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Type of Waste to be treated
Source Separated Organic Waste
Source Separated Packaging
Mixed MSW
Min Capacity (in tonnes)
>5.000
>12.500
>50.000
ALL
Secondary Products or Energy Production
FE
AL
Plastic
Paper
Glass
Wood
RDF
SRF
Compost
Biogas
Max Residues (%)
0-10%
10-20%
20-30%
30-40%
Capital Expenditure € tn of total feedstock on wet basis
0-100
101-200
301-400
601-700
Annual operation and maintenance cost € tn of total feedstock on wet basis
21-40
41-60
61-80
81-100
Land requirement m2 per tn of total feedstock
0-0.2
0.21-0.4
0.41-0.6
0.61-0.8
0.81-1
Water consumption m3 per tn of total feedstock
0-0.05
0.051-0.1
0.11-0.15
Mechanical Biological Treatment (MBT) PDF Print E-mail
1.  General
2.  Mechanical sorting component
3.  Biological processing compartment
4.  Mass and energy balances
5.  Market potential for products
6.  Environmental impacts
7.  Economic data
8.  Applicability
References

1.    General

A mechanical biological treatment system is a waste processing facility that combines a waste sorting facility with biological treatment methods e.g. anaerobic digestion and/or composting. MBT plants are designed to process mixed household waste as well as commercial and industrial waste. Therefore, MBT is neither a single technology nor a complete solution, since it combines a wide range of techniques and processing operations (mechanical and biological) dictated by the market needs of the end products. Thus, MBT systems vary greatly in their complexity and functionality. Figure 1 presents a process diagram of a Mechanical Biological Treatment facility.

 

 

Figure 1: Mechanical Biological Treatment flow chart

 

2. Mechanical sorting component

The "mechanical" element is usually an automated mechanical sorting stage. This either removes recyclable elements from a mixed waste stream (such as metals, plastics, glass and paper) or processes them. MBTs typically involve a combination of screens, magnetic separation, eddy current separation, optical separation and air classification.

The mechanical sorting processes recover a part of MSW as recyclable materials, while another part formulates a combustible product known as ‘Refuse Derived Fuel (RDF) which covers a wide range of materials sorted in such a manner in order to obtain high calorific value. RDF can be incinerated in power stations, pyrolysis and gasification systems, co-incinerated in other industrial combustion processes for energy production.

3. Biological processing compartment

The "biological" element includes the biological treatment of the biodegradable organic materials that has been sorted, since after the mechanical sorting stage. Therefore, the biological processing compartment refers to the following methods:

  • Aerobic treatment (composting)
  • Anaerobic digestion
  • Biodrying

 

By applying composting, the organic materials are treated with aerobic microorganisms. The microorganisms break down the organic compounds into carbon dioxide and a stabilized solid end product (compost). More details on the aerobic treatment of organic waste are given in the field on composting.

 

Anaerobic digestion breaks down the biodegradable organics to produce biogas (mainly methane) and a stabilized solid end product which has similar characteristics and potential applications with compost. The biogas can be used, after cleaning, to generate electricity and heat. More information on the anaerobic digestion of organic waste are given in the field of anaerobic digestion.

 

Biodrying of organic waste material involves the rapid heating of waste 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 combustion.

 

Some MBT systems incorporate both anaerobic digestion and composting treatment methods. This may either take the form of a full anaerobic digestion phase, followed by post - composting of the produced digestate. Alternatively a partial anaerobic digestion phase can be induced on water that is percolated through the initial substrate, dissolving the readily available organic matter, with the remaining material being sent to a windrow composting facility.

4. Mass and energy balances

A typical mass balance diagram of an MBT process with aerobic digestion is shown in Figure 2. According to the figure, for 100 tn of processing MSW, 46kg is assumed to be the biodegradable organic fraction which is mixed with additives producing a final end compost which accounts to 18tn (39% of the biodegradable waste treated). During the process of composting, 18tn are the mass losses due to the leachate and emissions production, while 9.6tn is the residue that remains after refining the mature compost.

 

 

Figure 2: Schematic presentation of inputs and outputs of a typical mechanical sorting component with aerobic digestion (Juniper, 2006)

 

Below a mass balance of an MBT with anaerobic digestion is presented. This is the process whereby only a fraction of 50% to 65% of the total organic fraction is actually digested, while the remaining 50 to 35% is bypassed and is not subjected to anaerobic decomposition. The digested residue is then intensively mixed with the non-digested organics. The dry matter concentration of 45% in the resulting mixture of the two fractions allows for efficient aeration and rapid aerobic decomposition. A plant treating 100,000 ton per year of residual solid waste is recovering recyclables and producing burnable fractions. About 28,000 tonnes per year of organics are diverted to digestion, to which also about 7,000 ton per year of non-digested dewatered sewage sludge is added. No wastewater is generated at the plant.

 

Figure 3: Schematic presentation of inputs and outputs of a typical mechanical sorting component with anaerobic digestion

 

A typical mass balance diagram of an MBT process with biodrying is shown in Figure 4. According to the figure, for 1 tn of processing MSW, during the biodrying process 250kg of gas products are produced, while the remaining material is separated mechanically to 550kg of Solid Recovered Fuel (SRF), 35kg metals and 165kg residues.

 

 

 

Figure 4: Schematic presentation of inputs and outputs of a typical mechanical sorting component with biodrying

 

The energy for reaching high temperatures and for drying during the aerobic phase is mainly provided by the fraction that was not digested. The energy balance for an MBT treatment process can be calculated through the energy balances of the composting and the anaerobic digestion process.

5. Market potential for products

The products of the Mechanical Biological Treatment technology are:

  • Recyclable materials such as metals, paper, plastics, glass etc.
  • Unusable materials (inert materials) safely disposed to sanitary landfill
  • Biogas (anaerobic digestion)
  • Organic stabilized end product
  • refuse derived fuel - RDF (High calorific fraction).

 

MBT systems can form an integral part of a region's waste treatment infrastructure. These systems are typically integrated with curbside collection schemes. In the event that a RDF is produced as a by-product then a combustion facility would be required. Alternatively MBT practices can diminish the need for home separation and curbside collection of recyclable elements of waste. This gives the ability of local authorities and councils to reduce the use of waste vehicles on the roads and keep recycling rates high.

6. Environmental impacts

The environmental impacts produced from the MBT can be drawn account being taken of the respective environmental impacts of the composting and the anaerobic digestion process.

7. Economic data

The treatment cost of a mechanical biological treatment system with a capacity of 150,000 tonnes per annum is around 45 €/tn (Neamt Master Plan, 2008). According to ARCADIS & EUNOMIA (2010) a typical mechanical sorting component with biodrying acquires a capital cost of €250 per tonne, with operating costs of €21 per tonne before residue disposal.

8. Applicability

The use of mechanical – biological treatment is for the management of municipal waste is a choice with a large number of applications around Europe. The crucial point about the feasibility of such schemes should take into account the fact that market should be available for the utilization of the produced RDF / SRF. Otherwise, in the case that there is also significant cost for the utilization of the produced SRF / RDF, then the total cost of this management option could be even higher than that of a thermal management option.

REFERENCES

Juniper Consultancy Services Limited. (2006). Independent Waste technology Reports, Bathurst house, Bisley GL6 7NH, England

Rambøll, Ficthner,PM, Interdevelopment, (2008), Master Plan 2008-2038 for integrated solid waste system in Neamt County, Bucharest