MBT 8-14: Conventional Recycling / Composting of the organic fraction

Process Description

This is an MBT configuration with 4 aims:

• Recovery of metals with the use of magnets and eddy current separators to recover ferrous metals and aluminium respectively
• Size separation of the organic fraction with the means of trommels or screens
• Production of RDF with the means of ballistic separators, or air classification
• Biological treatment (composting) of the organic fraction to produce CLO

Process Mass Flow Diagram

MBT 8-11: Conventional mechanical sorting for the recovery of metals and production of RDF, followed by a contained system for the composting of the organic fraction. The contained system for the composting of the organic fraction and maturation in covered windrows. The enclosed system may be in covered windrows (MBT 8), in tunnels(MBT 9), in boxes (MBT 10), or in closed halls (MBT 11). The post treatment (maturation) of the biostabilised organic fraction is done in covered windrows.

Process Photo

Linz MBT Facility / upper Austria: plant incorporating mechanical sorting and in-vessel (contained) composting.

Process Operational Data

Area

A MBT facility must be accommodated in a sufficient space for both mechanical and biological treatment processes. According to data from various existing facilities (DEFRA, 2009b) land requirements range from 0.18 to 0.36 m² per tonne input depending on the degree of mechanization and the type of biological treatment, while McDougal et al. (2002) report 0.40 m²/t and Golder Associates Ltd. (2009) 0.20-0.25 m²/t.

Energy

In MBT facilities energy is used for mechanical separation equipment operation (mainly electricity for shievers, ballistic separators etc.) and for biological treatment (electricity and fuel for the aeration of windrows, front end loaders etc.).

According to IPPC BREF (European Commission, 2006a) energy requirements of MBT facilities range from 4 to72 kWh per tonne of waste feedstock for electricity, while fuel requirements range between 1.4-36 kWh per tonne of waste feedstock. Ranges contain different types of installations with different degree of mechanization and more or less sophisticated gas treatments.

Water

MBT plants sometimes add water to the windrows, as moisture is lost during the aerobic digestion, which could otherwise lead to a shortage of water and halt the aerobic digestion process. This typically occurs during summer and winter months. In some cases, there is no net water consumption in the process. In the drying process, water is produced (350 litres -in vapour form- per tonne waste). During the aerobic digestion, temperatures of 50 - 60 ^oC are reached. Thus, water lost from the feedstock becomes water vapour (about 90 %) and is typically released to the air. However, in some cases, some of this water is condensed. The treatment of this condensation water is quite complex. The purified waste water (permeate) is used as process water in the cooling circuit. It is evaporated in the cooling tower. Tap water is only used in the cooling tower as make-up water (10 litres per tonne of waste). IPPC BREF (European Commission, 2006a) reports that the water consumption range from 260 - 470 litres per tonne of MSW treated.

Process Environmental Indices

Air Emissions

By contrast to composting plants treating green waste and separately collected biowaste, the material treated in MBT plants may exhibit a broad range of emissions (municipal waste). MBT exhaust gas may contain fluorinated chlorinated hydrocarbons, ammonia, mercury, methane, N₂O and other compounds. The MBT exhaust gas is partially produced during the mechanical treatment, but mostly is related to the biological process in which heat is released. Depending on the process, management temperatures from 30 up to 90 ^oC may be reached. Thus a great part of the moisture contained in the waste is driven out. Furthermore, the remains of solvents and of mineral oil carbohydrates can be driven out. Under these boundary conditions, the MBT exhaust gas contains at least the following material groups (European Commission, 2006a):

• water in the form of water vapour saturated process exhaust air which is not likely to be below saturation unless unsaturated hall air is added
• degradation products of organic decomposition which are also known from alcoholic fermentation, such as acetone, acetaldehyde, ethanol, methanol, butanol and other shortchained compounds
• solvents, especially benzene, toluole, xylene
• odorous terpenes, mainly limonene and alpha- and beta-pinene
• traces of mineral oil carbohydrates.

There is a limited amount of information available on emissions from MBT. The emissions of air pollutants and odorous substances of MBT plants are (European Commission, 2006a):

• waste specific (type, composition, age)
• treatment specific (aerobic degradation, fermentation)
• process specific (type of aeration)
• dependent on operational management
• influenced meteorlogically (weather conditions) in the case of open reactors.

CO₂ emitted from biological treatment is not fossil-derived, and therefore, it is not considered as a greenhouse gas emission.

Nitrogen in the waste can easily be converted to ammonia, and this is more likely to happen if the C:N ratios are unbalanced (too much nitrogen), or the mass becomes anoxic. For green wastes, this is more likely in the summer months with high levels of grass in the waste and insufficient woody material as a bulking agent. Enclosed aerobic digestion or biodegradation systems with a high forced air injection providing an excess of oxygen produce less ammonia than standard windrows. NH₃ emissions range from 5 to 3700 g per tonne of waste input to biological treatment, while N₂O emissions range from 11 g/t to 110 g /t and NOx to 100 g/t (European Commission, 2006a).

Sites undertake a range of riddling, sieving, grinding and turning operations. Particulate emissions occur, but there are no data to quantify them. It is known that most aerobic digestion operations generate a range of fungi, particularly aspergillus. Filters on the exit air will minimise particulate emissions (European Commission, 2006a).

Any volatiles in the feedstock will tend to be emitted to the air due to the temperature rises. The crude gas from MBT plants contains a large number of single organic compounds in relatively high but fluctuating concentrations. NMVOC emissions range from 0.7 to 600 g per tonne of waste input to biological treatment (European Commission, 2006a).

Methane will also be an issue, although the plant will usually be run in such a way so as to minimise this production. Methane emissions may range from 10 to 2000 mg/Nm3 (European Commission, 2006a).

Wastewater

Sites are unlikely to produce a surplus of liquid because the biological treatment process emits large volumes of water to the air and generally requires additional liquids as 'top-ups'. IPPC BREF (European Commission, 2006a) reports 0.260-0.470 m³ of generated wastewater per tonne of MSW treated in the facility.

Residual

Solid residues from MBT facilities arise from rejects of pre and post-treatment processes which are disposed to landfills. According to data from existing facilities residual is up to 0.2 tons per ton of feedstock.

The output of biological process is stabilised biodegradable fraction of MSW feedstock and it is also sometimes referred as Compost Like Output (CLO) or stabilised organic. CLO is not the same as the source segregated waste derived 'compost' or 'soil improver', which usually contains much less contamination and has a wider range of end uses. Thus CLO may also be disposed to landfill. According to data from existing facilities CLO is up to 0.2 tons per ton of MSW feedstock.