Process Features MBT normally comprises 3 stages 18 :

• Biological – Waste stabilisation, volume reduction;

• Material Separation – Mechanical separation of low grade aggregates, glass, stones;

• Resource Use – landfill cover, RDF, recyclables to reprocessors, etc.

The following flow diagram gives a generic overview of the various different processes and material flows that can be encompassed within an MBT plant.

Figure 8.1 Overview of MBT systems

18 _{McLanaghan (2002) Delivering the Landfill Directive: the Role of New and Emerging Technologies}

Municipal Solid Waste Mechanical Treatment • Shredding • Screening • Magnetic Separation • Air Separation • Pelletising • Conditioning Biological Treatment • Composting • Drying • Anaerobic Digestion End Products

• Refuse Derived Fuel • Soil Conditioner • Aggregates • Landfill Cover • Recyclables

Landfill Energy Markets

Recovery Sewage

8.2.1 Inputs

Unsorted Municipal Solid Waste (MSW) is delivered to the plant, either directly or via a bulking transfer station. The waste reception area is generally situated within a building or at the very least a covered area. Such areas are normally operated under negative pressure so that dispersion of dust and odours is minimised. The waste is deposited onto a tipping floor or into bays. At this stage, bulky materials and hazardous items may be removed prior to the waste entering the mechanical stage of the treatment process. The use of MBT is generally advocated for residual waste treatment after maximised recycling has taken place, although the plants are sufficiently flexible that they can withstand fluctuations in waste composition. If kerbside recycling is not maximised, an array of separation equipment can be installed as part of the mechanical treatment phase. However, it must be noted that recyclable materials separated from residual waste in this manner are generally regarded to be of lower quality and potentially lower value than that of source separated materials due to the level of contamination from the separation/treatment process.

8.2.2 Outputs

System outputs vary depending on the process employed and the degrees of separation desired, but generally consist of the following:

• Refuse Derived Fuel (RDF) or Solid Recovered Fuel (RDF); • Compost/soil conditioner/landfill cover;

• Non ferrous metals; • Ferrous metals; • Glass;

• Plastics;

• Stones (low grade aggregates).

All processes involve losses in terms of moisture and CO2 as a result of the volume reduction,

although some to a greater degree than others. The Bedminster composting process, for example is a net consumer of water as the desired end product is marketable compost, whereas

the Ecodeco process shows that 25% of the input weight is lost through water vapour and CO2

releases.

If landfilled, the RDF produced via the process will attract the standard rate of landfill tax, under current definitions. It is unclear whether classified fractions of the ‘stabilised organic’ fraction of the waste could be classed as inert for landfil tax purposes. It is suggested that further refinement of the fraction, by composting or digestion, would be necessary in order to achieve complete stabilisation for disposal to landfill or indeed to qualify as composted under BVPI. Current advice from the Environment Agency casts doubt on the ability of any MBT process to produce an end product that may attract the lower rate of landfill tax and there is still uncertainty about the end products from an MBT process being classified as non-biodegradable with regards to the Landfill Diversion target for BMW.

It should also be noted that the RDF arising from the MBT process is unlikely to be suitable for combustion in conventional EfW plant due to its enhanced calorific properties. Specialised combustion plant designed for the MBT derived RDF will therefore be required.

8.2.3 Cost & Footprint

Details of average costs and footprint size (where provided in the Juniper MBT Study13_{) for}

MBT plants based on those operating (at pilot or commercial scale) in Europe are listed in Tables 8.2 and 8.3. It should however be noted that the costs and sizes quoted are for bespoke MBT facilities of varying sizes and configurations.

Table 8.2 Costs & Footprint for MBT plants19

Technology Bio-Stab Splitting AD Splitting AD

CLO/Biogas Capex (£/tpa) Average Opex (£/t) Average gate Fee (£) Footprint (ha/t) Average

ArrowBio 86 16 0.1 Biodegma 46 21 BTA 0.1 Ecodeco 0.04 GRL 50 0.3 Herhof 0.1 Hese 268 0.1 Linde ISKA 180 47 Nehlsen 170 0.6 OWS 0.2 SBI Freisland 172 SRS 78 38 0.2 SUTCO 75 0.2 VKW 67 0.2 Wastec 22 17 0.1 Wehrle Werk 13 53 0.1

Data taken from Juniper MBT report (2005)13_{. Only systems where financial or footprint data (figures rounded to whole numbers) exists in the report are included in the table above}

Table 8.3 Average Costs and Footprints of MBT technology types (from Juniper Report) Capex (£/tpa) Average Opex

(£/t) Average gate Fee (£) Footprint Average (hectares/tonne)

MBT Bio-stabilisation 104.93 46.0 29.0 0.204

MBT Splitting 97.11 17.0 75.0 0.13

AD Splitting 176.79 12.5 34.3 0.105

AD CLO/Biogas 223.93 0.020 _{48.5 0.16}

As can be seen from the above table, from the cost data provided in the Juniper report13_{, the AD}

Splitting plant is the cheapest in terms of capex and opex. MBT Bio-stabilisation has the lowest gate fee, but the highest average footprint. These figures should however be reviewed with caution due to the small number of technology providers supplying information on their processes within the report.

MBT processes of one form or another have been in use for more than ten years. Worldwide, the 27 technology providers reviewed in the Juniper report13 _{claim around 80 operating plants,}

with a combined treatment capacity of 8.5 million tonnes per annum. With the growth of MBT, this worldwide treatment capacity is likely to grow to around 13 million tonnes per annum by the end of next year.