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The exhaust air pipes on the rotting box allow to split the exhaust flows of the biological treatment depending on the degree of exposure and to treat them separately.
© RWTH Aachen, Institut für Aufbereitung und Recycling
Projektinfo 15/2016

Process flow chart at the MBT Großefehn.
© RWTH Aachen University, Department of Processing and Recycling

Cross-section of the rotting box at the MBT Großefehn.
© RWTH Aachen University, Department of Processing and Recycling
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Reducing energy use from waste treatment

Residual household waste lands in the grey waste container. It is either thermally exploited in a waste incineration plant, or it is taken to a plant for mechanicalbiological waste treatment. Here, the waste is sorted: Energy-rich parts are used for energy, while metals are recycled. The residues are treated in rotting boxes before disposal on landfills. The exhaust air from the process has to be purified. This takes up almost two-thirds of the energy consumption of waste treatment plants. Researchers, in collaboration with their project partners, have developed a more efficient method.

For unavoidable waste, the German Closed Substance Cycle and Waste Management Act specifies the target hierarchy of prevention, preparing for re-use, recycling, other recovery and disposal. The mechanical-biological waste treatment (MBT) plants also operate according to this principle. There, the usable parts are first removed. The residues are then taken to the rotting boxes. In the boxes, the organically degradable portions are oxidised through aerobic microbiological conversion processes. The residues which cannot be used must be stabilised in this way before it can be stored in a landfill site. This pre-treatment prevents relevant emissions of methane and other climate-relevant gases from occurring. Researchers at the Department of Processing and Recycling at the RWTH Aachen University and the Institute for Sanitary Engineering, Water Quality and Solid Waste Management at the University of Stuttgart are studying how the amount of energy required for mechanical-biological waste treatment, in particular for biological process management and the subsequent exhaust air treatment, can be reduced. As project manager Erdogan Coskun explains, “MBT processes always include a biological treatment stage under aerobic conditions, which is designed as an intensive and/or subsequent rotting. For every ton of waste treated, an exhaust air flow of up to 8,000 m3 is created, which is charged among other things with volatile organic carbon compounds.” This exhaust air must be collected and purified in order to be able to uphold emission threshold values. The exhaust air is purified by using biofilters, washers and regenerative thermal oxidation units (RTO). However, since RTO units are not designed for the exhaust air from MBT plants, their use in MBT is only possible when a considerable amount of energy expenditure is involved. Around 100 kWh is needed for every ton of waste, of which around 65% is used for exhaust air treatment. The increased energy consumption for exhaust air treatment can be traced back to the use of supporting gas (e.g. natural gas, biogas) in the RTO units.

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Mechanical-biological waste treatment

In Germany, around 45 mechanical-biological waste treatment plants (MBT) process around five million tonnes of mixed household waste every year. Currently, there are about 330 plants operating in the EU, with a total capacity of 33 million tonnes per year. Experts predict that by the end of this decade, around 440 plants will be in operation, with around 46 million tonnes per year.

Around 40% of the quantity of domestic waste created every year is pre-treated before further use or deposition in landfill sites in MBT plants. The exhaust air which is created during stabilisation of the waste which is rich in organic matter through aerobic treatment must be purified in order to meet the valid emission threshold values. According to the 30th German Federal Immission Control Act, limits to the total carbon content of 55 grams per tonne of waste input and 20 to 40 milligrams per m3 of exhaust air must be observed.

In order to achieve these values, the exhaust air from the MBT is usually purified using regenerative thermal oxidation units (RTO). During the post-combustion, the organic pollutants contained are oxidised up to produce carbon dioxide. This process requires socalled supporting gas combustion. The fluctuating and comparatively low carbon load in the exhaust air is not sufficient on a permanent basis for autothermic oxidation.

Biological stabilisation of the material in the rotting box

In order to conduct practical tests with the MBT, researchers are working with the district of Aurich in Großefehn. Here, the specific energy consumption for the treatment of household waste is around 80 kWh per ton. At the test plant, two-thirds of this amount alone are accounted for by the need for supporting gas in the RTO. The researchers are developing measures for improving energy efficiency along the entire process chain, from the mechanical to the biological waste treatment through to the purification of the exhaust air. Their aim is to reduce the specific energy consumption by around a quarter to 60 kWh per ton. Transferred to all MBT plants in Germany, this amounts to an energy saving potential of over 100 GWh.

In the biological treatment stage during the aerobic degradation process of the organic constituents, a clear concentration peak of ethanol occurs in the exhaust air at the beginning of the process. After four days, nitrogen compounds are predominantly degraded, after which the ammonia concentration in the exhaust air increases considerably. After the material is transferred to the box, ammonia and carbon compounds are released at increased levels. During the rotting process, the organic load in the exhaust air consistently decreases. During the first seven days, over 80% of the carbon load is emitted, with even less than one per cent during the final two weeks of the rotting phase.

Projektinfo 15/2016:
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Addresses

Coordination, modification of the process chain in MT and BT
RWTH Aachen, IAR

Operator of the MBT system, test implementation
MKW

Development of exhaust air purification plant
PlasmaAir AG

Exhaust air monitoring:
Universität Stuttgart, ISWA

Info tips

Combining compost and biogas
BINE-Projektinfo 17/2014

Clean use of landfill gas
BINE-Projektinfo 11/2014

Links

powerstep.eu
Homepage of the project Powerstep

The Climate Change Mitigation Potential of the Waste Sector
Background information from the "Umweltbundesamt"