Baling and ensiling of wet cereal straw as combined storage and pretreatment for biogas production

Cereal straw is a large biomass resource which may potentially be pretreated for biogas production by combined ensiling and storage. Five pilot-scale ensiling experiments were performed with straw of wheat, barley and rye, baled in midi-bales (0.9*1.2*2.0 m) with various water contents (17–68 %), ensiling additives and ensiling periods (173–230 days). The water content in the bales differed depending on previous precipitation but also the time of day for baling. Temperature in wet straw increased to 30 and 63 °C during storage for bales with and without wrapping, respectively, and mean dry weight loss was 5.1 ± 5.9 % and 10.5 ± 7.3 %, respectively. During storage, pH dropped significantly in wrapped straw bales but not in bales without wrapping. In wrapped straw without additives, pH after ensiling decreased significantly with increasing initial water content, indicating the importance of water content above 30–40 % for ensiling of straw. Besides a more rapid pH reduction when applying acetic acid, there was no significant effect of acetic and formic acid, lactic acid bacteria and brown juice on pH after storage. Ensiling of wet straw had a significant effect on biochemical methane potential (BMP) in some cases, with up to 32 % increase. There was limited effect of ensiling additives on BMP, whereas increasing water content in the straw significantly increased BMP. Improper wrapping of straw during storage reduced BMP with 17–30 %. Overall, proper wrapping and good ensiling of wet straw may result in up to 25 % extra methane yield whereas poor storage may result in up to 37 % loss.