1. BMP mesophilic trials were performed to Salix samples pre-treated by steam explosion at different temperatures (170 to 230 °C) and time (5, 10 and 15 mins.) conditions. Biogas production was followed by manometric measurement [8] and its composition analyzed by gas chromatography. Co-digestion mixtures of steam exploded Salix and cow manure were studied in mesophilic, semi- continuously fed CSTRs of 6 L working capacity. Process water was recycled in one reactor after filtration (2 mm). The reactors were fed every 2 nd day with an organic loading rate of 1,5 g VS/L.d (9 gVS/d) and hydraulic retention time of 30 days. The feeding scheme was as follows: Reactor GA1: Salix (40% VS mixture’s content) and cow manure (60% VS mixture’s content) Reactor GA2: same amount of Salix as reactor GA1 and recycled filtrated effluent. Reactors GB1 and GB2: parallels fed with cow manure. Making biogas an attractive energy option in Norway implies optimizing the biogas conversion process. Salix viminalis (basket willow), a short rotation energy crop cultivated vastly in the Nordic countries, is not easily degraded during anaerobic digestion (AD). Salix should be pre-treated, as well as co-digested with animal manure, in order to balance nutrients levels and achieve the best methane yields. Steam explosion involves a high temperature heating combined with a rapid pressure drop that physically disrupts the lignocellulosic structure making cellulose more accessible for microbial degradation [1,2,3,4]. After biodegradation, the methane potential of the digestate represents up to 30 % of the total [5,6,7]. Logistic costs of a digestate (fertilizer) with higher dry matter content are less, in particular in countries with long winters as Norway. Recycling the digestate’s liquid fraction back to the AD process will decrease water consumption, may increase the efficiency of the digestion process as well as decrease the process’s effluent discharges. Introduction The authors would like to express their gratitude to the Norwegian Research Council for supporting this project. Special thanks to Zehra Zengin and Svein J. Horn (UMB) for their contributions. References: [1] Brownell and Saddler, Biotechnol. Bioeng. 29 (1987); [2] Ramos, Quim. Nova 26 (2003); [3] Sassner et al., App. Biochem. Biotechnol. 121-124 (2005); [4] Horn and Eijsink, Biosci., Biotechnol., Biochem. 74 (2010); [5] Hartmann et al., Water Sci. Technol. 43 (2000); [6] Angelidaki et al., Water Sci. Technol. 52 (2005); [7] Jagadabhi et al., Environ. Technol. 29 (2008); [8] ECETOC guideline, Stringer (1988); [9] Yadvika et al., Bioresour. Technol. 95 (2004); [10] Yen and Brune, Bioresour. Technol. 98 (2007). Conclusions Figure 2: Batch and CSTR set ups. Co-digestion gave good biogas yields. Reactor GA1 gave the highest biogas production, however, its methane content was lower compared to reactors GB1 and GB2, running on manure. In reactor GA2 the recycling of process water was performed. At day 10 th was evident the collapse of the reactor, translated into a small biogas production, low methane content, decreasing pH and low level of ammonium (Table 1, Fig. 4). These are indicators of poor buffer capacity leading to acidification, due to the lack of nutrients in the feeding. The C/N ratios of the substrates were 23 for the manure and 64 for the Salix. The co-digestion mixture fed in GA1 possessed a C/N ratio of 39, a higher value compared to literature ones [9,10] that gave a good methane production. Steam explosion of Salix increased the methane yield up to 50 %. Similar methane yields were obtained for all the treatments above 210 ºC. Co-digestion of steam exploded Salix and manure gave similar results to digestion of manure alone while a content of up to 40 % in VS of pre-treated lignocellulosic material is worth to consider. Recycling the filtrated digestate to co-digest it with treated Salix caused the system to collapse due to lack of buffer capacity. Future trials are aimed to study stability in longer digestion periods for co-digestion mixtures and combinations of substrates with recycled effluent. M. Estevez*, R. Linjordet** and J. Morken* M. Estevez*, R. Linjordet** and J. Morken* * Department of Mathematical Sciences and Technology, Norwegian University of Life Sciences, POB 5003, 1432 Ås, Norway. (E-mail: maria.estevez@umb.no; john.morken@umb.no) ** Bioforsk, Norwegian Institute for Agricultural and Environmental Research, Frederik A. Dahls vei 20, 1432 Ås, Norway. (E-mail: Roar.Linjordet@bioforsk.no) Aim Figure 1: Salix viminalis chopped and steam exploded. Optimization of the biogas production at the pre-treatment, process, and post-treatment level by applying steam explosion, co-digestion, and recycling of process water, respectively, and analyzing their effect on the methane yield. Methodology ReactorFeeding Acc. biogas production 1 (mL) Methane content 2 (%) Highest spec. methane yield (mL CH 4 /g VS) Ammonium 1 (mg/L) pH 1 GA1 Salix + manure 80020502466157.4 GA2 Salix + recycled 2299036634105.9 GB1 Manure 745505725110107.6 GB2 Manure 75880612709797.6 Results & Discussion Higher biogas yields were obtained for the pre-treated material, bigger difference was above 200 °C (Fig. 3). Methane content in the biogas varied from 45 to 56 % throughout the 57 days of the experiment. A 50 % increased over the methane yield obtained from non steam exploded Salix was achieved for the treated substrate. 1 Values after 30 days. 2 Average methane content throughout the 30 days. Table 1. CSTRs parameters Figure 4: Biogas (mL) and methane (mL/gVS) profiles for the CSTRs. Figure 3: Biogas production (mL/gVS) from steam exploded and untreated Salix.