Anaerobic Digestion of Hemp and Flax Straw and Shives and Rapeseed Straw by the Ruminal Microbiota

To mitigate greenhouse gas emissions and improve energy supply security, there is an increasing effort toward the use of non-fossil energy sources. Crop residues have a great potential to be exploited as biomass for biogas production. However, due to their lignocellulosic structures they are difficult to degrade and do not reach competitive performance. A feasible option to mine these substrates is present in the forestomach of ruminants. Therefore, the aim of the present study was to use rumen microorganisms to improve anaerobic digestion (AD) of crop residues. For this purpose, hemp straw, mechanically pre-treated hemp fibers and shives, flax straw, flax shives, and aged and fresh rapeseed straw were evaluated using the rumen simulation technique. The AD of the substrates was divided into three batches. In two batches, hay was added as a control substrate. In summary, none of the analyzed substrates had an equivalent performance as the control hay, but pre-treated hemp fibers and shives had better AD parameters compared to all other alternative substrates, with the lowest pH (mean: 6.81), highest short chain fatty acid (20.0 mmol/day) and H2 production (25.6 mM) and highest degradability (25.2%). Flax straw had the second-best performance (6.81, 17.4 mmol/day, 20.6 mM and 22.2%, respectively), followed by fresh rapeseed straw, hemp straw, aged rapeseed straw and flax shives. Therefore, hemp fibers and shives demonstrated to be the most suitable substrates for AD. However, since pre-treatment can represent significant additional costs for biogas production, flax straw also demonstrated to be a good alternative.