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MBT-Landfill as Carbon Sink - Evaluation of Longterm Incubation Tests

Target of MBT (mechanical biological treatment of wastes) is to mineralise and to stabilise waste organic matter. This leads to a reduction of emissions after landfilling the pretreated wastes. It is well known, that MBT-landfills can be seen as a carbon sink, but there is a deficit in knowledge about the time frame respectively which share of organic carbon will remain in the landfill over long period.

During last decades at ABF-BOKU approximately 750 incubation tests (according to ÖNORM S2027 part 2, 2012) were analysed. Many of these tests were carried out over much longer test periods than minimum duration (21 days + lag-phase) described in the Austrian standard. Duration was several 100 days up to eight years!
55 of these long-term tests were evaluated to get an idea about gas formation potential respectively the "end-carbon-content" in MBT-landfills.
In a first step, a prediction model for gas formation potential was developed. The late gas generation of ten samples was modelled by a log-normal distribution curve to gather the maximum gas formation potential. According to the log-normal model the observed gas sum curve leads to higher values than commonly used exponential decay models.
In step two, the gas formation potential of 43 MBT-samples and twelve samples from old landfills was estimated by this model. The MBT-samples were taken in twelve Austrian MBT plants. Thus feedstock materials and mechanical treatment as well as duration of treatment of the analysed samples differ very huge. It also has to be considered that incubation tests are done under optimised laboratory conditions (40 °C, grinding to particle size < 20 mm, wetted to water capacity). Thus there will be differences in the time frame between results in laboratory and real landfills. Nevertheless the evaluated gas formation potential will be the same.
For calculation of "end-TOC" (TOC = total organic carbon content) in MBT-landfills (TOC after gas formation has stopped) the TOC of input for incubation test and the carbon release by predicted amount of landfill-gas (1 g TOC leads to 1,868 l of landfill gas) were used. Carbon release by leachate runoff was not taken into account, because just 10 % of carbon are emitted by leachate (90 % of carbon are transferred into landfill gas).
RESULTS:
- Gas formation during incubation test can be predicted by a log-normal model,
- By analysing carbon content of the input material and estimating gas formation potential by model an 'end-TOC" of MBT-landfilles can be calculated and
- By evaluation of 43 MBT-samples we expect an 'end-TOC" of minimum 10 % DM for MBT-landfills were well pretreated waste (limit values of Austrian Landfill Ordinance are met) is landfilled.

Copyright:	© Lehrstuhl für Abfallverwertungstechnik und Abfallwirtschaft der Montanuniversität Leoben
Quelle:	Depotech 2012 (November 2012)
Seiten:	6
Preis:	€ 3,00
Autor:	Dipl.-Ing Erwin Binner Dr. Katharina Böhm O.Univ.Prof. Dipl.-Ing. Dr. Peter Lechner
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