of the waste.  As the body with statutory responsibility for determining the compliance of WDAs with their LATS targets, the Environment Agency consulted on possible methodologies in November 20045 and issued guidance on monitoring MBT and other pre-treatment processes for the landfill allowances schemes in England and Wales in August 2005.  This guidance recommends a suite of preferred tests and site-specific sampling and monitoring plans are required for each treatment plant with the frequency of testing depending on the likely variability of the waste stream.

A further important uncertainty is whether the residues sent to landfill are actually suitable for landfilling.  Research conducted for the Environment Agency6 has highlighted the fact that such residues are often more homogeneous and finer grained than untreated MSW, they have a lower 
biological activity, will undergo less settlement and have low gas and water permeabilities. Such 
properties are not ideal to actively manage and accelerate longer-term stabilisation, a requirement for sustainable landfilling.

MBT plants that produce a recyclable or marketable product

There may be three key outputs from the MBT process:  the recyclables which are removed during sorting, the organic fraction, and the RDF.  The success of MBT in delivering optimal levels of diversion relies on the WDA being able to find a destination other than landfill for these outputs.  The Juniper report4 states that "Concern about the marketability of the outputs from MBT processes is the single most significant factor constraining their use at present."

MBT has the potential to produce significant amounts of outputs which could be either recycled in turn or used for another purpose such as soil conditioning, or as a co-fuel for power stations and other industrial boilers and furnaces.  However, the certainty of these markets is far from guaranteed with, for example, legislative hurdles facing the use of RDFs in power stations and better quality composts available from elsewhere competing on the soil improver side.  If products of MBT cannot be sold for such use, then they are likely to have to be sent to landfill, with consequent impact on WDAs' BMW diversion performance.

The issue of product quality and consistency is paramount if a long-term market is to be secured.  Products developed from mixed municipal waste inputs are generally too contaminated to permit their use in anything other than basic, low value applications, be it a compost or an RDF.  This restricts the commercial viability of an MBT plant designed to handle unsorted "black bag" municipal waste, or mixed residual waste from households.

Conclusion

MBT can contribute to delivering targets for reducing the amount of BMW sent to landfill.  The technology can be of help to local authorities in meeting their recycling targets, even where kerbside recycling is already employed.  MBT is also likely to help authorities meet targets for diversion of BMW under the LATS.  However, because MBT is not a complete solution, its viability as an alternative to technologies such as direct incineration of residual waste will depend on how markets develop for the outputs of MBT and whether life cycle studies show the use of MBT does actually assist in contributing  to sustainable development objectives.  Proposals for MBT plants are in many cases being held back because waste management companies are unwilling to take on the level of risk which would be required under the Government's Private Finance Initiative2. The commercial development of MBT processes in the UK is largely dependent on the following factors:

  • a reversal of the current hostility towards dedicated energy from waste applications
  • the opening and maturation of a long term market in alternative fuels
  • a long term market in biologically stabilised products destined for application to land
  • a demonstration that MBT has a role to play in meeting sustainable development objectives.

If such market development can be achieved then the completeness of the MBT solution may be improved.  DEFRA are consulting on whether composts and digestates derived from non-source segregated biodegradable waste should be spread to land, in the recognition that this market is critical for the long-term viability of MBT.

The present situation is one where there is limited capacity for the use of RDFs due to stringent air quality standards concerning the burning of waste in power stations, meaning that only the cement and paper industry represent significant markets for co-firing.  Biomass fuel may provide part of a solution to this problem, should it be made eligible for ROCs, but high product quality requires a relatively clean waste input, generally only achievable through source segregation of municipal waste or by restricting the input to selected industrial and commercial waste streams.  Organic outputs also face fierce competition from similar products in the marketplace.  If MBT is to deliver its full potential, it is important that solutions to these challenges are found.

References

  1. Prime Minister's Strategy Unit, Waste background Papers Annex G: Treatment and Disposal of Residual Waste.  MBT in Context, 2002
  2. ENDS Report, MBT market slows - but interest in outputs soars, April 2006
  3. Environment Agency data, www.environment-agency.gov.uk
  4. Juniper, Mechanical-Biological Treatment: A Guide for Decision Makers - Processes, Policies and Markets, March 2005
  5. Environment Agency, Assessing the diversion of biodegradable municipal waste from landfill by mechanical biological treatment and other options - A consultation related to monitoring the utilisation of landfill allowances, November 2004
  6. Robinson, H.D., Knox, K. and Bone, B.D. Improved definition of leachate source term from landfills. Phase 1: review of data from European landfills. Science Report P1-494/SR1.  Environment Agency, Bristol.

October 2006

Note: CIWEM Policy Position Statements (PPS) represents the Institution's views on issues at a particular point in time.  It is accepted that situations change as research provides new evidence.  It should be understood, therefore, that CIWEM PPS's are under constant review, that previously held views may alter and lead to revised PPS's.

 

Upcoming Events

 

Chartered Institution of Water and
Environmental Management (CIWEM)

106-109 Saffron Hill, London, EC1N 8QS  
Tel: 020 7831 3110 Fax: 020 7405 4967

Banner Photographs by: