Database of Waste Management Technologies Life

MBT 5-7: Advanced Recycling / AD of organic fraction

 

Process Description

This is an MBT configuration with 4 aims:

  • Recovery of recyclables with a combination of advanced sorting equipment: optical sensors, magnets and eddy current separators to recover specific streams of materials
  • 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 (anaerobic digestion) of the organic fraction to produce biogas+CLO respectively

 

Process Mass Flow Diagram

Figure 1

MBT 5-7: Advanced mechanical sorting for the recovery of recyclables & the production of RDF followed by Anaerobic Digestion (AD) of the sorted organic fraction. The anaerobic digestion process may be a dry AD (MBT 5), a wet AD (MBT 6), or a complete dry AD system (MBT 7). Usually, the dewatered digestate undergoes a maturation phase (aerobic composting). The most common approach for this maturation step is to incorporate a contained composting system such as covered windrows.

 

Process Photo

Figure 2

MBT-AD - Farington Waste Recovery Park – Lancashire (UK): advanced Mechanical Biological Treatment plant (MBT) that will recover materials from general rubbish such as paper, plastics and metals. The technology uses both anaerobic digestion and composting to produce both biogas and a compost-like soil improver.

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.

The energy requirements of MBT can be covered by energy recovery from biogas generated during anaerobic digestion. Gas yields for dry AD systems range from 50 m3 to 150 m3 per ton of waste input to digester (HYDER CONSULTING, 2007; ARI et al. 2008; RIS et al. 2005).

Water

IPPC BREF (European Commission, 2006a) reported that the water consumption in MBT facilities ranges from 260 - 470 litres per tonne of MSW treated. It must be noted that the amount of water added in the digester in dry systems is less than in "wet" systems and it is calculated according to the moisture content of the input material and the desired moisture in the digester. For some plants, some of the water from the dewatering process goes back to plant, while other facilities recirculate liquid from the digester either within the digester or reuse it as process water (RIS et al. 2005). According to IPPC BREF (European Commission, 2006) the total fresh water consumption for "dry AD" treatment of 1 tonne of organic waste is 78 kg, while HYDER CONSULTING (2007) reports 185 kg per ton of input to digester.

The amount of water added in the digester in wet systems is up to two times greater than in "dry" systems (HYDER CONSULTING, 2007). According to data from various existing facilities (RIS et al. 2005) is up to 200 kg per ton of input to digester.

Unlike "classic" AD, "complete dry AD" does not require the adding of water to the organic waste treated, since it operates at c.50% dry matter content. Thus, water requirements of the process are minimized.

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 °C 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

MBT plants incorporating anaerobic digesters will produce wastewater mainly coming from the process of dewatering the digestate. According to IPPC BREF for Waste Treatment Industries (European Commission 2006) the volume of generated wastewater is 0.47 m3 per tonne of organic waste treated, while HYDER CONSULTING (2007) reports 0.52 tn per ton of input to digester and Economopoulos (2007) reports 0.437 tn per ton of organic feedstock.

If a "complete dry AD" is used, then as no water is added in the process, leachate issues are minimized and the volume of generated wastewater is estimated around 0.05 m3 per ton of organic feedstock.

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.