Many advantages of dry digestion

20 July 2017

Monica Westman Svenselius

Sweden’s first dry digestion plant is located at Mörrum, where food waste in brown bags is converted to useful biogas. The waste is digested at the consistency as it has when it arrives. One bag of food waste is sufficient for 2.5 km of fossil-free transport.

The visitors got inside one of the large digestion chambers (800 cubic metres) and could observe both the heat-exchange coils in the walls and the large stirrer.Scandinavia’s first dry digestion plant was opened in the spring of 2013 by VMAB, Västblekinge Miljö AB, at Mörrum. The industry was highly sceptical of the new technology, but research carried out at LiU shows that it can compete with established technology. In his doctoral thesis, LiU researcher Roozbeh Feiz compares dry digestion with the wet methods that are most common in Swedish biogas plants, and he shows that dry digestion has several advantages. The substrate needs less pretreatment and the process is more robust, while the yield of biogas is approximately the same.

“There are several advantages of dry digestion, but the comparison is subject to a certain amount of uncertainty. The optimal method in a particular case depends on many factors, and can be determined by calculation,” says Roozbeh Feiz.

Food waste from three minicipalities

Food wasteVMAB converts 15,000 tonnes of food waste a year from the municipalities of Karlshamn, Sölvesborg and Olofström to 2.4 million cubic metres of vehicle fuel. The fuel is sold to E.ON, which has several refuelling stations for its own and the municipality vehicles in the region, one of them right next to the VMAB plant.

The food waste arrives in brown bags, which are unloaded from a large container and loaded onto a conveyor belt for sieving and sorting. Anything that is not food waste is sent for combustion, while the food waste is loaded into one of the two digestion reactors, which both have a volume of 800 cubic metres. Heat exchangers on the walls ensure that the temperature in the digester lies between 37 and 40 degrees, and a large stirrer in the centre distributes the heat evenly.
The link below shows a video (in Swedish) in which the visitors get inside an empty digester, while methane bubbles up towards the ceiling in the neighbouring digester.

In this case, the remains from the digestion are not used as fertiliser, but as compost for the final cover for landfill sites. If the digestion residue is to be used as fertiliser, it will be necessary to use a thermophilic process, in which the digestion takes place at a significantly higher temperature. This also sanitises the residue.

“During the summer, we will investigate the possibility of using thermophilic treatment. It currently takes 27 days for waste to be converted to biogas, but an individual particle can go through the process in 10-12 hours,” says Robert Lundgren, giving the guided tour around VMAB.

Johan Ekelund, VMAB — Nearly all (90%) of the food waste is from single-family houses. The more a householder sorts waste, the cheaper it becomes. The most advanced collection service comprises eight fractions, which are collected at different intervals. “We must make it easy for the householder,” says Johan Ekelund of VMAB.

Substrat på väg ini reaktorn — The waste is sieved and sorted such that only organic material enters the reactor.

Rötningen kollas via en glugg — A small observation port allows you to see methane bubbling in the neighbouring digester, which is in operation.

Biogas tour, 10 stops in 3 days

A biogas bus takes a tour

More than 30 researchers and representatives for companies involved in the Biogas Research Center, BRC, spent three days this summer touring southern Sweden to exchange experiences with companies and decision-makers.

Biogas and fertiliser from fish sludge

Biogas solved problems with slimy and foul-smelling wastewater from the fishing industry in Västervik. The sludge is now used as a source of both fuel for vehicles and top-quality fertiliser. There are, however, several other benefits.

Moving to biogas saves Kalmar money

When Kalmar County invited tenders for operating public transport, biogas was the obvious preferred option. “Our starting point was that we believed it to be best for the county,” says Ulf Nilsson, chairman of the Kalmar Regional Association.

National research center

Biogas Solutions Research Center

BSRC is a national competence center for biogas research administered by the LiU. It's base for the development of innovative and resource-efficient biogas solutions often with positive local and regional effects on the environment and the economy.

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