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The effects of a mechanical pretreatment, carried out by a two stages mill followed by fractionation into two different products, were determined on the biodegradability and methane yield of wheat straw. Straw was chopped in a knife mill to an average length of about 30 mm, and a portion was used as reference material. Pretreatment reduced the size of the processed straw, whose median particle size was 300μm and 1200μm for the fine and the coarse fractions, respectively. Untreated and pretreated materials were anaerobically digested in batch reactors under mesophilic conditions for 28 days. The methane yield of raw material was 167.8 Nm 3 tVS -1 , whereas the pretreated material reached an average methane yield of 250.6 Nm 3 tVS -1 , showing a 49.3 % gain on VS basis compared to the feedstock. The finest fraction of the processed material reached a methane production of 264.7 Nm 3 tVS -1 . The specific electric energy consumption was 66.6 kWh per ton of processed material. The energy efficiency was high, since consumption was only 23.7 % of the gross electric energy gain available after pretreatment. These results proved that the investigated pretreatment could be successfully implemented to improve methane production efficiency in anaerobic digesters.

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