‘Steam explosion as a pretreatment method to improve biogas production from wheat straw’

Wheat straw (WS) is a lignocellulosic residue with high biogas potential. However, the composition and the crystalline structure of this residue is the main reasons for its low anaerobic biodegradability. As a method to increase the biomethanation of WS, autoclaving, was conducted with a retention time of 30, 60 and 90 min. The subsequent digestion was conducted on a batch mode at different organic load rates (OLR) (2, 4, 8, 12 kg VS/m3 ). The 60 minutes pretreatment seemed to be superior to all other tested treatments offering the highest enhancement on the biomethane production. Besides, for every OLR assessed, the addition of the liquid fraction generated during the pretreatment was found as inhibitory to the methane production. The inhibition effect was decreased along with the increase in the OLR. Furthermore, a biomethane potential test (BMP) was conducted to evaluate the effect of steam explosion pretreatment, which conducted under 13 different pretreatment conditions, on the enhancement of the anaerobic biodegradability of WS. The severity factor (SF) for the pretreatment ranged between 2.61 and 3.35. The highest biomethane yields were offered from samples treated under SF of 2.76, 2.9 and 3.05. As a result, no clear conclusions can be made regarding the optimisation of the pretreatment conditions. The steam explosion pretreatment of WS was also evaluated in a continuous AD system at two different OLR (2 and 5 g VS/L day-1 ). According to the results from this experiment, for both the examined OLR, the steam-exploded samples offered increased methane yields on an average percentage of 20% compared to the yields produced after the digestion WS. During the same experiment, the composition and possible changes in the microbial populations were monitored throughout the experimental period for both feedstocks. The identified microbial populations were similar for the digesters fed with WS and steam-exploded straw (SE). For both systems when the OLR was set to 2gVS/l the most dominant order was Clostridiales while after the increase of the OLR to 5 gVS/l the most abundant order was found to be Bacteroidales. A combination and comparison between the steam explosion and a mechanical pretreatment were also evaluated. On a batch digestion mode, both pretreatments increased the produced gas yields while the combination of the two did not provide any significant enhancement. Similarly, for the continuous digestion mode experiment, the steam-exploded feedstock achieved higher gas yields compared to the mechanically pretreated ones. The two reactors digesting steam-exploded straw demonstrated higher instability with fluctuations on the pH values, fast accumulation of VFAs and low buffering capacity but they seemed to recover fast after the addition of buffer solution and a low-level re-inoculation. Regarding the effect of the two pretreatments on the microbial populations of AD, the steam explosion seemed to affect the microbiology of the system to a higher extent compared to the mechanical pretreatment. In this study it was also evaluated the adjustment of the high C/N that WS usually has, with the use of NH4Cl, combined with the steam explosion pretreatment as a method to further enhance the biogas yields from WS. A co-digestion of WS in various ratios with protein-rich food processing by-products [dried distillers’ grains with solubles (DDGS) and rapeseed meal (RM)] was evaluated. According to the results of this experiment, the addition of NH4Cl was more beneficial for the steam-exploded rather than the untreated WS. For the co-digestion of WS and SE with DDGS and RM, an increase in the cumulative methane production was noted when higher amounts of DDGS and RM were added. On the other hand, the biodegradation of WS and SE was higher when lower amounts of food processing by-products were co-digested in the system. Overall this work increases the knowledge on the steam explosion pretreatment of WS prior to AD and proposes additional techniques and methods for increasing the biodegradability of this type of biomass. According to the results, WS has potential for commercialisation as an AD feedstock while it can partially replace traditional AD feedstocks and potentially increase the financial and environmental sustainability of a full-scale biogas plant