Effect of substrate pretreatment on particle size distribution in a full-scale research biogas plant

•The particle size distribution in fermenting substrate was measured.•Mechanically pretreated and untreated substrate high in fiber was fed.•Fine fraction of particles were measured to a large extent.•The disintegration had an effect on larger particles, not on smaller particle sizes.•A uniform distribution is not achieved and no indications for stagnant were found. The objective of this study was to investigate the pretreatment effects of high-fibre substrate on particle size distribution in a full-scale agricultural biogas plant (BGP). Two digesters, one fed with pretreated material and one with untreated material, were investigated for a period of 90days. Samples from different positions and heights were taken with a special probe sampling system and put through a wet sieve. The results show that on average 58.0±8.6% of the particles in both digesters are fine fraction (4mm was measured in the untreated digester. However, the volume distribution over all positions and heights did not show a clear and uniform distribution of particles. These results reveal that substrate pretreatment has an effect on particle size in the fermenting substrate, but due to the uneven distribution mixing, is not homogeneous within the digester.