Technological background

Mechanical-biological waste pre-treatment (MBP) technology is a very recent option in waste management. The first facilities started operation in the mid-1990s. MBP comprises the processing or conversion of waste from human settlements, and waste that can be managed like waste from human settlements, with biodegradable organic components, via a combination of mechanical and other physical processes (e.g. cutting or crushing, sorting) with biological processes ("rotting" or "decomposition", fermentation), on which

• biologically stabilized waste is produced for deposition or prior to thermal treatment,

• thermally valuable components or substitute fuels (refuse derived fuels, RDF) are obtained for recovery, or

• biogas is generated for energy recovery (BMU, 2001).

The technology is now being widely implemented in Europe, especially in Germany and Austria. In Germany, in 2006 about 3.8 Mio t of residual waste were operated in 46 MBP facilities. The total treatment in Europe was about 13 Mio t. In developing and emerging countries, such as China, Vietnam, and Brazil, MBP projects were built to improve the waste management situation (Nelles, 2007).

The typical process is characterized by mechanical separation steps to produce i) the RDF fraction which typically amounts to about 40 percent of the original waste. ii) 35 percent by mass of a fine fraction is produced which is rich in organics, but has a low heating value. After separation, this fraction is biologically treated to reduce the organic content to a low value so that MBP treated waste can be deposited at low risk of emissions (Soyez, 2001). The residual organics must not exceed a certain level for the respiration activity (AT4<5 mg/g) or alternatively gas formation in a laboratory test (GB21 <20 l/kg). The upper thermal value of the waste deposited must be lower than 6,000 kJ per kg. Thus an energy rich fraction is separated.

A typical process scheme is given in figure 12.6.

Figure 12.6 Process schema of RDF production by MBP (Soyez, 2001)

Two types of processes for the biological treatment or combinations thereof can be applied according to the MBP technology: aerobic and anaerobic proc esses. In the case of aerobic processes the organic residues are oxidized. This is similar to composting. However the product is not "compost" because its quality is poor due to heavy metals and toxic substances. The energy content of the waste in this case is lost. In case the biological treatment is executed by an anaerobic process, methane will be produced and transformed into electricity to cover the needs of the process itself or for energy marketing. Waste heat can be used for district heating or for technical processes such as drying.

As another important aspect, in MBP facilities a material recovery takes place. Especially ferrous and non-ferrous metals are recovered by about four and one percent, respectively. Optionally a recovery of glass is possible and is applied depending on the market needs. The recovery of these materials avoids climate impacts in the production processes and is thus considered as a climate benefit of waste pre-treatment.

There are MBP variants established which only produce RDF and recyclable materials such as metals and glass, and no waste for deposition. The plant type may be directly combined with a combustion unit to produce energy, or it produces an RDF fraction for marketing in the energy sector as well as for cement or methanol production. A MBP facility situated in Dresden (Germany) is shown in figure Figure 12.7.

Figure 12.7 Total view of an industrial scale MBP facility in Germany

The output flow of such a process is characterized by about two-thirds of the fractions for industrial re-use; only 4 percent are deposited, relevant to 100 percent input, not including process losses (see table 12.11).

Table 12.11 Output flow composition of MBP technology (Soyez, 2002)

Output fraction

Percent of total input

Fraction for industrial re-use

RDF (calorific value 15-18 MJ/kg)

53

Ferrous metals

4

Non-ferrous metals

1

Batteries

0.05

White glass

5

Brown glass

0.5

Green glass

0.5

Minerals

4

Others

Fine grain and dust (to be deposited)

4

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