Sustainability Issue #2 April 2012

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This is how biogas can be produced better

Fuel with an environmental focus. What must be done is to find the right level, where biogas production is maximised with as little environmental impact as possible, says Jenny Gustavsson of Linköping University who has recently been awarded her doctorate.  Photo: Göran Billeson

This is how biogas can be produced better

It is known that the addition of trace metals can improve biogas production. A recent thesis at Linköping University shows which metals these are and how to apply them.

Small additions of the metals cobalt and nickel are needed for the production of biogas from the raw material, grain fermentation residue, to work well. Both metals are also needed simultaneously, since the process deteriorates if only one of them is used. Jenny Gustavsson shows this in her thesis at the Department of Thematic Studies, Water and Environment, Linköping University.

-It is well known that the production of biogas deteriorates when there is a shortage of nutrients, such as trace metals, she says. Fatty acids are formed and pH drops so that in the worst case the entire reactor has to be closed down.

Residual product of ethanol

Jenny Gustavsson wanted to see what metals affect the production of biogas from fermentation residue and in what form it can be utilised by the microorganisms in the process. Fermentation residue is a by-product in the production of ethanol that contains relatively large concentrations of sulphate which is converted into sulphide in the biogas reactor. Gustavsson shows that both cobalt and nickel are needed for the production of biogas to proceed well. But with only nickel or cobalt the process deteriorates. She has also investigated the addition of selenium and tungsten without finding any effect.

She has also analysed the chemical form of cobalt and nickel in relation to how the substances are taken up by the microorganisms in the biogas process. She found that cobalt mainly binds to sulphides/organic matter, but in spite of this it is taken up by the microorganisms. This is a somewhat surprising result, she says, which poses questions regarding bioavailability.

Balance is needed

-We usually think that metals must be in dissolved form so that they may be taken up in biological processes. But my results raise the question of whether precipitation of metal sulphides really affects bioavailability. It seems that the metals can be taken up despite being strongly bound to sulphides.

Metals in the process also raise the question of how environmentally friendly biogas production is.

-It is a matter of finding a balance – finding the right level, where production is maximised with as little environmental impact as possible, says Jenny Gustavsson, and continues:

-In this context, greater knowledge of the bioavailability of trace metals is an important part in improving the understanding of the effect of metallic additions, so that we may know what metals, and in what quantities, should be added in different processes.

Responsible for this page: Birgitta Bruzelius

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