“Squeezing every last bit out of the digestion residue”

The biogas system offers many benefits, but there is still room for improvement. Everything that passes through a biogas facility does not get decomposed. For this reason, researchers from the Biogas Research Center have studied what is actually left and how the decomposition process can be maximised.

The digestate is a residual product that is left over when methane is extracted from organic material.

A research group from the Biogas Research Center (BRC) – which includes researchers from Linköping University, the Swedish University of Agricultural Sciences, the regional waste and energy company Tekniska verken i Linköping and other actors in the biogas sector – have analysed how biogas facilities can maximise the decomposition of organic waste so as to extract as much biogas as possible. Their article was recently published in the Journal of Biotechnology for Biofuels and Bioproducts.

The work has been ongoing since 2016, and the original plan was to publish two or three articles based on the material collected. However the authors have now published what they call a “monster article”, where all the collected material is presented in a single study.

“Everything is interconnected and interdependent in such a system. We have looked at nine large facilities in order to cover the decomposition of agricultural residue, food waste and sludge, and have investigated every aspect that we've thought of that can affect the digestion residue”, says Eva-Maria Ekstrand, formerly postdoctoral researcher at Linköping University and now process consultant at AFRY.

The group has collected data on what goes into, what is inside of, and what comes out of the biogas reactor. This data has then been analysed to determine which factors can affect how the various parts of the organic waste are decomposed, and what remains in the waste that is left, i.e. the digestion residue. This helps them understand why some parts of the organic waste are not decomposed, and what can be done to extract more gas.

“We were processing enormous volumes of data”, says Jan Moestedt, R&D engineer at Tekniska verken i Linköping, and continues: “We studied the ‘goo’ that comes out. If there is organic material left, it's obvious that it hasn't turned into gas, and so there is room for improvement. We also looked at how we can squeeze out the last bit from the ‘dishrag’, and correlate this to the question of why is it left, perhaps it is because of these things?”

The results showed that there are two main reasons why the digestion chamber could not decompose all the organic material, and create gas. One reason was that the biogas facility had an unstable process, or that too little of the trace elements was added. Trace elements are needed for the microorganisms to function optimally. With insufficient trace elements, the microorganisms are not able to decompose the biomass to methane.

It also emerged that protein often remained, regardless of which facility they looked at. One reason for this is the difficulties in decomposing certain proteins, such as keratin. Another reason is that microorganisms inside the digestion chamber grow and multiply as they decompose the organic waste. This creates a new type of biomass which contains protein that has not been decomposed. This is a problem today, but it's also an important discovery that can contribute to an improved biogas system.

“Proteins contain amino acids which consist of ammonium, among other things. If we can achieve a better decomposition of our protein, we will increase the ammonium content of our fertiliser. And what do the farmers want, what do they pay for? The answer is ammonium, the nutrient in the fertiliser for their plants. So it goes hand in hand: with more efficient decomposition, which means less protein going out and more gas going out from the material, we will also get a better quality fertiliser”, says Jan Moestedt.

“The reason why it's bad to have a lot of organic material left in the digestion residue is that when it goes into the fields, methane can form. It's the same for those who store their digestion residue in an open fertiliser tank. They can end up with loads of methane if they have a low rate of decomposition in the reactor”, says Eva Maria Ekstrand.

The solution, according to the researchers, is after-treating the digestion residue, or what is called post-digestion – a solution not yet established internationally but which research recently published by the Biogas Research Center supports.

“Whichever conference you go to, there's a lot of talk about pre-treatment to get out more methane, but we think it's more efficient to let the reactor decompose everything that is already easy to break down, and then devote energy and money to the difficult part – what's left. Then we can get to that microbial biomass that has formed in the digestion chamber. This is something the BRC continues to study”, says Eva-Maria Ekstrand.

Translated by Martin Mirko

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