25 June 2019

There are calls to increase the production of biogas, but thus far such an increase has been associated with a number of problems. Luka Šafarič from Linköping University has studied the problem of anaerobic digester sludge thickening in the biogas reactor. His doctoral thesis shows that the organic material that is fed into the reactor affects the thickness of the sludge, and ultimately, the reactor’s efficiency.

Bio Gas Installation on a farm processing Cow DungBiogas plant Photo credit CreativeNature_nl“It was not clear why some biogas reactors had problems with stirring sludge fluids, and why their thickness – viscosity – increased. Since this reduces the efficiency of the biogas process, I wanted to find out why”, says Luka Šafarič, who recently completed his PhD at Linköping University.

Increasing biogas production involves substrates – the organic materials that are digested in the process. By introducing new substrates, and increasing the input of the substrates in the biogas reactors, production can rise.

However trials with new substrates and increased input has led to problems such as the sludge fluids in the biogas reactor sometimes becoming more viscous, and harder to stir.

In his doctoral thesis, Luka Šafarič elucidates the interactions between substrate type, microbial community and its metabolic activity, and sludge rheology – how sludge flows and deforms.

In two studies, he has examined the problem of sludge thickening in the biogas reactor.

In the first study, which focussed on substrates, Luka Šafarič took samples from various reactors that had different types of substrates, such as sewage sludge, food waste and agricultural waste, and mixtures of substrates (co-digestion), and then compared the viscosities of the fluids. In the second study he investigated the microorganisms, and to what extent their metabolism affected the sludge fluids.

The results showed that the substrate one chooses significantly affects the rheology of the fluids, and that this results in large differences in stirring power and mixing efficiency. Digestion of food waste generally led to the highest viscosity, while digestion of paper mill waste and the co-digestion of various substrates at increased temperatures both led to the lowest viscosity.

“The thesis shows that one should consider how a new substrate affects rheology, in addition to the direct effects on the biogas process. This way the reactor could be adapted to the specific substrate, or one could evaluate if pre-processing is required, in order to achieve high production levels and a stable process.”

Avhandling: Anaerobic Digester Fluid Rheology and Process Efficiency: Interactions of Substrate Composition, Trace Element Availability, and Microbial Activity (2019), Luka Šafarič. DOI: 10.3384/diss.diva-156030



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