MBT alone ‘might’ meet council landfill targets

Mechanical-biological treatment could meet local authorities’ landfill diversion targets by itself, according to the most comprehensive study to date.1 There is only "limited" potential to send process residues for combustion, and quality standards for low-grade soil improver applications should be developed.

The study by Juniper Consultancy Services was funded using landfill tax credits by SITA Environmental Trust and the Association for the Sustainable Use and Recovery or Resources in Europe (ASSURE). It reviews MBT processes offered by 27 companies and which are in use at 80 facilities world-wide, processing over 8.5 million tonnes of waste per year.

MBT is the umbrella term for processes that use mechanical and biological techniques to sort, separate and treat municipal waste.

It is being promoted as an answer to local authorities’ waste problems, enabling them to meet tough EU targets for the diversion of biodegradable municipal waste (BMW) from landfill without having to turn to incineration (ENDS Report 361, pp 25-28 ).

The report is unambiguously in favour of the technology. MBT, it says, is an "attractive" option for local authorities having the potential to divert more than 90% of BMW from landfill in a best-case scenario.

As a result, local authorities "might" be able to meet their EU targets without other forms of waste treatment - a conclusion that could have "far-reaching consequences for UK waste management".

Even if MBT cannot meet diversion targets by itself and is simply used as pre-treatment for incineration, it will reduce the need for new incineration capacity by half.

The report’s conclusions are based on an analysis of the eight main types of MBT plants, differentiated on the basis of their outputs. These vary from those that produce refuse-derived fuel to those that produce biogas and a digestate.

Seven of these options can achieve a BMW diversion rate of at least 85% if markets are available for their outputs (see table). But if markets are not available, diversion could be as low as 6%.

Correspondingly, the report paints an unfavourable picture of MBT systems that generate RDF or compost - the options that have to date received most promotion in the UK thanks to companies like Shanks and Herhof.

"Our analysis indicates that the challenges associated with using the output as a compost or fuel are significant," it says. "The longer-term viability of MBT projects that do not take these issues into consideration is questionable."

The difficulty of using it as compost is well known - outputs from MBT plants cannot qualify for the PAS1000 quality standard.

For fuel applications, though, the report says that outputs are "simply less attractive to users than other fuels".

Co-firing RDF in power stations, for example, is ruled out due to "some 20 different technical challenges" facing it. These range from possible damage to boiler tubes through to increased risk of fire.

The potential for using RDF in cement kilns is also "limited" to perhaps as little as 125,000 tonnes per year due to competition from other waste-derived fuels.

Even if only 10% of Britain’s waste is put through MBT plants, it would produce 1.25 million tonnes of RDF, significantly more than that capacity.

In spite of this assessment, Joe Schwager of Juniper says the report should not be read as dismissing such systems.

"You could make the leap [from the report’s conclusions] and say that some companies are promoting the wrong system, but that isn’t what we’re saying. We’re just saying that the MBT systems most visible today may not be the best in the future."

The "best in the future", according to the report, is more likely to be one of three other types of system:

  • Those that produce land remediation material.

  • Those that produce biogas using anaerobic digestion. Electricity generated qualifies for renewable obligation certificates.

  • Close-coupled MBT and gasification systems that burn refined-RDF, known as solid recovered fuel, on-site. This removes the risk of having to find markets for the material and also qualifies for ROCs. The report also backs the use of MBT residues on land in low-grade soil applications.

    Admitting that its conclusions are "somewhat more positive than those reported by others", it says there are "significant" markets for MBT outputs in forestry applications and for use as a soil improver on brownfield sites.

    It also projects that MBT outputs - totalling six million tonnes per annum if 50% of Britain’s waste is put through such systems - could be utilised as a mulch on the verges of trunk roads, assuming it is re-spread every five years.

    The main barrier to such uses is the Environment Agency’s proposal that MBT residues spread on land will only count as diverted from landfill if they are "part of a genuine recovery operation" - one where they provide "agricultural benefit or ecological improvement" (ENDS Report 359, pp 46-47 ).

    As a result, the report urges the Government to develop quality standards for such applications "so that MBT plants can be designed accordingly and confidence [be] built into the market."

    The withdrawal of proposals for a EU biowaste Directive allows the UK to develop such standards by itself (see p 60 ).

    The report’s focus on land applications has not been received well by industry or local authorities, according to Joe Schwager. The report has also taken people by surprise for its views on what tests should be used to measure the performance of MBT.

    It throws its weight firmly behind the loss on ignition (LOI) tests preferred by the Agency, instead of those biological tests generally preferred by industry. Fears that LOI tests will lower the performance of MBT plants because they are unable to distinguish between biodegradable waste and the biodegradability of waste are given short shrift by Mr Schwager. They are caused by a lack of full understanding of the test methodology, he says.

    Indeed, the report says, LOI could make "the contribution from some types of MBT system…far higher than generally appreciated."

    The recycling performance of MBT processes and their contribution to Best Value targets for recycling and composting is also discussed in the report. The recovery of dry recyclables by MBT plants is "somewhat limited", it says, at around 3-15% of weight. However, MBT plants could contribute a lot more than this, argues the report, because moisture and carbon dioxide lost from the process qualify as "composting" provided that the residues are used for a genuine recovery operation.

    The report does not reach conclusions as to the relative costs of MBT systems compared with other forms of waste treatment. The attractiveness of MBT hinges on markets being found for its outputs.

    Proposals from the Department of Trade and Industry to make the biodegradable component of RDF eligible for ROCs (see pp 44-45 ) will offer a boost.

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