Peter Malmros

ABSTRACT
Biogas potential of the reject fraction from the biogas plant
Kungsängens gård
Peter Malmros
The biogas production in Sweden in 2009 was 1,4 TWh and 22 % of the biogas was
produced in co-digestion plants. Since 2005 it is prohibited to deposit organic waste and
this has resulted in an increased biogas production from this type of waste materials in
recent years. The total biogas potential in Sweden is approximately 15 TWh and 70 %
of that comes from agriculture wastes. Sewage sludge, waste from food industry and
food waste accounts for the rest. It is also these three commodity groups that have the
greatest profitability with today's technology. Among these groups, food waste have the
largest remaining biogas potential. The problem with food waste is that it often contains
impurities that must be sorted out in order to avoid operational problems. The out
sorting process often causes problems and typically organic material is lost in the reject
fraction.
The biogas plant Kungsängens gård is a co-digestion plant that produces biogas from
different types of organic residues. In 2009 about 7 536 tons of material were treated, of
which 58 % was food waste. Of the incoming material 938 tons were sorted out as
reject. In 2011 the amount of incoming material is expected to increase. In addition, the
proportion of food waste is expected to increase to 90 %, which will result in even
larger amounts of reject. The purpose of this study was to perform analysis and batch
digestion experiments to determine the chemical composition and methane potential of
the reject from the biogas plant Kungsängens gård. An investigation was also made to
examine methods and technologies that potentially could be used to produce biogas
from the reject.
The results from the batch digestion experiments showed that the methane potential of
the different reject fractions was high and it can be compared with what might be
expected of source separated food waste. If the reject that was produced in 2010 would
be used for biogas production, this methane potential is equal to 10 % of the total
production of biogas at Kungsängens gård in 2010. The investigation shows that dry
fermentation with a batch system is the only technology that potentially could produce
biogas from the reject in its existing form. There are no dry fermentation plants in
Sweden and therefore the level of knowledge is relatively low. Because of that it is hard
to estimate the profitability and efficiency of dry fermentation plants.
Keywords: biogas, biogas potential, food waste, reject handling, dry fermentation.
Department of Microbiology, The Swedish University of Agriculture Sciences,
Genetikcentrum, Genetikvägen 5, SE- 750 07 Uppsala
ISSN 1401-5765
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